AN OBSERVATIONAL STUDY OF PERSISTENT TEMPERATURE ANOMALIES
OVER THE NORTH AMERICAN REGION
by
Bradfield Lyon
B.S., Meteorology, University of Lowell
(1984)
Submitted to the Department of
Earth, Atmospheric and Planetary Sciences
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
IN METEOROLOGY
at the
MASSACHUSETI'S INSTITUTE OF TECHNOLOGY
August 1991
© Massachusetts Institute of Technology, 1991
All rights reserved
Signature of Author
Center fo Aeteorology and Physical Oceanography
31 August 1991
Certified by
Randall M. Dole, Thesis Supervisor
Accepted by
H. Jordan, Department Chairman
4VIET
-4
L.JumWAES
AN OBSERVATIONAL STUDY OF PERSISTENT TEMPERATURE ANOMALIES
OVER THE NORTH AMERICAN REGION
by
BRADFIELD LYON
Submitted to the Department of Earth, Atmospheric and Planetary Sciences
in partial fulfillment of the requirements for the Degree of
Doctor of Philosophy in Meteorology
ABSTRACT
The primary objective of this thesis is to identify the key physical and dynamical
processes that are responsible for the development and maintenance of persistent
temperature anomalies (PTA's). In order to do this, we have performed detailed
observational studies of the behavior of PTA's, focusing primarily on events occurring
over the US and Canada. Both winter and summer events are considered. For two
summertime heat wave/drought cases, we also investigate potential physical processes that
may contribute to the enhancement or maintenance of these anomalous events.
We first examine the geographical distributions and regional persistence characteristics
of PTA's for both winter and summer seasons. Results for winter indicate that both POS
and NEG PTA's tend to occur over land, with a local maxima in frequency located over
northwestern Canada. Removing high frequency transients from the temperature data
increases the number of events, but does not substantially alter the general geographical
distribution. For winter events, there is no strong tendency toward the recurrence of PTA
events of a given sign (e.g., a cold event is just as likely to be followed by a warm event as
another cold event). In some regions temperature anomaly frequency distributions do
exhibit significant departures from normality, including bimodality.
Similar analyses for summer PTA events in the US and Canada reveal several
similarities but also some significant differences from the winter results. In contrast with
the winter events, geographical distributions of the number of summer PTA events vary
with the sign of the anomaly and with the values of the selection criteria. As in the winter
data, temperature anomaly distributions in some locations display multiple frequency
maxima, although for stations in the Great Plains, temperature anomaly distributions are
unimodal. In certain regions such as portions of the Great Plains there is also a bias
towards the recurrence of POS events (i.e., in summer a POS event tends to be followed
by another POS event).
More detailed synoptic and diagnostic analyses are then performed for winter events
located in the region of most frequent PTA occurrence over northwestern Canada (the
"key" region). The temporal behavior of 850mb temperature anomalies indicates that for
both POS and NEG cases the temperature anomalies develop primarily in situ over the key
region. Moreover, corresponding composite 500mb height analyses indicate that the
developments of the PTA events are often associated with particular anomalous large-scale
atmospheric circulations. For the POS and NEG PTA cases, these anomalous flows
closely resemble, respectively, the "PAC NEG" and "PAC POS" height anomaly patterns
described by Dole (1986a). In the POS PTA cases the height anomaly pattern develops
approximately contemporaneously with the temperature anomalies, while in the NEG PTA
cases the upstream height anomalies develop well prior to PTA onset.
Analyses of the vertical thermal structures indicate that, for both POS and NEG events,
the largest temperature anomalies are typically located near the surface. The first two
empirical orthogonal functions (EOF's) of the vertical thermal structures indicate that the
temperature anomalies are mainly associated with disturbances having deep vertical
structures, with the dominant EOF showing a single reversal in the sign of the anomalies at
the tropopause. Individual events, however, can have vertical structures that depart
markedly from this pattern. For example, the thermal anomalies for the NEG events may
be quite shallow, occurring in the lowest 150-200mb, and sometimes reverse sign above
that level. In these cases warm air intrusions aloft lead to an extremely stable boundary
layer with temperature inversions of up to 20* C.
To further assess the mechanisms responsible for the development of the wintertime
PTA events, 850mb heat budget analyses were performed for 5 POS and 5 NEG cases.
The results show that in the NEG cases horizontal temperature advection and diabatic
cooling are the largest contributors to development, with the diabatic term being the same
order as the advection. In contrast, in the POS cases both horizontal and vertical
temperature advection contribute and are opposed by a small diabatic cooling term. For
both POS and NEG cases the results point to the importance of orography, with shallow
cold air trapped to the lee of the Rockies in the NEG cases with downslope flow associated
with adiabatic warming contributing to the development of the POS cases.
We next turn to an examination of summertime PTA's, focusing on two extreme heat
wave/drought cases that occurred during 1980 and 1988. An initial examination of the
synoptic features associated with these cases reveals that the early stages of both events
were associated with anomalous stationary wave patterns. In an attempt to identify
potential source regions for these anomalous stationary waves we apply a diagnostic
technique developed by Plumb (1985). For the 1980 case, the results suggest that an
anomalous mid-latitude wave source is located over the central North Pacific ocean. For
the 1988 case, there are indications that at early stages anomalous wave activity emanates
from upstream over the western North Pacific; however there is little evidence for an
anomalous tropical source at this time contrary to the suggestion of Trenberth et al. (1988).
For both the 1980 and 1988 cases, the magnitude of the anomalous waves decrease
markedly during the middle and latter stages of the heat wave/drought.
We therefore examine the possibility that these summertime events were enhanced or
prolonged by local changes in the surface energy budget associated with reductions in
evapotranspiration (ET) over the two drought regions. Water vapor budget results indicate
that there is a systematic decrease in monthly-mean ET from June to August during both
events, with an associated reduction in latent heat fluxes from the surface amounting to
roughly 50 W/m 2 in both cases, implying an increase in sensible heating of similar
magnitude. Estimated ET rates for two non-drought summers suggest that the ET rates at
the later stages of the 1980 and 1988 heat wave/drought cases are in fact anomalously low.
Overall, the results suggest that the behavior of wintertime PTA events over the US and
Canada is predominantly determined by large-scale dynamical processes, with orographic
influences having a significant modifying effect. In contrast, in the summer heat
wave/drought cases, both dynamical forcing from a remote source and anomalous
boundary conditions in the drought regions appeared to contribute during the evolution of
the events, with the former being particularly prominent at early stages and the latter
assuming increased importance at later stages.
Thesis Supervisor: Dr. Randall M. Dole
Title: Associate Professor of Meteorology
Acknowledgements
I foremost wish to thank my advisor, Prof. Randall Dole, for his suggestions, support
and unflagging patience during the course of my study at MIT. With so many demands on
his time, Randy's commitment to teaching was also inspirational. My work has also
benefitted from scientific discussions with my other thesis committee members, Kerry
Emanuel, Peter Stone, Earle Williams and Paola Rizzoli. Frederick Sanders, and fellow
students Rob Black, Chris Davis, Peter Neilley and Michael Morgan also provided valuable
input.
The completion of the thesis would not have been possible without other assistance I
have generously received from many people. I am particularly grateful to Rob Black, Peter
Neilley and Chris Davis for their support, advice and friendship over the years. I have
also greatly enjoyed and benefitted from experiences shared with Michael Morgan, Roger
Atkinson, Josh Wurman and Randy Mackie. Administrative assistance from Tracey
Stanelun, Joel Sloman and especially Jane McNabb has been invaluable (and their gracious
tolerance of my many anecdotes appreciated). Diana Spiegel and Peter Neilley have
rescued me innumerable times from the clutches of Unix, VMS and the Cray, while Eddie
Nelson has endured years of my "punmanship".
In addition I owe a great deal to my brother, Jonathan, whose encouragement and
friendship have never faltered and whose scientific endeavors have been inspiring. And
finally, I wish to thank my wife Judy for her patience, understanding and support.
This work was supported by NASA, grant NAG5-927 and the National Science
Foundation, grant NSF 8820938-ATM.
6
TABLE OF CONTENTS
ABSTRACT
............................................................................
3
ACKNOWLEDGEMENTS
..........................................................
5
TABLE OF CONTENTS
..........................................................
7
CHAPTER I: INTRODUCTION
.................................................
11
.......................................
15
CHAPTER II: BACKGROUND
A. Persistent Flow Anomalies
B. Persistent Surface Temperature Anomalies
1. Persistence Characteristics
...............................
2. Relationship to the Large-Scale Flow
3. Anomalous Surface Boundary Effects
.....................
......................
C. Summary and Objectives .................................................
Figures
...................................................................
17
19
20
22
25
CHAPTER III: METHODOLOGY
A . D ata
....................................................................
29
.........................................
B. Case Definition and Statistics
..................................................
C. Compositing Method
..................................................
D. Research Questions
1. Dynamical versus Diabatic Effects
.......................
.....
2. Soil Moisture Deficit and Heat Wave Perpetuation
30
31
31
31
32
3. Tropical Forcing and the 1988 Heat Wave/Drought
.....
Figures
....................................................................
32
33
CHAPTER IV: GEOGRAPHICAL DISTRIBUTION AND REGIONAL
PERSISTENCE CHARCATERISTICS
...........................................................
A. Introduction
B . D ata
....................................................................
35
35
C. Winter Cases
1. Geographical Distribution ..........................................
36
.................................
2. Persistence Characteristics
........................
3. Temperature Anomaly Distributions
38
39
.........................................
4. Contingency Analyses
........................
5. PTA's and Anomalous Snow Cover
40
40
D. Summer Cases
1. Geographical Distribution ..........................................
2. Summertime Persistence Characteristics .........................
........................
3. Temperature Anomaly Distributions
4. Contingency Analyses ............................
............................................................
E . D iscussion
.....................................................................
Figures
CHAPTER V: WINTER CASES
............................................................
A . Introduction
....................................................................
B . D ata
C. Thermal Structure
........................................
1. Horizontal Structure
..........................................
2. Vertical Structure
D. Synoptic Analyses and Time Evolution
........................
1. Evolution of the Thermal Anomalies
2. Corresponding Flow Anomalies .................................
.................................
3. Surface Circulation Features
4. Case to Case Variability .........................................
...................................................
E. Heat Budget Analyses
F. Summary and Discussion ...................................................
..............................................
Table 5.1
.........................
......................
Table 5.2
Figures
.....................................................................
CHAPTER VI: SUMMER CASES
...........................................................
A. Introduction
...............
B. General Characteristics of Heat Waves and Droughts
...............................................
C. Data
D. Case Descriptions
...................................................
1. The 1980 Case
2. The 1988 Case
...........
......................
42
44
44
45
46
49
67
67
68
70
73
74
76
78
79
82
85
86
87
113
113
115
115
116
E. Sources of Anomalous Stationary Wave Activity ........................
...................................................
F. Water Vapor Budgets
G . Summ ary .....................................................................
.............................................................
Table 6.1
.....................................................................
T able 6.2
Figures
.......................................................
CHAPTER VII. SUMMARY AND CONCLUSIONS
A. Summary .......................................................
........
...................................................
B . Conclusions
APPENDIX A. DATASETS .................................................
117
121
124
127
128
129
141
144
. 147
APPENDIX B. TEMPORAL FILTERING ...........................................
149
...........................................
151
APPENDIX C. WINTER CASE DATES
APPENDIX D. EOF ANALYSES OF VERTICAL THERMAL STRUCTURE
APPENDIX E. HEAT BUDGET CASES
...........................................
APPENDIX F. WATER VAPOR BUDGET CALCULATIONS
REFERENCES
................
155
157
159
167
10
I.
Introduction
Observational studies indicate that during most seasons, anomalies in the large-scale
circulation develop (manifestations of departures from climatological values of the upperlevel geopotential height field on a given day) which may persist well beyond the periods
associated with synoptic scale variability (e.g. Dole and Gordon, 1983, hereafter DG;
Shukla and Mo, 1983). Extreme events can last for most of a season, such as during the
winter of 1976-77 (e.g. Namias, 1978), although this behavior is rare. More common are
cases where major flow anomalies persist for durations of a few weeks to a month.
In recent years, a number of investigators have studied these persistent and anomalous
upper-level circulations (hereafter persistent flow anomalies, or PFA's). Considerable
progress has been made toward identifying characteristic aspects of the temporal and
structural behavior of PFA's and determining potential mechanisms for their development.
So far, most observational studies have focused on upper-level geopotential height and
flow anomalies associated with PFA's of the extratropical Northern Hemisphere wintertime
circulation. Examples of these investigations of PFA's are analyses of their geographical
distribution and persistence characteristics (DG; Shukla and Mo, 1983; Knox and Hay,
1985), their time-mean structures (Blackmon et al., 1984a; Dole, 1983b, 1986a) and
temporal evolutions (Blackmon et al., 1984b; Dole, 1986b; Dole and Black, 1990; Black,
1990).
Although these and other studies have described important aspects of PFA's, our
understanding of the mechanisms which generate and maintain persistent weather
phenomena is still far from complete. For example, it is unclear to what extent large-scale
dynamical processes alone can account for the development and maintenance of long-lived
surface weather anomalies, or whether other factors, such as anomalous boundary
conditions (either local or remote to the region) also play significant roles (see e.g.,
Wallace, 1987).
In the field of extended-range weather prediction, a problem of particular practical
importance is to identify those factors which contribute to the formation and maintenance of
persistent temperature anomalies (PTA's). Particularly extreme surface PTA events may
have enormous social and economic consequences.
For example, the direct losses
attributed to the abnormally cold and persistent weather during the winter of 1976-77 are
estimated at $40 billion (Hughes, 1982). Nevertheless, our understanding of the processes
leading to the development and maintenance of these events is quite limited.
In this thesis we will adopt an observational approach to investigate the characteristics
and mechanisms responsible for the development and maintenance of PTA's. We will
examine PTA events occurring in both winter and summer seasons, focusing primarily on
major cases occurring over the US and Canada.
Among our principal objectives are to:
1) Objectively define PTA's and to assess their geographical distributions and
regional persistence characteristics.
2) Identify systematic features in the large-scale flow proceeding and following the
development of major cases of PTA's.
3) Analyze the three dimensional structure and time evolution of PTA's.
4) Estimate, through diagnostic analyses of near-surface heat and energy budgets, the
relative importance of dynamical and diabatic processes in the development and
evolution of the events, and
5) ascertain whether anomalous boundary conditions are likely to play key roles
the development and/or maintenance of PTA's.
Among the specific questions we will address are:
*What are the geographical distributions of PTA's, and how do they vary with the
seasons?
. Do the persistence characteristics of PTA's vary depending on the sign,
magnitude or duration of the anomalies?
in
- What are the relationships between PTA's and PFA's? To what extent are surface
PTA's specifiable from upper-level PFA's?
*Do internal dynamics or anomalous physical processes appear to play the dominant
role in the development and maintenance of PTA's? To what extent is the answer to
this question dependent on the type of event (i.e. warm vs. cold), location, and the time
of year in which it occurs?
The thesis is organized as follows. In the next Chapter, we first present a brief review
of research related to PTA's. This review indicates that previous studies have largely fallen
into two broad areas: PFA's at mid-tropospheric levels, and statistical analyses of
monthly-mean surface temperature anomalies. We briefly discuss basic results for both of
these areas. For surface temperature anomalies, results of prior correlation studies are
described.
We also consider some previous results indicating the potential role of
anomalous boundary conditions in enhancing and maintaining surface temperature
anomalies.
Chapter III outlines certain hypotheses for PTA development and maintenance that we
will examine, and describes aspects of our methodology that will be employed in the
remainder of the thesis. Chapter IV then presents results on the geographical distribution
and regional persistence characteristics of PTA's. Both wintertime and summertime events
are considered.
Chapter V examines in more detail the synoptic and diagnostic
characteristics of winter PTA cases. Characteristic aspects of the vertical structure of
PTA's are identified using empirical orthogonal function (EOF) analyses. A heat budget
analysis is also performed to identify the dominant factors in the development of the
thermal anomalies.
In Chapter VI two summertime POS PTA events are examined using a case study
approach. After providing a synoptic overview of the two events, we employ diagnostic
techniques to identify potential sources of anomalous stationary wave activity that occur
with these cases. We are particularly interested to see whether there is persuasive evidence
14
that anomalous wave propagation emanating from the tropical Pacific played a key role in
the onset of the 1988 heat wave/drought event, as suggested by Trenberth et al. (1988).
We also obtain estimates of monthly-mean evapotranspiration rates to assess the potential
role of anomalous surface boundary conditions in the maintenance of these events.
Finally, Chapter VII summarizes our main results and presents our primary
conclusions.
II.
Background
A thorough review of prior research on topics related to this study would be both
lengthy and impractical. Instead, we will briefly review some of the more significant
results of related work, stressing important unanswered questions to be addressed in the
thesis. We have organized the previous studies into two broad categories: a) Persistent
flow anomalies (PFA's) and b) Persistent surface temperature anomalies (PTA's). At the
end of this Chapter, we summarize some of the outstanding questions in these areas.
A. PersistentFlow Anomalies
Previous observational studies of PFA's have primarily emphasized anomalous flow
characteristics at mid-tropospheric levels, with the "blocking" studies of Rex (1950a,
1950b) being a prototypical example. However, in these earlier studies, varying and
mainly subjective criteria were used to identify cases (see Dole, 1982 for a review).
Dole and Gordon (1983, hereafter DG) described an objective procedure for identifying
PFA's which they then applied in a study of the Northern Hemisphere wintertime
circulation. For a given location, DG defined a "persistent anomaly" if the magnitude of
the anomaly exceeded a specified value M for a specified duration T (fig. 2.1). We will
later apply similar criteria to identify PTA "events". DG found that PFA's tended to occur
over three geographical locations: the central North Pacific Ocean, the North Atlantic
Ocean to the southeast of Greenland, and the northern Soviet Union (Fig. 2.2). These
regions also roughly coincide with the regions of maximum temporal variance in the 500mb
height field (Sawyer, 1970; Blackmon,1976).
DG further studied the statistical characteristics of PFA's within their key regions.
Among their findings were:
- For corresponding values of the selection criteria, positive height anomalies (POS)
and negative height anomalies (NEG) events had comparable frequencies and similar
geographical distributions;
* The number of POS and NEG events decreased approximately exponentially with
increasing durations;
- There were no strongly preferred anomaly values for anomalies of either sign;
. The average duration for POS and NEG events was approximately
two weeks, although no preferred durations were identified.
The horizontal and vertical structures of composite PFA's were examined in subsequent
studies by Dole (1983b, 1986a). We will primarily discuss his results for PFA events over
the central North Pacific (his PAC cases) for, as will be shown in Chapter V, PAC PFA
events are often directly related to major PTA events over the US and Canada. The 300mb
composite height anomaly patterns for the PAC POS and PAC NEG events are shown in
fig. 2.3. The height field for POS cases is reminiscent of that seen in earlier blocking
studies (e.g. Rex, 1950a) with an anomalous ridge over the key region. In contrast, the
NEG cases are associated with an enhanced zonal flow across the mid-latitude North
Pacific Ocean extending from the Asian coast eastward to just south of the key region. The
corresponding height anomaly analysis for the POS events shows that the anomalous ridge
over the key region is one part of a larger PFA pattern, with a train of anomalies of
alternating sign extending downstream from the key region. The PAC NEG anomaly field
is similar to the POS pattern but with opposite polarity. The vertical structure of both the
PAC POS and PAC NEG case-mean height and temperature anomalies (Dole, 1986a)
display only modest vertical tilts with maximum anomaly values near the tropopause.
The fully developed PAC POS and PAC NEG height anomaly patterns closely
resemble the Pacific North American (PNA) teleconnection pattern described by Wallace
and Gutzler (1981). This PNA pattern has long been associated with abnormal, timeaveraged (monthly and seasonal mean) surface weather conditions over North America.
The use of long-term time averages, however, limits their potential applicability in
determining the proximate mechanisms responsible for PTA development. For example,
life cycle studies of PFA's by Dole (1986b, 1989) indicate that development of the PFA's
often occurs rapidly (in a few days). In addition, the vertical structure of the mature
anomaly pattern may not be indicative of the key processes occurring during its
development.
For instance, Dole and Black (1990) and Black (1990) have shown
baroclinic processes are likely to play an important role during the early stages of PAC
NEG development, even though at later stages, the structure is nearly equivalent
barotropic.
B. PersistentSurface TemperatureAnomalies
Our discussion of prior work will focus on three topics central to this thesis: 1)
persistence characteristics of PTA's, 2) relationships of PTA's to large-scale flow features
and 3) possible influences of anomalous surface boundary effects on PTA's. We will
briefly summarize prior research on these topics and note how we will extend prior studies
in these areas.
1. PersistenceCharacteristics
Early investigators utilized persistence as the simplest method of extended-range
forecasting (e.g. Namias, 1953). The most frequently used measure of persistence in these
studies has been the correlation of anomalies in successive monthly means (Reed, 1933;
Namias, 1952, 1953; Dickson, 1967; Van Den Dool, 1984; Van Den Dool et al. , 1986).
For the US, correlations have been calculated for both annual and seasonal data, (Van Den
Dool et al. , 1986). Fig. 2.4 shows a typical example. We see that correlations between
monthly means are typically modest (0.1 to 0.3), with the largest values occurring during
the winter and summer seasons and usually along coastal regions.
Van Den Dool et al. (1986) have shown the observed persistence in annual temperature
data for widely different climatic regions is greater than would be expected from averaging
daily weather fluctuations, which they modeled as a linear first order (Markov) process 1 .
This result is similar to the findings of Madden and Shea (1978) who identified large
regions of the country where the observed variability of monthly mean temperature is larger
than that expected from a first-order Markov process 2 . They suggested that this enhanced
low frequency variance could arise from either persistent anomalies in the large-scale flow
pattern, or through forcing from anomalous boundary effects (e.g., snow cover, soil
moisture, sea surface temperatures, etc.), or through some combination of the two. We
will later review prior work on these potential contributors.
It is important, however, to first consider certain assumptions and limitations involved
in analyzing PTA's through correlations of either monthly or seasonal anomalies. First,
correlations do not distinguish between positive and negative events; however, there is no
reason a priori to expect that anomalous events of opposite sign should have similar
persistence characteristics. Also, lag relationships between monthly anomalies may depend
on the magnitude of the initial anomalies (i.e., be non-linear in amplitude). Further,
contrary to the assumptions of numerous studies, temperature distributions may not be
Gaussian. McIntyre (1950), for example, has provided evidence of bimodality in 700mb
wintertime temperature frequency distributions at selected locations in the US. Finally, as
monthly anomalies are often a manifestation of considerably shorter-lived events, some of
the month-to-month persistence may be accounted for simply by the arbitrary choice of
1The appropriateness
2 They termed these
of this assumption has been questioned by Straus and Halem (1981).
regions as having potentially predictable monthly means assuming daily weather not to
be predictable on monthly time scales (see Leith, 1975).
calendar dates. A more detailed and less restrictive analysis of the development of these
events is therefore a fundamental goal of this thesis.
2. Relationshipto the large-scaleflow
Numerous observational studies primarily of monthly or seasonal mean data have noted
both lagged and contemporaneous relationships between anomalies in surface temperatures
and large-scale flow anomalies. Namias (1953), for example, has provided monthly
frequencies and spatial descriptions of 24 distinct weather anomaly "types" for the US. To
supplement these descriptive relationships, statistical methods have been used to develop
specification equations which relate weighted, time-averaged values of the upper-level
height field (or height anomalies) to average surface temperatures (or temperature
anomalies) at selected stations (e.g. Martin and Leight, 1949; Namias, 1953; Klein et al.,
1959).
Though in recent years a number of observational and theoretical studies have been
concerned with the development and maintenance of PFA's, the relation of anomalous
upper-level flows to surface temperature remains problematical and, for the most part,
based on statistically-derived results. For example, specification equations are still widely
employed operationally in extended range predictions (Klein, 1983, 1985; Kline and Klein,
1986; Van Den Dool et al., 1986). These studies indicate that, depending on the season
and geographic location, from 40 to 80 percent of the variance in observed temperatures in
the US is accounted for by values specified from the 700mb height field.
Of course, there is no reason a priori to expect that the full time-mean behavior of
surface temperatures should be determined from a time-mean height field (typically 700mb)
alone. For instance, DG have shown that PFA's in the 500mb height field often develop
rapidly. Since these PFA's may develop during any part of a particular month, the utility
of relating monthly mean surface temperatures to upper-level heights is likely to be
reduced. Perhaps more importantly, variations in the vertical structure of the thermal
anomalies may also be quite significant. A particularly shallow temperature anomaly, for
example, is unlikely to be well reflected in the 700mb height field. As mentioned earlier,
the character of surface boundary conditions, such as snow cover or soil moisture may also
be important.
In the thesis we will focus particular attention on identifying circulation features and
vertical thermal structures associated with the development of major PTA episodes. How
the relationships vary with the sign and the season of occurrence of the temperature
anomalies will also be examined.
3. Anomalous surface boundary effects
Both local and remote anomalous surface boundary conditions (ABC's) may influence
surface temperature anomalies. Locally, the surface temperature depends on the surface
energy balance, so factors which systematically alter this balance may play a dominant role
in the development or maintenance of the anomalies.
Two potentially important
contributors which we will examine are anomalous snow cover and soil moisture. Remote
ABC's may also alter the large-scale flow (by altering the diabatic heating distribution
associated, for instance, with convection) which, through mechanisms such as Rossby
wave propagation, may subsequently affect the surface temperature in areas well removed
from the ABC's. For example, anomalous sea surface temperatures (SST's) (both tropical
and extratropical) have long been suggested as having important influences on remote flow
and temperature patterns (e.g., Hoskins and Karoly, 1981; Trenberth et al., 1988).
For the purposes of the thesis, we limit our discussion to local effects of ABC's. Local
ABC's have been suggested as contributing to the month-to-month persistence in surface
temperatures, for example, in winter cold "spells" (Lamb, 1954; Namias, 1962) and
summer heat wave/doughts (Namias, 1962, 1978, 1982, 1991).
Possible influences of ABC's have been studied both observationally and through
sensitivity studies with numerical models. The role of anomalous soil moisture in
modifying surface temperature was suggested in early observational work by Namias
(1960). He indicated that there was a negative correlation between spring precipitation
amounts and summer temperature anomalies over the central US. Madden and Williams
(1978) have subsequently shown a similar relationship for concurrent seasonal
temperatures and precipitation. Walsh et al. (1985), using specification equations of the
type mentioned in Section 2, attribute errors of ~ 0.50 C in specified monthly mean air
temperature to anomalous soil moisture. These results are consistent with the findings of
Hao and Bosart (1987) who, studying the US heat wave of 1980, show a secular
enhancement of surface temperature occurring in unison with decreasing evapotranspiration
rates.
Sensitivity experiments with a general circulation model (GCM) by Mintz and Shukla
(1982) have also shown a large response in surface temperature to changes in soil
moisture. They indicate that modeled July surface temperatures were 15* to 25' C higher in
their "dry" soil versus "saturated" soil runs. GCM studies by Wolfson et al. (1987) and
Oglesby and Erickson (1989) have stressed the role of soil moisture in perpetuating the
1980 and 1988 US summer droughts. Other model studies have considered changes in
albedo, interactive versus non-interactive soil moisture, and response to regional anomalies
(see e.g. Mintz, 1984; Shukla, 1984).
The effect of variations in snow cover on short-term climate fluctuations have also been
considered by a number of investigators. Local effects have been studied by Wagner
(1973) and Heim and Dewey (1984), who have shown a negative correlation between
anomalies in snow cover and surface temperature using monthly and weekly data,
respectively. Walsh et al. (1985) attribute errors in "specified" monthly temperatures of 10
to 20 C to anomalous snow cover. Errors of 50 to 100 C in model output statistics (MOS)
for short-range forecasts have also been attributed to anomalous snow cover by Dewey
(1977). Among numerical studies, Walsh and Ross (1988) indicate the largest response to
anomalous snow cover is at low levels in the atmosphere, coincident with the region of the
anomaly.
Many of the previous investigations have considered effects of anomalous soil moisture
and snow cover in isolation, rather than in relation to particular PTA events.
An
unanswered question is how ABC's act in conjunction with other processes in the
development and/or maintenance of these events.
We will consider this question
observationally when we examine soil moisture variability and associated surface energy
balances during the summer heat wave/droughts of 1980 and 1988. We will also discuss
some statistical relationships obtained between anomalous snow cover and and wintertime
PTA events.
C. Summary and Objectives
Investigations of PFA's have primarily emphasized characteristics of development and
evolution at mid-tropospheric levels, typically at 500mb. Previous studies of PTA's have
been largely statistical, with time-averaged surface temperatures "specified" from a mean
large-scale flow (usually at 700mb).
Most studies of PTA's have employed lag correlations of monthly anomalies. This
approach is likely to be inadequate for discerning the time evolutions and three-dimensional
structures of many important cases of PTA's. As a result, knowledge of the key
physical/dynamical mechanisms responsible for the development and subsequent
maintenance of PTA's remains incomplete.
These shortcomings have motivated us to undertake an objective and systematic study
of PTA's. Among our specific objectives are to:
1) Objectively define PTA's and to assess their geographical distributions and
regional persistence characteristics.
2) Identify systematic features in the large-scale flow proceeding and following the
development of major cases of PTA's.
3) Analyze the three-dimensional structure and time evolution of PTA's.
4) Estimate the relative importance of dynamical and diabatic processes to the
development and maintenance of PTA's.
5) Ascertain whether anomalous boundary conditions are likely to play a key role
during the development and evolution of PTA's.
Chapter III describes our basic methodology and datasets.
Chapter IV presents
analyses of the geographical distributions and persistence characteristics of PTA's. More
detailed analyses of winter and summer cases are given in Chapters V and VI, respectively,
with the overall results summarized in Chapter VII.
24
onomoty
- -
-I
M(pos)
- -
_ -
- -
0
T
-
Fig. 2.1. Method used by Dole and Gordon (1983) to define persistent anomalies. A
persistent anomaly event was identified when an anomaly exceeded a specified
magnitude (M) for a specified duration (T). From Dole and Gordon (1983).
Ft
d
of
-
ts
ot
90w
3
D
1
9
v
16
41
.90
Fig. 2.2. Wintertime distribution of the sum of the POS and NIEG PFA's identified by
Dole and Gordon for the 14 winters from 1963-1964 to 1976-1977. Events met the
M) T criteria (± l00m, 10 days) for low-pass filtered data.
300MB HEIGHT ANOMALY (M) DAY +5 (PN)
0,
L
I
2
A
4A4W
Fig. 2.3. Composite 300mb height anomalies (m) for the fully developed a) PAC POS
and b) PAC NEG height anomaly patterns. From Black (1990).
.4
%~~~__
X.
4
22
'LI
.
-n
\
V5\
.3
-'
~p
'
-'
J
'25
A
2
(h
.
.~.....
Fi.2..Lg oreainsbtwe
onhymensufc arteprtue fra wne
and monhs.
b summrFom Va Den ool Ta.(18)
28
III.
Methodology
Our general goals are to document the geographical and regional persistence
characteristics of PTA's, identify their typical structures and temporal behaviors and to
discern the physical/dynamical mechanisms responsible for their development and
subsequent maintenance. Cases will be separated into classes that depend upon the sign of
the anomaly and the season in which the event occurs.
A. Data
Three basic datasets are used in the study: gridded objective analyses, climatological
station data, and radiosonde sounding data (for more details of these sets, see Appendix
A).
These sets were obtained from archives maintained at the National Center for
Atmospheric Research (NCAR).
The gridded data are based upon objective analyses
derived at the National Meteorological Center (NMC) and the European Centre for Medium
Range Weather Forecasts (ECMWF). Sounding data were obtained from NCAR archives,
while the climatological station datasets were obtained from NCAR and the Atmospheric
Environment Service (AES) in Canada.
The NMC and ECMWF gridded datasets both consist of twice daily (0000 UTC and
1200 UTC) final analyses of temperature, geopotential heights, and u and v wind
components for all standard pressure levels between 1000mb and 100mb. The ECMWF
grids also contain an initialized "vertical" motion field. The NMC grid is 20 latitude by 5'
longitude over latitudes 20'N to 904 N and covers the 24-year period from 1963 through
1986. The ECMWF grid is approximately 4.5* by 7.54 with global coverage, with the
ECMWF set extending over the 14-year period 1976 through 1989 (Appendix A).
The climatological station data consist of daily values of maximum and minimum
temperatures, precipitation and observed snow cover for approximately 400 stations in the
US and for 175 Canadian stations. Time series at individual stations typically have lengths
ranging from 30-50 years, with some records extending back over 100 years1 . The dataset
for US stations extend through December 1986, and for Canadian stations, through
December 1988.
The sounding data consist of standard, twice-daily (0000 UTC and 1200 UTC)
radiosonde observations for US and Canadian stations for the 14-year period 1976-89. All
mandatory and significant level temperature, humidity and wind data are included.
In some of our analyses, the data have been filtered in time in order to remove higherfrequency fluctuations. Unless otherwise specified, we have used a four-point filter for
this purpose. This filter has a response of 0.5 at a period of roughly 6 days; details of the
filter and its response function are provided in Appendix B.
B. Case Definition and Statistics
For this study winter and summer seasons are specified as the periods 1 December - 28
February and 1 June - 31 August, respectively. To define PTA events, we have applied the
threshold-crossing method of DG to surface temperature data. As in DG, we define an
event as a run of anomalies at a given point that exceeds specified magnitude (M) and
duration (T) criteria (fig. 3.1a). We will refer to the date when the anomaly first crosses
the threshold value as onset or "day(0)". Other dates will be described relative to the onset
date: e.g., 5 days prior to onset is day(-5), 5 days after onset is day(+5). In addition, we
define the magnitude of an event as the sum (integral) of the temperature anomalies over
the case duration (fig. 3. 1b). These basic definitions allow us to directly extend the work
of DG to the analysis of surface temperature data and will also provide a more
comprehensive documentation of the geographical distribution and regional persistence
characteristics of PTA's than has so far been available. In addition, this approach allows
1Many
of these time series contain "gaps" due to missing data. The handling of missing data is described
in Appendix A.
us to more directly compare persistence characteristics of temperature and 500mb height
anomalies.
C. CompositingMethod
To aid in identifying the systematic features of PTA development, we have used
compositing in several analyses. The composites are obtained by averaging a given field
over multiple events at times synchronous relative to the onset time. For example, in
Chapter V composite evolution analyses are presented of 850mb temperature and 500mb
height anomalies. Both composite means and variances have been determined, with the
statistical significance of the composite anomalies estimated using a two-sided t-test.
In addition, composite heat budget analyses of winter events have been performed.
Here the terms in the thermodynamic energy equation are first determined from the case
data and then composited over all events.
D. Research Questions
Some of the specific research questions we will address in subsequent Chapters are:
i) the roles of dynamical versus diabatic effects on the development of wintertime NEG
events; ii) the role of soil moisture deficits in the maintenance of summer heat waves; and
iii) observational evidence for tropical forcing of anomalous stationary waves during the
development of the 1988 heat wave/drought. We expand on these ideas below.
i. Dynamicalversus DiabaticEffects on Winter NEG Development
The role of radiational cooling in the development of continental polar air has long been
recognized (see e.g., Petterssen, 1956). In more recent years, however, there has been an
increased emphasis on the role of large-scale dynamics on the development of anomalous
surface weather conditions. In Chapter V we examine whether dynamical or diabatic
processes (most directly in the form of longwave radiative cooling) play the primary role in
the development of anomalous cold air over parts of the US and Canada during wintertime.
Our results suggest the two effects may be complementary.
ii. Soil Moisture Deficit and the Persistenceof Heatwaves
The enhancement and/or maintenance of summer heat waves due to anomalously low
soil moisture has long been suggested (see Oglesby and Erickson, 1989). In Chapter VI
we address this question through analysis of monthly mean evapotranspiration rates for the
1980 and 1988 heat waves in the US. Using a surface energy balance approach, we
examine whether changes in evapotranspiration rates occur during the heat wave/droughts
that could lead to significant changes in surface sensible heating. Although our results
cannot prove causality of the persistence of the heat waves, they serve as a measure of the
potential magnitude of this effect on the surface energy balance.
iii. TropicalForcing and the 1988 Heat Wave/Drought
Trenberth et al. (1988) and Palmer and Brankovic (1989) have argued that an
anomalous wave train associated with the 1988 heat wave/drought was forced by an
anomalous sea surface temperature distribution in the tropical east Pacific Ocean. In
Chapter VI we investigate this possibility through analyses of anomalous stationary wave
activity fluxes. This technique provides us with one means for examining whether
anomalous wave propagation from the tropics is likely to have played a primary role in the
development and maintenance of the 1988 North American heat wave/drought.
(a)
( M ----- - ------- --(+>M'--~~
T
D
0
+-
------------------------ ----Time
C
(-)M --
(+)b)
................
0
.
.
T
----------
----
-----------------
D.
-
-
....
[.
Time
(-)M
Fig. 3.1. Method for defining a) PTA events and b) event magnitude. An event is
defined as in Dole and Gordon; the event magnitude is defined as the integral of the
temperature anomalies over the event duration, stippled in (b).
34
IV.
Geographical Distribution and Regional Persistence Characteristics
A. Introduction
In this Chapter we examine how PTA's are distributed geographically. For selected
locations over North America, we also present more detailed statistical analyses of the
persistence characteristics. Results are obtained for both winter and summer seasons, and
are compared with corresponding results obtained previously for 500mb height anomalies.
B. Data
Our primary data consist of gridded 850mb temperatures from the NMC dataset and
climate station surface temperature data for selected US and Canadian stations. The
gridded data enables us to examine certain general characteristics of temperature variability
over the extratropical Northern Hemisphere. Our surface data consists of observations
from US and Canadian stations. In analyses with gridded data, we have used 850mb
instead of 1000mb temperatures, due to the generally poorer quality of the latter (Trenberth
and Olson, 1988a).
For the 850mb data, temperature anomalies are defined as the difference between 1200
UTC values and a climatology determined at each gridpoint as the sum of the first four
Fourier harmonics of the annual cycle. The Fourier coefficients for the NMC data are
based on the 23-year period 1964-1986 and were supplied by Dr. Robert Black. For
surface temperature analyses daily mean values were used, the daily mean being defined as
the average of the maximum and minimum temperatures. Anomalies were defined as the
difference between observed daily mean values and a climatology determined as the sum of
the first four Fourier harmonics over the 25 year period 1954-1978.
Many of our
calculations are performed using light low-pass (LLP) filtered data to remove higher
frequency (periods < 10 days) fluctuations from the data (for details of the filter response
function, see Appendix B).
Note that we have not normalized the anomalies by their standard deviations as done in
many previous studies (e.g. Madden and Shea, 1978; Van Den Dool, 1986). This allows
us to readily identify events which are of large magnitude in an absolute sense. These
large, sometimes extreme, events are of interest both scientifically and practically. Such
events provide a large signal for analysis which might also be obscured by normalization if
the anomalies occur in a region of large variance.
C. Winter Cases
1. GeographicalDistribution
We first examine the geographical distribution of wintertime PTA's at 850mb for the
Northern Hemisphere extratropics.
Fig. 4.1 shows the distribution of the number of events in the 23 winters (1963-64 to
1985-86) meeting the magnitude (M) and duration (T) criteria of (± 5' C, 10 days) for both
unfiltered and LLP filtered data. We see that, as might be anticipated, PTA events display a
strong continentality bias. For the unfiltered NEG cases, maxima in frequency are located
in an elongated region extending from western Canada northwestward toward Alaska (we
will hereafter refer to this as the CAN region) and over Greenland. Approximately colocated maxima are seen for the POS cases, with an additional maxima located just to the
east of Hudson Bay. Note that results over Greenland and south-central Asia should be
interpreted with considerable caution, given that 850mb temperatures in these regions are
often based on extrapolations to levels below the local terrain heights (Trenberth and Olson,
1988a).
Removing the higher-frequency transients (fig. 4. 1c and 4. 1d) increases the number of
events in both POS and NEG cases. For the NEG cases, the overall pattern remains
unchanged, with the number of CAN events increasing by roughly 40%. For the POS
cases, the basic pattern also remains unchanged but the CAN maxima becomes even more
dominant, with the number of events there increasing by a factor of 2. The relatively larger
increase in POS events for the filtered data suggests high-frequency transient interruptions
are more common during POS than NEG regimes. The distributions for the filtered POS
and NEG cases are quite similar, with an almost equal number of events in the
approximately co-located CAN maximum. These patterns are also generally similar to
those of the low-pass variance in 850mb temperatures (periods > 10 days) presented by
Blackmon (1976) and shown in fig. 4.2.
The regional patterns of PTA frequency contrast markedly with those of the 500mb
PFA events of DG (cf. fig. 2.2), who identified local maxima in PFA's over the North
Atlantic and North Pacific oceans and the northern Soviet Union and a relative minimum
over the North American continent. The lack of larger magnitude (near) surface PTA
events over the oceans is quite understandable, given the large differences in heat capacity
between the ocean and land, and the tendency for sensible heat fluxes to rapidly modify air
which has a temperature that differs significantly from that of the underlying waterl.
We have examined similar distributions for different values of M and T. Fig. 4.3
shows the regional frequency distributions for LLP filtered anomalies for criteria (± 20 C,
10 days). We see that a relaxation of the anomaly threshold (while holding the required
duration constant) leads to an increase in the events everywhere, including over the oceans.
However, the general patterns for POS and NEG events remain quite similar.
Fig. 4.4 presents results for LLP filtered anomalies meeting the criteria (± 50 C, 5
days). As expected, decreasing the required duration also increases the number of events.
1For
example, see Chen et al. (1985) pg 359-360.
Again, however, this does not substantially alter the overall geographical distribution, with
the dominant maximum in events again located in the CAN region.
Finally, we have computed the number of PTA events occurring in "zero runs", where
only the sign of the anomaly is considered (not its magnitude). Results for LLP filtered
anomalies for zero runs of POS and NEG events lasting at least 10 days are presented in fig
4.5. The geographical distribution of these events differs from our previous distributions,
the most striking feature being a significant increase in the number of both POS and NEG
events over the oceans. Although there remains a slight continentality bias the CAN
maximum is not clearly indicated, especially for the NEG events. Asymmetries between
the number of POS and NEG events are most evident over North America. Overall these
zero runs indicate PTA's of small magnitude are prevalent outside the CAN region,
including over ocean areas.
2. PersistenceCharacteristics
For stations within the CAN region, we have examined whether there are preferred
durations of events and if so, whether they depend on the sign or magnitude of the
anomaly. To do this we have followed the method of DG and have examined the
cumulative frequencies of events as a function of duration. The results for two such
stations are shown by the heavy solid lines in fig. 4.6. The distributions seen in these lognormal plots are, to first order, straight lines. Although individual distributions do display
some curvature, we find no systematic changes in the slope of the distributions with
increasing duration. We do see, however, a tendency for fewer POS events at larger
magnitudes and durations for the two stations.
For anomalies of either sign our results provide little evidence for preferred anomaly
durations. Straight lines on these log-normal plots indicate that the probability of an event
of duration n days lasting n+1 days is constant. This behavior may be closely modelled as
a linear first-order autoregressive (or Markov) process. Using the method of Flocas (1981)
the cumulative number of events, Fc, of duration r may be written as
Fc = N Pr- 1
(4.1)
where N is the total number of observed events, P the probability that an event of n days
will last n+1 days, and r is the duration in days. Knowing f(r), the cumulative frequency of
events of duration r, the value of P is determined by equating the observed mean of the
distribution to the model mean
f(r) -r
X f(r)
1~
.
and solving for P. A decay rate for the number of events may also be determined as -1/(2.3
logP). Markov fits to the observed frequency distributions are shown as dashed lines in
fig. 4.6 with their associated decay rates also indicated.
3. Temperature Anomaly Distributions
Fig. 4.7 displays temperature anomaly distributions for four stations in western and
northwestern Canada. These plots each consist of two curves, one for the observed
distribution and the other of a normal distribution having zero mean and the observed
variance. Also indicated are the standard deviation, skewness and kurtosis (the first three
moments about the mean of the distributions). For reference, a normal distribution has
zero skewness and a kurtosis of 3.
In all four plots we see departures from normality that are suggestive of bimodality,
especially for the more southern stations (fig. 4.7a and 4.7b). The distributions tend to be
flatter than normal distributions (kurtosis < 3) and are negatively skewed. Using the
statistical tables of Brooks and Carruthers (1953) and assuming 5 days between
independent samples (Madden and Shea, 1978), the observed (low) kurtosis in all four
plots is statistically significant at the 95% confidence level, although the negative skewness
is not. In Chapter V we discuss aspects of PTA development which are consistent with
these observed temperature anomaly distributions.
4. Contingency Analyses
For the CAN region we have tested for other possible behaviors suggestive of nonlinearity. In fig. 4.8 are contingency plots for event magnitudes, where the magnitude of a
given event is plotted against the magnitude of a subsequent event. Recall that the event
magnitude is the sum of the temperature anomalies over the case duration. For POS (NEG)
cases the event magnitude is therefore a positive (negative) number. These plots indicate
whether a bias exists for recurrence of events of a given sign.
From fig. 4.8 we do not see a strong preference for recurrence of events of a given
sign. For example, it is about as likely that a NEG event will be followed by a POS event
as it is to be followed by another NEG event. This result holds as well for other stations in
the region (not shown).
5. PTA's and Anomalous Snow Cover
The role of anomalous snow cover in enhancing winter "cold spells" has long been
suggested (see Cohen and Rind, 1991). Snow has a high albedo and emissivity and a low
thermal conductivity, all factors which may favor a reduction in surface temperatures
compared to those occurring over bare ground. For example, a fresh snow may reduce the
amount of solar radiation absorbed by an otherwise bare surface by 50% (see Sellers,
1965), while the high emissivity may act to increase the longwave radiative heat loss. The
low conductivity of snow inhibits the flux of heat from the underlying ground.
We have examined the relationship between anomalous snow cover and the occurrence
of NEG PTA events in regions of the US that are near the climatological-mean snow
"edge". The snow cover data have been obtained from the US climate station dataset, with
the snow edge defined as a line delineating where the long-term (we use the 25-year period
1954-78) average monthly snow cover is an inch or more from where it is an inch or less.
Studies by Walsh et al. (1985) indicate that, for monthly means, regions near the snow
edge in the Midwest and northern Plains have the largest potential response in surface
temperatures to anomalous snow cover.
In fig. 4.9 snow cover anomalies are plotted against event magnitude for three US
stations near the climatological winter snow edge. The event criteria were (-5' C, 10 days)
and the temperature data were temporally filtered. The snow cover anomaly is defined as
the difference between the average snow cover during an event and a 25-year mean value
for the month in which the event occurs (i.e. December, January or February). If an event
occurs during parts of two different months, the mean snow cover for the month having the
majority of days in the event is used.
We see in fig. 4.9 that for all three stations the smaller magnitude PTA events are about
as likely as not to be accompanied by anomalous snow cover. For larger magnitude
events, the behavior for the three stations is different, with some stations (fig. 4.9a) being
frequently associated with anomalous snow cover while others are not (fig. 4.9c).
Although in general there is a suggested correlation, it is difficult to discern a clear role for
anomalous snow cover in the development and/or maintenance of NEG events from these
results given that 1) the correlations vary greatly with location (true of other stations not
shown) and 2) it is problematic to separate the effect of anomalous snow cover from
possible dynamical influences associated with concurrent large-scale circulation anomalies.
Similar difficulties in interpretation are found in other observational studies relating
anomalous snow cover to surface temperatures. For example, Walsh et al. (1985) found a
l-24C difference between specified (from 700mb heights) and observed monthly mean
surface temperatures during months having anomalously high snow cover in the northern
Plains and Midwest, attributing the difference to the snow cover. It is not clear, however,
the extent to which the 700mb height field is independent from the low-level thermal field
as assumed in their calculations.
Given the inherent linkage between dynamical and thermodymamical processes in the
atmosphere, the problem of assessing the role of anomalous snow cover may be best
addressed by numerical modelling studies. To date such studies have been modest in
number and have produced contradictory results concerning the feedback of anomalous
snow cover onto the large-scale flow (see Cohen and Rind, 1991).
D. Summer Cases
For the summer PTA events, we limit the domain of our statistical analyses to the US
and Canada. Only surface climate station data were used in this portion of the study, with
temperature anomalies and PTA events defined using the methodology described
previously.
1. GeographicalDistribution
The geographical frequency distribution of summer PTA events meeting the criteria (i
5' C, 5 days) and (± 10 C, 10 days) for the temporally filtered data are shown in fig. 4.10.
This figure displays the sum of the events occurring over 25 summers from 1954-78,
excluding those events which began before the first day of the season (1 June) or ended
after the last day (31 August). Figure 4. 10a for POS events (+5* C, 5 days) indicates that
there are local maxima in frequency over the Pacific Northwest of the US, over the
northern Midwest extending westward to the northern Plains, and in the vicinity of Hudson
Bay and over the Northwest Territory of Canada. There is a noticeable lack of events along
the east coast and the entire southern US.
Figure 4.10c displays the number of POS events (+10 C, 10 days). The maxima over
the Pacific Northwest is still present, although not as distinctly. Maxima in frequency are
again seen near Hudson Bay and in the Northwest Territory of Canada. A fourth
maximum is located over the central Great Plains of the US. The southeastern parts of the
US are again characterized by a relative minimum in the number of events.
NEG events meeting the criteria (-5* C, 5 days) are shown in figure 4.10b. As seen,
these occur most frequently to the east of the central Rockies of the US and near Ontario.
This pattern stands in contrast to the corresponding POS events (fig. 4.10a). A minimum
of NEG events occurs over the southern and eastern US and in western/southwestern
Canada.
For the events satisfying (-1* C, 10 days) shown in fig. 4.10d, there is a
pronounced increase in events in British Columbia and Alberta. A broad maximum extends
across north-central sections of the US, extending northward through the Great Lakes to
Hudson Bay. The southeastern US again has the fewest events.
The geographical distribution of summer PTA events is noticeably different from the
winter cases.
Winter cases showed a single maxima in frequency occurring over
northwestern Canada, a result unaffected by the precise values of M and T. In contrast the
summer PTA cases exhibit multiple maxima which vary in location depending on the sign
and values of the selection criteria.
2. Summertime PersistenceCharacteristics
We have selected the US Great Plains region for a more detailed analysis of persistence
characteristics of summertime surface temperatures. In Chapter VI, processes contributing
to two heat wave/drought events which occur over this region are studied. As for the
winter cases, we first examine the cumulative frequency of events as a function of duration.
In fig. 4.11 distributions are shown for stations in Kansas and South Dakota using
temporally filtered data for the 65-year period 1914-78. For summer the cases we examine
only two anomaly magnitudes (± 14 and
±50
C) due to the smaller temporal variance in
temperature.
The distributions shown in fig. 4.11 behave similarly to those for the winter cases, and
on this log-normal plot are again well approximated by straight lines, suggesting that there
are no preferred durations for the anomalies. We do note, however, that POS cases of
large duration occur somewhat more frequently at both stations than do NEG events.
Using the same methodology as in the winter cases, we have made Markov fits to the
observed distributions which are indicated as dashed lines in the figure, with the decay
rates for the number of events indicated.
The Markov fits are seen to be good approximations to the observed distributions in
fig. 4.11 with the possible exception of POS cases at large duration, as seen in fig. 4.11 c.
However, examination of distributions at other stations (not shown) revealed no systematic
departures.
3. TemperatureAnomaly Distributions
Fig. 4.12 shows summer temperature anomaly frequency distributions for four US
stations located in North and South Dakota, Kansas and Oklahoma, along with
corresponding normal distributions having zero mean and the observed variance. Measures
of the standard deviation, skewness and kurtosis are also indicated. For stations in North
and South Dakota (figs. 4.12a and 4.12b), the distributions are close to normal, with only
slight positive skewness. The kurtosis is also slightly less than normal (= 3.0).
The other two stations (figs. 4.12c and 4.12d) are also close to normal distributions
with the exception of a tendency toward negative skewness. Under the same assumptions
as for the earlier winter analyses, this negative skewness is statistically significant at the
95% confidence level (Brooks and Carruthers, 1953).
4. Contingency Analyses
We have also investigated possible asymmetries in the recurrence of summer PTA
events by examining event magnitude contingency plots. As for the winter cases, these are
constructed by plotting the magnitude of a given event against the magnitude of a
subsequent event at a specified station.
Fig. 4.13 shows scattergrams for the same two stations examined earlier. We notice
the tendency for POS events to recur successively at both stations. For both stations this
bias toward recurrent POS events was found to be statistically significant at the 95%
confidence level using a chi-square test (e.g., Freund and Walpole, 1980). A similar bias
was also found at other stations in the Plains (not shown), although the tendency toward
POS event occurrence was not not always as pronounced. A few stations in the Plains
region showed a slight negative bias, due mainly to events of small magnitude.
The tendency for summertime POS events to be followed by subsequent POS events
contrasts with the wintertime behavior and suggests that different mechanisms may play a
role in the development and/or maintenance of winter versus summer events. For example,
if PTA's are forced solely by internal dynamics which has a "memory" of its initial state of
at most a few weeks then, in the long term, there should be no significant tendency toward
the recurrence of PTA's of a given sign. Indeed, if the low-frequency variability is
dominated by quasi-oscillatory behavior, there may even be a tendency for an event of a
given sign to be followed by an event of opposite sign (there are some hints of just such a
behavior in the wintertime data). However, a bias toward event recurrences may be
introduced if anomalous boundary conditions do play a role in PTA development and/or
maintenance, if these conditions persist on monthly to seasonal time scales (as can be the
case for soil moisture). In Chapter VI we investigate this possibility in more detail by
analyzing changes in latent and sensible heating during the course of summer heat waves.
E. Discussion
DG found that wintertime PFA's in the extratropical 500mb height field occurred most
frequently in three locations: the central North Pacific, the eastern North Atlantic and over
the northern Soviet Union. This distribution contrasts markedly with our winter PTA
frequency pattern, which shows a strong continentality bias, with a local maxima in both
POS and NEG events over western and northwestern Canada. We note, however, that this
result does not preclude the possibility that PFA's and PTA's can be closely associated.
Indeed, some of the persistence characteristics of PFA's and PTA's are found to be
similar. For example, DG find the number of PA events of a given sign and magnitude
decrease nearly exponentially with increased duration. This result holds true for PTA's as
well. For both PFA's and PTA's there are no strongly preferred anomaly durations, with
the probability of an event of n days lasting n+1 days being nearly constant.
Although DG did not test for asymmetries in occurrence of PFA's, they did find POS
and NEG PFA's occurred about as frequently in their "key" regions, similar to our findings
for PTA's. Our contingency plots for winter PTA cases did not indicate any strong
preference for recurrence of POS or NEG events.
During winter, PTA events in 850mb temperatures occurred most frequently over
western and northwestern Canada. This was true for both POS and NEG events and for
different values of M and T. Analysis of summer PTA's in the US and Canada using
surface temperature data reveal different regional frequency patterns which vary with the
sign of the anomaly and with the specific values of the selection criteria.
The surface temperature anomaly distributions for winter and summer are also quite
different.
Winter distributions for the region of maximum PTA occurrence are
characterized by significantly low kurtosis, sometimes manifested by clear bimodality.
Summer distributions for stations in the Great Plains of the US are more Gaussian in
character, but displayed statistically significant negative skewness in the more southerly
stations. Summer PTA events also display a bias toward the recurrence of POS events.
No strong bias toward the recurrence of events of either sign is found during the winter
season.
Both the winter and summer events display similar persistence characteristics, as
revealed through cumulative frequency versus duration analyses. The number of events
decreases exponentially with increased duration in both cases. This behavior is closely
modelled by a Markov process, with associated decay rates indicating somewhat more
rapid decay rates during summer than winter for anomalies of the same magnitude.
The recurrence characteristics of PTA's differ significantly for winter and summer
seasons. For winter cases there is little evidence for the recurrence of either POS or NEG
events. Local feedbacks from anomalous snow cover are not strongly indicated for stations
located near the climatological-mean snow edge. Taken together, these results suggest that
internal dynamics are likely to play the dominant role during the wintertime events. In
contrast, for summer events over the Great Plains of the US, a tendency for recurrent POS
anomaly events is observed. This summer bias raises the possibility that, on longer time
scales, mechanisms other than internal dynamics may be playing a role in the development
and/or maintenance of these events. One such possibility is anomalous boundary forcing
due to a reduction in evapotranspiration following prolonged heat wave/drought events.
48
We will investigate this possibility in Chapter VI, where we consider changes in surface
latent and sensible heating rates during the heat wave/droughts of 1980 and 1988.
49
1
(
RMP AVNOM
?.3
\IH--
(a)
Fig. 4.1. Number of PTA events identified in the 23 winter seasons from 1963-64 to
1985-86 for positive (POS) and negative (NEG) cases meeting the criteria (± 50 C,
10 days). Individual plots for a) unfiltered POS cases, b) unfiltered NEG cases, c)
filtered POS cases and d) filtered NEG cases. Contour interval 3.
C
bb
.r
V.
(b)
----- ------
-
...0
Fig. 4.2. Standard deviation in wintertime 850mb temperatures for a) unfiltered and b)
low-pass filtered data. From Blackmon et al. (1976). Contour interval 1 in a) and
0.5 in b).
-
*
--
--
Fig. 4.3. Number of wintertime PTA events meeting the criteria (± 2* C, 10 days) for a)
POS cases and b) NEG cases. Filtered data, contour interval 5.
850MR
Tru
MANM(5D.
231 WTNT PTT.T
307.
Fig. 4.4. Number of wintertime PTA events meeting the
criteria
POS cases and b) NEG cases. Filtered data, contour interval (±50 C, 5 days) for a)
5.
iV T
~~-k-(a)
r(b
/
7-
46..0\
Fig. 4.5. Number of "zero run" events having durations of 10 days or more for a) POS
and b) NEG events. Filtered data, contour interval 5.
2101100 Frequency vs Duration 30 Wint Filt
1000
(a)
5
0
10
20
15
25
30
Duration (days)
2101200 Frequency vs Duration 30 Wint Filt
1000
Decay Rate. (days
+1 7.8
+5
5.0
+10 3.5
+1
+
+
1
10
0
5
10
20
15
Duration
25
(days)
Fig. 4.6. Cumulative frequency of events as a function of duration for two stations in the
CAN key region. Distributions are for a) and b), POS cases and c) and d) NEG
cases. Decay rates for Markov fits to the distributions are displayed in the upperright of the figures.
2101100 Frequency vs Duration 30 Wint Filit
1000
(c)
0
5
10
15
20
25
30
Duration (days)
2101200 Frequency vs Duration 30 Wint Filt
1000
01
a.)
100
LL
5
10
15
Duration
20
(days)
Fig. 4.6. (continued)
25
30
STATION 4015320
STATION 3015400 VAR 3 WINT 20 YRS
VAR 3 WINTER 25 YRS
SKEY
-0.12
KURT 2.14
35
7
30
25
~TJ
20
-40
-30
-20
-10
-30
C
-20
-10
MOALY (DEGQ
STATION 210i300
0
10
20
30
40
ANOMALY
(DEGC)
VAR 3 WINTER 25 YRS
STATION 1183000
VAR 3 WINTER 25 YRS
60
50
40
30
20
10
-40
Q
(DEG
ANO$iALY
-30
-20
-10
0
10
ANOMALY
(DEGC)
20
30
40
Fig. 4.7. Winter temperature anomaly distributions at four Canadian stations near the CAN
key region . In addition to the observed distributions, normal distributions having the
same mean and variance are displayed. Data for the period 1959-1978 in (a) and
1954-1978 in (b) through (d).
(a)
C, 5 days Flit Data 30 Wint
1128580 ± 5*
0 04--
600
400
200
0
Event Magnitude
-200
-400
-600
(b)
2500600 ±
5*
C,
5 days Flit Data 30 WInt
60 0N
(23)
(15)
400.
+
200.
+
+
-
++
+
++
+
~.
+
+
+A
+
-200.
+
+ 47T
*0*+
++
+
+
-400.
(23)
(26)
U-U1
-600
-400
-200
0
Event Magnitude
200
400
600
Fig. 4.8. Scattergrams of event magnitudes plotted against subsequent event magnitudes
for two Canadian stations near the CAN key region. For each point the value along the
abscissa is the magnitude of a given event, with the ordinate value being the magnitude
of the subsequent event. The total numbers of the four possible transitions are also
indicated in each quadrant.
59
11 2193 77yrs Winter Filt (end 1978)
2,
(a)
-2
* 4.
-350
-300
.250
-200
-150
-100
-50
0
Event Magnitude
33 2791 35yrs Winter Filt (end 1978)
14
12
10
8
6
(b)
4
2
0
-2.
-
4,
-220
-200
-180
-160
-140
-120
-100
-80
-60
-40
Event Magnitude
Fig. 4.9. Distribution of snow cover anomalies (in.) with NEG PTA event magnitudes
for three stations in the Midwest a) Decatur, Illinois, b) Findlay, Ohio and c) Aberdeen,
South Dakota. The length of the time series used for each station is indicated.
60
39 0020 40yrs Winter Flt (end 1978)
10.
8.
6.
4
>2
'00
Event Magnitude
Fig. 4.9. (continued)
61
-
8
3
-
j4
--
j
3~
+
-
an:)NGcss
and
NGcssCotuinevl2i
d)
±
ae
otu
an
itra
ZI
]L
-
-- .
-
(1
fra)PS
C
I.) +
<1-
-
C,5das
-0'1 -~
/
b)
('.4
NEG
cae
n
6and
na
n
)adbad
1*
)
_-A
C,
10ay)focOScae
(a)d)
c)
A
i-
nc
n
)
(d)
Fig. 4.10. (continued)
63
14 3954 KS Cum Freq 65 Summers Filt
1000
(a)
0
5
10
15
20
25
30
Duration (days)
39 1076 SD Cum Freq 65 Summers Filt
1000
L..
(b)
0
5
10
15
20
25
30
Duration (days)
Fig. 4.11. As in fig. 4.6 but for two stations in the Great Plains during summer. Here a)
and b) are for POS events, c) and d) are for NEG events.
64
14 3954 KS Cum Freq 65 Summers Flit
1000
(c)
0
5
10
15
20
25
30
Duration (days)
39 1076 SD Cum Freq 65 Summers Flit
1000
(d)
0
5
10
15
20
Duration (days)
Fig. 4.11. (continued)
25
30
STATE
32
STATION
1871VAR
STATE
3 SUMME
39
STATION
1076
VAR 3 SUMME
130
120 -
3(c
8.8
$LY
0.01
610
100
20-
70
40
-30
- 0
-20
-10
o
0
20
30
--
SEC
2.38
3954
t' STATION
VAR 3 SUMmE
5.4L
STATE
(c)I
5t?1* -0.29
Ktii
t20
31 STAT[ON
ss2_*T-0 42
XtWr,2.97
(140
\
ifo
r
10
k' \3
80
So
'7
20
40
-30
-20
t to
-to
;sOSt.T
cOE F1
20
20
io
(d)
/j \~\.
Jj \j
~1 \*~~
120
100
-40
30
3 SUMME
2912 VAR
I
3
-.
2.$6
-1
120
20
3
ANOMA.Y1OECFl
ANGMALT
(BEGF1
STATE
10
0
-10
-20
-30
-40
.0
20
-3
Z
/
7k
-10
Af
0
1.
10
to
0CEG F1
20
20
30
30
Fig. 4.12. As in fig. 4.7 but for four stations in the Great Plains during summer.
0o
66
14 3954 KS Summer Filt 1914 - 1978
200
150
100
(a)
-100
-150
-200 -
-
-200
-150
-100
-50
50
0
100
150
200
Event Magnitude
39 1076 SD Summer Filt 1914-1978
(b)
i
-200
-200
-150
-100
-50
0
50
100
150
200
Event Magnitude
Fig. 4.13. As in fig. 4.8 but for two stations in the Great Plains during summer.
V.
Winter Cases
A. Introduction
The previous statistical analyses indicate that wintertime PTA's frequently occur over
northwest Canada (the CAN key region). We have therefore selected events in this region
for further synoptic and diagnostic study. Whereas previous studies have mainly examined
monthly-mean analyses, we will instead consider the characteristics of events at various
times relative to their objectively-defined onset dates (i.e. day 0). To emphasize the
systematic features of these events, we have adopted a compositing approach. Our goal is
to document the characteristic structures and time evolutions of the thermal anomalies of
PTA's and to investigate the related changes in the large-scale flow during these events. In
addition, we have performed heat budget analyses to identify the roles of various
mechanisms during the development of PTA's. A summary and discussion of our primary
findings is provided at the end of the Chapter.
B. Data
The principal datasets used in this portion of the study are the climate station data and
the NMC and ECMWF gridded analyses. The events selected for study are based on mean
daily surface temperature anomalies observed at Mayo, Yukon Territory (63.6'N,
135.9'W). Mayo was chosen as our reference station because it is located in our CAN key
region and has a nearly continuous surface temperature time series from 1947 to 1988.
C. Thermal Structure
1. HorizontalStructure
For Mayo we have identified 24 POS and 24 NEG events meeting the M, T criteria of
(
50
C, 10 days) in LLP filtered anomaly data (details of these events are listed in
Appendix C). From these cases we have constructed from the gridded NMC analyses
composite maps of selected fields at one-day intervals relative to PTA onset.
Fig. 5.1 displays composite 850mb temperature anomalies averaged from day 0 to day
+5 for POS and NEG PTA events along with estimates of the statistical significance of the
anomalies. These plots indicate that the largest anomaly values for both POS and NEG
events are located over the CAN key region, with upstream anomalies of opposite sign at
high latitudes near the dateline. These features are all statistically significant at or above the
90% confidence level (fig. 5.1c and 5.1d). For the NEG cases statistically significant
positive temperature anomalies also occur over the southeastern US.
For reasons
discussed previously, results in the vicinity of the Himalayas should be treated with
caution.
The anomaly patterns in fig. 5.1 very closely resemble those identified in
corresponding analyses (not shown) of temporal anomalies in the zonally asymmetric
component of the 850mb temperature field. That is, T' ~ T*' where T is temperature, T a
temporal anomaly and T* is the departure from the zonal average at a given latitude. The
comparable values of T' and T*' suggests that the PTA events we are studying are
associated with major changes in the stationary wave pattern. It is also interesting to note
the similarity of the POS and NEG temperature anomaly patterns. This is clearly indicated
in fig. 5.2, which displays the difference (POS minus NEG) for the composite anomalies
in fig. 5.1. Again, the largest anomalies are observed over the key region, with anomalies
of opposite sign located farther west near the dateline. A much smaller anomaly center is
also indicated over the southeastern US.
Studies of wintertime PFA's conducted by Dole (1982, 1986a) and Dole and Black
(1990) have identified thermal anomaly patterns somewhat similar those presented for our
PTA events. Fig. 5.3, taken from Black and Dole (1990) and Black (1991, unpublished),
shows composite 700mb temperature anomalies for the fully developed PAC POS and
PAC NEG flow anomaly patterns. The temperature anomalies associated with the PAC
POS pattern (fig. 5.3a) broadly resemble those for our PTA NEG cases, with negative
anomalies located over western Canada and positive anomalies near the dateline and over
the southern US. The PAC NEG cases (fig. 5.3b) and our POS PTA cases also show
considerable similarity, although our analyses do not display the negative anomaly center
over the southeastern US present in their composites. In both cases there are some
additional differences, the foremost being the tendency in the PTA cases for the temperature
anomalies near the dateline to be much broader in scale, of larger magnitude and located
farther south than in the PFA cases.
Caution should be exercized in directly comparing our results with those of the PFA
studies. Events in the PTA and PFA studies are defined using different parameters, and
different numbers of cases were used in the respective composites. Also, the composites
for the PFA's and PTA's were constructed at times relative to their respective onsets and
are therefore unlikely to be synchronous (i.e., day 0 of the PFA cases will not in general
coincide with day 0 of the PTA cases). Nonetheless, the general similarity of results
strongly suggests that there is a close link between wintertime PFA's over the central North
Pacific and PTA's occurring in the CAN region. This possibility will be investigated
further when we analyze characteristic synoptic features during the time evolutions of
PTA's.
2. Vertical Structure
We next consider the vertical thermal structures of PTA's in the CAN key region. Our
primary data for this purpose are climate station data for Mayo, YT and gridded NMC
heights and temperatures. Temperature anomalies were calculated (using the NMC data) at
10 vertical levels: the surface (from Mayo, YT data) and the 9 mandatory pressure levels
from 850mb to 100mb.
We first examine the thermal anomalies for a vertical column centered over Mayo, with
anomalies above the surface being averaged over the four closest gridpoints in latitude and
longitude. Composite temperature anomalies were computed in two steps for this column
from 27 POS and 29 NEG PTA cases1 . First, the temperature anomalies were averaged
for each case and pressure level over the 5-day period centered on the time when the largest
anomaly values were observed at the surface. From these case averages, composites were
then obtained by averaging over the 27 POS and 29 NEG cases, respectively, at each level.
The results are shown in fig. 5.4, with the standard deviations about the composite mean
also indicated.
Fig. 5.4 indicates that for both POS and NEG cases the largest temperature anomalies
are typically located at the surface then decay with height, with a tendency to reverse sign
near the 250mb level. The variance of the composite anomalies is largest near the surface
and at upper levels, with minimum values observed near the tropopause. Overall, the
composites display a tropospheric deep structure which is nearly symmetric for the POS
and NEG cases.
To extract additional information relating to the vertical thermal structure of the cases,
we have performed empirical orthogonal function (EOF) analyses for the pooled data
1These include the same 24 POS and NEG cases
used previously and additional events meeting the same
criteria (±5* C, 10 days) but which began before 1 December or ended after 28 February.
obtained by combining the 27 POS and 29 NEG cases used to construct the composites in
fig. 5.4. The main objective here is to identify the EOF's (or eigenvectors) that account for
the majority of the variance in the observed structures. A discussion of the application of
EOF analysis techniques is given in Horel (1981), with a more complete description of the
present analyses given in Appendix D.
Fig. 5.5 displays plots of the first two EOF's for the pooled temperature anomaly data
where the EOF's have been normalized to have a maximum value of ±1. The first EOF,
which accounts for 54.2% of the variance, indicates a deep thermal structure with
anomalies reversing sign near the tropopause. The second EOF, accounting for 34.6% of
the variance, reveals anomalies of like sign extending from the surface to the 100mb level.
Taken together, the first two EOF's account for over 88% of the total variance, with eight
additional EOF's accounting for the remaining 12%.
We have examined the case-to-case variability in structure by computing the percentage
of total variance explained by each of the first five EOF's for individual PTA cases. Our
results for POS and NEG events are listed in Table 5.1 and Table 5.2, respectively.
Consistent with our previous results, these tables indicate that the first two EOF's typically
account for the majority of the variance. There are occasional events, however, which
significantly depart from this pattern. Two interesting examples are shown in fig. 5.6
which exhibit particularly shallow anomalies for POS case 25 and NEG case 2.
To gain additional insight into the three-dimensional thermal structures we have
constructed cross sections of composite temperature anomalies along a latitude circle at 62N
for the longitudes 160E, eastward to 100W using the NMC gridded analyses. The eastern
portions of these cross sections traverse the CAN key region, while longitudes farther west
are included to examine the associated anomalies near the dateline. The composite
anomalies in this analysis were obtained from 24 POS and NEG cases and are time
averages from day 0 to day +5. The "surface" anomalies in these plots are determined as
departures of the 1000mb to 850mb mean layer temperature from climatology (as defined in
Chapter IV) . The estimated surface topography is indicated at the bottom of these plots.
For both the POS and NEG cases, we see (fig. 5.7) that the maximum temperature
anomalies are located near the surface, with a tendency to reverse sign near 250mb. From
hydrostatic considerations, these deep thermal structures suggest that positive (negative)
height anomalies at upper tropospheric levels occur over the key region during POS (NEG)
events, with height anomalies of opposite sign located upstream. It is also interesting to
note that the surface anomalies for both POS and NEG events are located to the lee of the
Rockies. The possible role of orography during PTA development will be considered in
the next section and in heat budget studies presented later.
Finally, we present some examples of the temporal variability observed in the vertical
thermal structure of PTA's for individual cases. Fig. 5.8 displays temperature anomalies
as a function of pressure and time for two PTA cases. Fig. 5.8a shows an example of a
warm air intrusion which occurs during the course of a persistent NEG event. Initially deep
negative anomalies are subsequently observed to be confined to near the surface and then to
deepen again at later stages. Fig. 5.8b shows a very rapid transition between two surface
temperature regimes, going from a cold regime to a warm regime in a period of only a few
days. Such rapid transitions between otherwise persistent and anomalous conditions,
which are suggestive of air mass and frontal transitions, are likely to provide at least a
partial explanation for the bimodal character of the wintertime temperature anomaly
distributions, with large anomaly values observed more frequently than near-normal
conditions.
D. Synoptic Analyses and Time Evolution
1. Evolution of the Thermal Anomalies
The previous sections have emphasized the structural characteristics of fully developed
PTA patterns.
We now examine the temporal evolution of thermal anomalies and
associated synoptic and large-scale circulations during the development of PTA's. For this
purpose, composite analyses were constructed of our 24 POS and NEG cases from the
NMC gridded data.
Fig. 5.9 presents the composite 850mb temperature anomalies and their associated
statistical significance for the POS PTA cases at two-day intervals from day -3 to day +3.
At day -3 small positive anomalies are present over eastern Siberia, while in the CAN
region conditions are near normal. At day -1, positive anomalies begin to develop over the
Gulf of Alaska. By day +1, the dominant feature is the positive anomalies which have
developed in situ over the key region. On day +1 smaller regions of weaker negative
anomalies are located near the dateline at high latitudes and downstream over the southcentral US. By day +3, the positive anomalies centered in the key region have broadened
in size and become slightly larger in magnitude, while negative anomalies persist in the
vicinity of the dateline and over the southern US. Overall, POS PTA development is
typically characterized by the apparent in situ development of positive anomalies over the
key region near day 0, with a nearly contemporaneous development of weaker negative
anomalies near the dateline.
The evolutions of the NEG cases (figs. 5. 10a - 5. 1Od) display several similarities to the
POS cases, but also some significant differences. At day -3 positive anomalies are located
over the Gulf of Alaska and the southeastern US, while negative anomalies are situated
over the western US. These anomaly centers resemble a wave train pattern, with centers
that are statistically significant at or above the 95% confidence level. By day -1, negative
anomalies develop over the CAN key region with the positive anomalies previously located
over the Gulf of Alaska retrograding towards the dateline. By day +1, the fully developed
temperature anomaly pattern is present, with positive anomalies centered over extreme
eastern Siberia and the Bering Sea, negative anomalies over the key region and positive
anomalies over the eastern US. This pattern persists, with anomalies of larger magnitude
developing by day +3. As in the POS cases, the temperature anomalies over the key region
appear to develop in situ. However, the temperature anomaly pattern observed at day -3
for the NEG cases suggests that there is a pre-existing anomalous large-scale flow pattern
before onset of these cases. If the anomalies in the large scale flow are nearly equivalentbarotropic in structure (as in Dole, 1986a) we would anticipate that the height anomaly
pattern will be of like sign and approximately co-located with the thermal anomalies.
2. CorrespondingFlow Anomalies
To investigate characteristics of the large-scale flow associated with PTA's, we have
constructed corresponding composite 500mb height anomaly analyses for POS and NEG
events. This allows us to directly compare our results with those of prior studies of PFA's
(Dole, 1986a).
Fig. 5.11 displays, for the POS cases, composite 500mb height anomalies and their
statistical significance at days -3, -1, +1 and+3.
At day -3 (fig. 5.11 a), a broad area of
weak negative anomalies is located over the North Pacific. By day -1, a height anomaly
pattern has developed with anomalously low heights centered just south of the Alaska
Penninsula and positive height anomalies centered near the CAN key region. By day +3,
the height anomalies strongly resemble the PAC NEG pattern of Dole (1986a), with
anomaly centers of alternating sign located near the dateline, over western Canada and the
southeastern US. An anomalous south to southwest geostrophic flow, inferred from the
height anomalies, is located over the CAN key region beginning near day 0. Note that this
flow pattern favors transport of relatively warm maritime air into the key region from the
Pacific.
For the NEG cases (fig. 5.12) at day -3, positive height anomalies are located near the
Gulf of Alaska, while
(southeastern) US.
negative (positive) anomalies located over the western
These features are consistent with the thermal anomaly pattern
observed at this time and are statistically significant at the 99% confidence level. By day 1, anomalously low heights have developed near the key region, while the region of
positive anomalies located near the Gulf of Alaska at day -3 have expanded westward
toward the dateline. At days +1 and +3, the height anomalies resemble the PAC POS
height anomaly pattern, although the positive anomalies near the dateline are located farther
north than in the composite plots of Dole (1986a). In the CAN key region an anomalous
north-to-northeasterly geostrophic flow is inferred from the composite anomalies beginning
near day -1.
The composite height anomaly analyses indicate that PTA's occurring in the CAN key
region are associated with large-scale PFA's. The implied geostrophic flow associated
with the observed 500mb height anomalies indicates an anomalous northerly (southerly)
component over the key region during the NEG (POS) cases. In the key region these
anomalous upper-level winds imply downslope motion for the POS cases and upslope flow
for the NEG events. Also, the existence of upstream height anomalies strongly suggests
that processes remote to the key region are playing an important role in the development of
PTA's. For the POS PTA cases, the height anomaly pattern apparently develops nearly
contemporaneously with the thermal anomalies, while the height anomalies lead the
development of the thermal anomalies by a few days the NEG PTA cases. This result may
have implications for the mechanisms responsible for development of the respective PTA
events, which will be examined further in Section E.
3. Surface CirculationFeatures
We have identified PFA features similar to the Dole (1986a) PAC POS and PAC NEG
patterns during the development of the CAN PTA events. General characteristics of the
surface circulation features associated with the PAC POS and PAC NEG PFA
developments have been documented elsewhere (e.g. Black, 1990). However, we will
consider the synoptic features occurring during the developed stages of the PTA events and
also describe characteristics of the low level flow during the evolution of NEG PTA events.
Fig. 5.13 presents composite sea level pressure and 850mb temperature analyses at day
+5 of the POS and NEG PTA events. A large low pressure system centered over the
Aleutian Islands dominates the low level circulation over the North Pacific Ocean during the
mature stages of the POS cases (fig 5.13a), with the accompanying southerly geostrophic
flow producing strong warm advection over northwest Canada and Alaska. For the NEG
PTA cases (fig. 5.13b) the pressure pattern over the Pacific is markedly different, with the
climatological Aleutian low being replaced by two cyclone centers, one located near
Kamchatka, the other in the Gulf of Alaska. High pressure is located over the CAN key
region, with a strong geostrophic flow paralleling the coast of British Columbia and
Alaska. Thermal advection over the key region is weak at day +5.
The development of the cyclone in the upstream Gulf of Alaska is an interesting
phenomena that deserves further comment. In several cases, it is observed to be more
intense than indicated in the composite shown in fig. 5.13b. The cyclone tends to be of
small scale, and develops while remaining quasi-stationary in the northeast Gulf. The
typical evolution is revealed in the composite fields of sea level pressure and 850mb
temperature presented in fig. 5.14 for day -4, -2 and 0 relative to NEG PTA onset.
At day -4 (fig. 5.14a) a single low pressure center near the dateline is located over the
Pacific, with a trough in the pressure field extending east into the Gulf of Alaska. At day 2, (fig. 5.14b) this cyclone becomes more clearly defined, while by day 0 (fig. 5.14c), the
fully developed pattern is observed, with high pressure located over the CAN key region,
the low in the Gulf and strong pressure and thermal gradients established in between.
Although detailed analysis of the development of this Gulf low is beyond the scope of
the present study, we do note that during the development there is an anomalous upperlevel northerly flow that passes over the Alaska Range, (see fig. 5.14d) and apparently
favors a form of lee cyclogenesis over the northeastern Gulf of Alaska.
We have investigated the association between the low-level circulation and the vertical
thermal structure of PTA's by correlating the sea level pressure field with the time
coefficients of the dominant EOF identified for each of the 27 POS and 29 NEG cases used
in Section C.2. Applying the procedure used to examine the vertical structure, we first
average, for each case, the sea level pressure obtained from the NMC grids over the 5-day
period when the largest surface temperature anomalies occur. The time coefficients of the
dominant EOF are then correlated, at each gridpoint, with the sea level pressure field, with
POS and NEG cases considered separately.
Fig. 5.15 displays the correlations obtained for the POS and NEG cases. Also shown
are composite sea level pressure analyses for both types of events and the difference in the
fields between types (NEG minus POS) . To aid in interpretation, we have multiplied the
correlations for the NEG cases by -1. For the positive cases, a broad region of negative
correlations is observed over the North Pacific with the largest values (-0.83) located in the
vicinity of the Aleutian low. For the NEG cases a large region of positive correlations
(recalling these have been multiplied by -1) is observed over the North Pacific at high
latitudes with the largest values (0.63) located over the Bering Strait.
We can loosely associate the sign of the correlations in these plots with the sign of the
pressure perturbations occurring during these events. For the POS cases this indicates the
deep thermal structure (represented by EOF 1) in these cases most highly correlates with
anomalously low pressure to the south of the Aleutians. For the NEG cases, this thermal
structure most highly correlates with anomalously high pressure centered over the Bering
Strait. We note that these two locations roughly correspond to those identified by Dole
(1986a) as having the largest 1000mb height anomalies for the developed PAC NEG and
PAC POS PFA patterns. These analyses suggest the low-level flow has an anomalous
southerly component in the vicinity of the CAN key region during POS PTA events, while
an anomalous northerly component is observed during the NEG PTA cases.
The difference in composite sea level pressure (NEG - POS, fig. 5.15e) indicates the
largest coherent pressure changes are centered over the Bering Sea and Alaska during PTA
events in the CAN key region. The tendencies for high pressure to become established
over the key region for the NEG cases and for a low to be centered in the Gulf of Alaska
are also indicated. The coherent (and persistent ) surface pressure features indicated in this
analysis suggest the importance of the trajectories of air parcels during the development of
PTA's, with an enhanced maritime flow over the key region during POS events and
primarily continental trajectories for the NEG cases.
Observational studies by Lau (1979) indicate that the Gulf of Alaska and Canadian west
coast are regions of climatological-mean warm advection during winter. Our analyses
imply that an enhancement of this warm advection occurs during the POS PTA cases, with
values reduced (or even slightly reversed) from climatology during NEG cases.
This
possibility will be addressed more quantitatively in the heat budget analyses.
4. Case to Case Variability
It is important to note that individual events sometimes display important variations
from the composite characteristics presented previously. We will briefly demonstrate this
by showing an example of very shallow negative temperature anomalies which actually
reverse sign above the 850mb level during the course of their evolution. The vertical
structure of this case is shown in fig. 5.8a.
Fig. 5.16 presents the sea level pressure and 700mb heights averaged over a five-day
period beginning when the anomalies reversed sign at the 850mb level for this case. At the
surface a cold-core high pressure center is located over the CAN key region while aloft a
classic blocking structure is observed, with an anomalous ridge centered over Alaska. In
our composite results 500mb heights an anomalous ridge is typically located over the
Bering Sea during the NEG cases. In this case, however, it is located farther east,
allowing abnormally warm air aloft to move in over the CAN key region. This creates a
very stable boundary layer which helps trap the cold air near the surface. Fig. 5.17
displays a cross-section (constructed similarly to fig. 5.7) of the thermal anomalies in this
case, with the thermal signature of the block observed to the west, with cold air trapped
near the surface to the lee of the Rockies.
E. HeatBudget Analyses
Heat budgets were computed for 5 POS and 5 NEG PTA cases from the ECMWF
gridded analyses. The cases are for the CAN key region and are based on temperature
anomaly data for Mayo, Yukon Territory. Additional details of the cases used are listed in
Appendix E. The ECMWF dataset has been employed for our analyses since it includes
vertical velocity analyses (in pressure coordinates), satisfactory values which were not
obtainable by other means. The temperatures, heights and winds in this dataset are
initialized values obtained from the ECMWF four-dimensional data assimilation scheme
(Bengtsson et al., 1982). As Trenberth and Olson (1988b) indicate, this initialization
process tends to reduce the magnitude of the vertical motion field. They also indicate,
however, that the initialized values appear superior in quality to vertical motions calculated
by alternative procedures.
We will consider the onset period of the PTA cases. Our basic approach is to first
identify the dates where the largest surface temperature changes at Mayo were observed
during development. This period typically ranged from three or four days prior to onset to
two days after onset. Using the twice-daily ECMWF grids, we then calculated, at each
gridpoint and for each case, the terms in the thermodynamic energy equation at the 850mb
level:
-=
at
A
-V-V-0>--ap cp
B
C
(5.1)
D
In (5.1), term A represents the local time rate of change of potential temperature, terms B
and C are advection by the horizontal and vertical (in pressure coordinates) flow and term D
is the diabatic term, which is determined as a residual. The terms in (5.1) are then averaged
over the onset period for each case at the four gridpoints closest to the region of largest
observed potential temperature change. The results are then composited over the 5 POS
and 5 NEG cases.
Fig. 5.18 indicates that the largest contributors to the change in 0 during POS onset are
warm advection by the horizontal wind (term B) and adiabatic warming associated with
descent (term C). These terms are opposed by a smaller diabatic cooling term (term D).
For comparison, the climatological values (based on a 10 winter average, 1978-79 to 198788) of these terms are shown in fig. 5.18c. Based on our previous synoptic analyses and
more quantitiative estimates of the topographically forced vertical motion (not shown), the
large vertical advection term is apparently associated with downslope flow to the lee of the
Rockies. Thus, in the vicinity of the key region, changes in the low-level circulation
associated with PTA development produce both warm advection and downslope flow
leading to adiabatic warming.
In contrast, the negative cases reveal a markedly different situation. The largest
contributors to the observed 0 decrease are cold advection by the horizontal flow and
diabatic cooling, presumably due to net longwave radiation to space. The composite
vertical motion is near zero for the NEG cases. The contribution by the diabatic cooling
term in the NEG cases is consistent with the previous synoptic analyses, which indicated
that an anomalous ridge develops upstream from the key region prior to PTA onset,
reducing or eliminating the climatological mean warm advection in the region (cf. fig
5.18c) that normally helps balance the radiative cooling (Lau, 1979).
It is also interesting to note the potential role of orography in the NEG PTA cases. Our
previous analyses have indicated the maximum negative temperature anomalies are usually
located near the surface to the lee of the Rockies and can be quite shallow. A shallow coldcore high is typically located to the east of the mountains in these cases and is associated
with a very stable boundary layer and a weak slope-parallel or upslope low-level flow.
Overall, this configuration traps the cold air against the mountains, inhibiting downward
mixing of warm air to the surface.
For both the POS and NEG cases we have further evaluated the horizontal advection
term in (5.1) by expanding 8 and V into their climatological mean (the average over 23
winter seasons) and anomalous (departures from the winter climatology) values utilizing
the NMC gridded data. Our calculations are based on composite wind and temperature data
for the 24 POS and 24 NEG cases (described previously) averaged from day (-3) to day
(+3).
We find that for both the POS and NEG cases the largest contributor to the
horizontal advection of 0 is from the anomalous velocity advecting the climatological-mean
0 distribution. The anomalous advection patterns for the composite POS and NEG cases
are presented in fig. 5.19 along with the climatological mean horizontal advection. The
figure indicates that the anomalous advection patterns for the POS (fig. 5.19b) and NEG
(fig. 5.19c) cases are quite similar, but of opposite sign. In the vicinity of the CAN key
region, the anomalous 0 advection is observed to enhance the climatological mean values
(fig. 5.19a) in the POS cases, while it opposes the mean values during NEG events as
suggested earlier.
F. Summary andDiscussion
We have identified characteristic features of the structures and temporal evolutions of
PTA's occurring in the CAN key region, emphasizing systematic aspects as identified in
composite analyses. Some of the potential mechanisms responsible for development of
wintertime PTA's have been inferred from our synoptic analyses and heat budget studies.
Among our findings are:
1) The temperature anomalies for the POS and NEG events were identified with similar
large-scale persistent flow anomaly patterns, with height anomalies of opposite sign located
upstream near the dateline and, especially for the NEG cases, downstream over the eastern
US.
2) The vertical thermal structures are similar for the POS and NEG cases, with largest
anomaly values located near the surface then decaying with height and often reversing sign
near the tropopause. In both cases the largest anomaly values occur to the lee of the
Rockies.
3) The temporal evolutions indicate that the thermal anomalies develop locally over the
key region, in association with anomalous circulations in the large-scale flow which, for
the POS (NEG) events, resemble the PAC NEG (PAC POS) flow anomaly patterns
identified by Dole (1986a).
4) Heat budget analyses indicate that warm advection and adiabatic warming by
downslope flow are responsible for the development of POS events, while cold advection
and diabatic cooling play dominant roles in the onset stages of the NEG events. The
horizontal advection in both POS and NEG cases is dominated by advection of the
climatological mean potential temperature field by the anomalous flow.
Our results confirm some findings of previous work on PTA's but have also identified
some subtleties not revealed in studies where monthly mean anomalies were employed.
For example, both POS and NEG PTA's occurring in the CAN key region are associated
with anomalous and persistent large-scale flows, but for the NEG PTA cases, upstream
height anomalies develop well prior to PTA onset, while for POS PTA cases, they develop
nearly contemporaneously. The precursory height anomaly pattern observed for the NEG
cases suggests that their onset may be easier to predict than POS events, since for the NEG
(POS) cases, the forecast is related to the persistence (development) of a large-scale flow
anomalies. In addition, transitions between persistent anomalous temperature regimes can
be quite rapid (occurring in less than a week) and PTA's can also be very shallow, limiting
the usefulness for predictive purposes of correlations between surface temperatures and the
mid-tropospheric height field.
Our heat budget analyses suggest the reduction of the climatological warm advection
near the key region during NEG PTA cases allows diabatic processes (mainly radiative
cooling) to play a significant role in their development. For both POS and NEG events, the
importance of orography is evident, with downslope flow over the Rockies enhancing the
surface warming during POS events and the trapping of cold air by the mountains and
anomalous weak upslope flow aiding in the development of the NEG events. The very
stable boundary layer associated with cold air damming in the NEG cases tends to inhibit
advection of warm air into the key region by synoptic-scale transient disturbances. This
may help explain the result of Chapter IV that showed that when the data were temporally
filtered to remove high frequency disturbances, the number of PTA events increased more
dramatically for the POS cases than for the NEG cases.
Overall, the results suggest that there are two primary reasons why PTA's occur most
frequently in the CAN key region. First, persistent large-scale flow anomalies often
develop upstream over the central North Pacific. These flow anomalies have associated
circulations and thermal advection patterns that are conducive to the development of PTA
84
events downstream over the CAN region. Second, local topography downstream in the
CAN region enhances the effect of the anomalous PFA circulations with downslope flow
enhancing warming in the POS cases and cold air damming aiding in the persistence of
NEG events.
TABLE 5.1
Percent of variance explained by each of the first five EOFs for the vertical
thermal structure of 25 POS PTA cases and the cumulative
variance explained by the first two EOFs
EOF 4
16.3
25.1
9.3
1.1
EOF 5
10.5
7.1
4.6
9.2
9.7
17.8
11.7
Sum (1 and 2)
61.4
62.2
78.3
61.4
60.0
45.4
17.1
45.1
1.6
5.4
19.2
12.1
12.6
72.8
5.0
2.7
47.0
65.8
32.0
12.3
2.1
22.9
26.6
12.4
21.0
5.8
9.4
3.3
10.1
8.9
8.0
10.0
15.9
12
0.1
51.6
17.3
7.2
21.2
79.2
67.9
59.3
67.9
54.9
51.7
13
14
41.2
27.4
13.9
9.6
14.1
15.6
16.7
27.6
13.0
16.3
55.1
37.0
15
49.5
5.4
37.8
2.1
18.4
24.3
12.0
16
14.3
1.3
32.5
63.8
39.1
17
18
19
20
21
22
23
24
64.5
20.3
71.5
5.6
0.6
62.2
75.2
3.2
5.5
1.2
3.3
29.2
2.7
4.7
12.5
8.9
13.0
6.2
8.4
12.9
0.8
7.5
7.2
12.0
18.9
21.0
9.7
9.0
76.3
77.5
73.0
69.3
39.4
71.7
11.8
57.2
1.5
63.7
38.8
12.4
8.9
1.5
6.6
7.2
12.0
73.2
25
6.5
18.3
29.3
7.3
36.3
24.8
EOF 1
41.7
49.6
47.7
8.7
EOF 2
19.7
12.6
30.6
52.7
5
22.3
37.7
6
7
8
27.4
62.1
22.8
9
10
11
Case
1
2
3
4
EOF 3
10.8
4.6
7.5
27.5
74.6
84.1
TABLE 5.2
Percent of variance explained by each of the first five EOFs for the vertical
thermal structure of 25 NEG PTA cases and the cumulative
variance explained by the first two EOFs
Case
1
EOF 1
72.3
EOF 2
3.3
2
12.9
29.8
3
4
57.2
87.2
5
6
EOF 3
5.9
EOF 4
9.6
EOF 5
8.2
Sum (1 and 2)
75.6
8.7
17.7
22.5
12.6
0.2
7.8
5.8
9.8
3.0
4.2
5.7
16.1
78.1
80.8
1.9
0.7
5.1
0.4
42.7
59.1
6.5
1.0
6.7
87.9
83.2
81.2
7
8
59.6
68.3
14.0
4.0
73.6
53.8
9.4
10.6
8.0
12.0
10.4
9
10
4.4
11.5
1.3
6.7
19.6
72.3
63.2
32.8
28.1
3.1
19.3
14.8
60.9
11
12
86.2
1.2
0.6
7.7
3.8
87.4
21.7
64.6
9.4
1.8
1.7
13
14
15
16
30.9
80.7
8.0
63.0
47.4
5.3
26.2
17.5
4.9
2.1
10.2
7.4
6.4
8.0
24.5
5.5
8.9
3.1
26.4
86.3
78.3
86.0
34.2
17
18
19
20
21
22
23
0.5
67.4
61.0
13.4
71.5
34.6
5.1
81.5
18.3
12.3
38.6
3.2
26.2
46.9
11.9
3.1
0.0
6.0
2.6
80.5
82.0
3.3
10.1
20.7
85.7
5.3
6.6
4.6
9.2
2.4
4.9
21.7
13.7
11.7
5.5
10.7
8.4
11.0
22.2
74.7
60.8
52.0
24
81.2
1.0
2.2
5.8
9.6
82.2
25
66.5
6.0
8.0
9.2
9.8
72.5
73.3
52.0
87 -
(a)
R0MCOP
TUMP
ANQM(Ql YPDAY1
(b)
TO DAY(5)
AnM
\N/
OUM
P ANOM(C)
AY)
YV
'TO DlAYn)
;
Ile
-7
~\\--
rQNr
RSQMR
TEMlD ANOM YTP nAY4ni TO
DAY(+5
,\\L/
/
/
-
-/'
-/s
CN.A0-,EP
--
N
YTN
DAY(01
\7V
Tf) nAY(.s-)
ft
\/
NNr
Fig. 5. 1. Composite 850mb temperature anomalies (*C) averaged from day 0 to day +5
for a) POS events and b) NEG events. Corresponding confidence levels are shown in
c) and d). Anomaly contour interval is 2, negative anomalies are dashed and the zero
contour has been omitted. Confidence shown as 1, 2 or 3 indicate the 90, 95 and 99%
percent levels, with dashed lines indicating confidence of negative anomaly values.
88
85MB
AR ANOM DTF(C)YTP-YTN (0 TO +5)
- 0- -/--
$00K$,
/
\---5
7-
\
Fig. 5.2. Difference between POS and NEG composite 850mb temperature anomalies
shown in fig. 5.1. Contour interval 2, negative anomalies dashed.
V
(a)
700MB TEMPERATURE ANOMLY (DEG C) DAY +5 (PN)
-
0
2-
Lq
-
Fig. 5.3. Composite 700mb temperature anomalies for the developed a) PAC POS and b)
PAC NEG flow anomaly patterns. Contour interval is 1 *C. From Black (1990).
400
(a)
600
800-
1000 --10
-5
0
5
10
15
20
Temperature Anomaly (C)
(b)
600
800-
1000
-20
-15
-10
-5
0
5
10
Temperature Anomaly (C)
Fig. 5.4. Vertical profile of composite temperature anomalies (*C) near the CAN key
region for a) POS cases and b) NEG cases. Standard deviation about composite
mean also indicated. See text for additional details.
EOF 1 Var: 54.2%
100
200
300
400500-
(a)
600700 800 900 -
1000 -1.2
1
-0.8
I
-0.4
0.0
EOF 2
0.4
0.8
1.2
Var: 34.6%
200
400
(b)
600
1000 !
-1.2
I
I
i
1
-0.8
-0.4
0.0
0.4
'
0.8
|
1.2
Fig. 5.5. The first two EOF's obtained from the pooled POS and NEG anomaly data
normalized so maximum value is ± 1. See Appendix D for more details.
92
POS Case 25
(a)
-10
0
-5
10
5
15
20
Temperature Anomaly (C)
NEG Case 2
(b)
1000 i
-30
i i
-25
-
I
I
I
i
-20
-15
-10
-5
0
Temperature Anomaly (C)
Fig. 5.6. As in fig. 5.4 but for an individual a) POS event and b) NEG event.
93
TEMP ANOM(C) COMP YTP 24 CASES
1
T -
I
I
I
I
-I
100
i
--
150
-200
------
250
-
300
2
400
D
(
C4
-
~
'.850
700
.oo
~
-
SFC
160 E
LONGITUDE
100 W
TEMP ANOM(C) COMP YTN 24 CASES
'100
150
200
-- *
.--
0.0~
-
00
250
300
---------- --- 300cc
-
--
-oo-
400
500
-
700
S850
SFC
160 E
LONGITUDE
100W
Fig. 5.7. Cross section at 62N for longitudes 160E to 100W indicating composite
temperature anomalies (*C) for a) POS cases and b) NEG cases. Contour interval is 1,
estimated topography indicated.
94
2100700 TEMP ANOM(C)-FILT 1-25-75 (-5)
100
1
8
8.co--0..o
300
--
'
/
I
1
I
200
300
1j
IV
q
.400.'|'
..
0
40-
500
(a)
CA,
K.
700
850
3
SFC
-. 7a.
1
3.
1.
2100700 TEMP ANOM(C)
1
.
-.
c
1
49
3
5
7
9
1
BRKDWN 12-19-80 (-10)
0-4..0
400
(b)
T
700
~ ~ ~ ~ a1.
.
- 4. -
850
--- - .
'
.; -' :.
I* -
SFC
.
3.
5.
7.
9.
It.
13.
15.
17.
19.
21.
23.
25.
27.
29.
31.
Fig. 5.8. Temperature anomalies as a function of pressure and time indicating a)
variability in a NEG case and b) rapid transition between NEG and POS events. See
text for additional details.
(a)
CO"
850MB TEMP ANOM(C)YT? DAY(-3)
K5.9.Com
m\ e)th
(b)
M
COW
A OM
850
) Y
B P TD Y(1
t 85
g
COWP 850MB
hi
t
COM 850MB TEMP ANOM(C)YTP DAY(-1)
-ai2oned
..
COP
TEMP ANOM(C)YTP DAY(1I
Kl
(\~I/N~(
1'~k
8 0MB TEMP
AN OMC)
YTP
DAY
)
COWP 850MB TEMP ANMCYTPDAY 3
18"
N
,.N
Fig. 5.9. Composite 850mb temperature anomalies ('C) for POS events at day a) -3, b)
-1,c) +1 and d) +3, contour interval 2. Corresponding confidence levels are displayed
in e) through h) contoured as in fig. 5.1.
(f)
(e)
(g)
(h)
Fig. 5.9. (continued)
(a)
(b)
(a)
(c)
Fig. 5.10. As in fig. 5.9 but for NEG cases.
(d)
(e)
(f)
(g)
(h)
Fig. 5.10. (continued)
(a)
(b)
500MB COMP RT ANOM(M)WIr TTP DAY(-3)
500MB COMP HT ANOM(M) WNT YTP DAY(-1)
/
----
-
HJ
dx ;
~-
/-22.5
(c)
(d)
500MB COUP HfTANOM(M)WINTYTP DAY(3)
500MB COMP HT ANOIL(M)WENTITP DAY )
H
evns/
otu mevli
5 zeroontou omited
s
I'
4 .0
Z
Fig. 5.11. As in fig. 5.9 but for 500mb height anomalies (in) associated with POS PTA
events. Contour interval is 25, zero contour omitted.
100
(e)
(f)
(g)
(h)
Fig. 5.11. (continued)
101
(a)
(c)
(b)
(d)
Fig. 5.12. As in fig. 5.11 but for NEG PTA events.
102
(e)
(f)
(g)
(h)
Fig. 5.12. (continued)
103
c
-
-
()R
102
4:
-J
-'
0
COMP SLP(MB) AND 860MB TEMP(C) YTN DAY(6)
f
/1.
*-
ZI-
-
Fig. 5.13. Composite sea level pressure (mb) and 850mb temperature (C) for a) POS and
b) NEG PTA events at day + 5. Contour intervals are 4mb for pressure and 5*C for
temperature.
104
(a)
(b)
coUP stP(B) AM 5OUB TEMP(c) 'N DAY(-4
COUP SLP(MB) AND
850MB
TEMP(C) YTN DAY(-2)
00
(c)
(d)
COMP SLPMB) AND 850MB TEMP(C) YTN DAY(0)
2
--
-
--
500MB COMP HT( 1
WINT YTN DAY(0)
5 40
Fig. 5.14. As in fig. 5.13 but for composite NEG cases at days a) -4, b) -2 and c) 0. Also
indicated in d) are the composite 500mb heights (contour interval 60m) at day 0 of the
NEG cases.
105
(a)
(b)
fN.
gA
4'~
LAAf)~
Nn
/.\
%
\~
~'N ~
/:/NK
.
/
~
-A
01
Aro
F~~
/
m'f.FVR
~~~~~0
JEG-
/
\7
,
!S"
1/'.-..r-'-
F\<
Fi. .1.omostese lvl resue ordeelpe
X
r
7\~
) 05an c NG assCn
forlpe b)P05 and c E ae n
coreaton betwee CopstecofienfrEO1ad sea level pressure fiel
d) NEG cases. Correlations for NEG cases have been multiplied by -1. Sea level pressure
difference (NEG - POS) shown in e). Contour interval is 4mb for pressure and 0. 1for
correlations. See text for more details.I
106
(e)
Fig. 5.15. (continued)
107
~\~V~H((
700MB 14r
GT
N
AS
'R
a)
15
-(b)
Fig. 5.16. Sea level pressure (a) and 700mb heights (b) for shallow cold air case.
Pressure contour 4mb, heights 60m.
108
TMP ANOM(C) 2-1 TO 2-7 NEG 15
100
150
200
250
300
400
500
700
850
SFC
160 E
LONGITUDE
100W
Fig. 5.17. As in fig. 5.7 but for case of warm
air intrusion aloft discussed in text.
'
109
ECMWF Average Warm Cases Onset
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0-1.5
-2.0
-2.5-3.0-
(a)
A
8
C
D
Term
ECMWF Avgerage Cold Cases Onset
0.5
0.0
-0.5-
(b)
-1.0 -
-1.5
-
-2.0
-
A
8
C
o
Term
Fig. 5.18. Composite terms and standard deviations (bars) in 850mb heat budget for a)
POS events and b) NEG events. Terms indicated are A) local time rate of change in
potential temperature, B) horizontal advection, C) vertical advection and D) diabatic
term estimated as a residual. Climatological values displayed in c). See text.
110
10 Year Climatology Heat Budget Terms
-0.0
\/E
(c)
-4A
B
C
Term in (5.1)
Fig. 5.18. (continued)
D
111
(a)
850MB TMP
ADV(C/ DAY)
wTNTER CLMO
HN-
/'P~
--
t ......
.
Fig. 5.19. Horizontal advection (K/day)'of 850mb potential temperature (0). a) advection
of the climatological mean 0 field by the climatological mean flow, b) the anomalous
flow advecting the climatological 0 distribution for composite POS ITA cases and c)
as in b) but for composite NEG PTA cases. See text.
112
(b)
R50M11 TMP
I
"(N
*\.**~
-K
ADV(('/DAY)
COMP NGFr VP T-RAR
I
/
/
'K
/
-.-.
/
/)
\7
-N
Fig. 5.19. (continued)
( c)
113
VI.
Summer Cases
A. Introduction
In this Chapter, we focus on the observational characteristics and potential mechanisms
contributing to two exceptionally intense US heat wave/droughts that occurred during the
summers of 1980 and 1988. These events had dire socioeconomic consequences for large
regions of the country. Their unusual severity makes them particularly interesting cases
for diagnostic study.
Before proceeding with our investigation, we will first review some of the more general
characteristics of summer heat waves and droughts in the US. We will then provide a
descriptive overview of the 1980 and 1988 cases. Following this, we will present further
diagnostic analyses in order to identify key mechanisms responsible for these cases.
B. General Characteristicsof Heat Waves and Droughts
Chang and Wallace (1987) have emphasized the distinction between heat waves and
droughts, noting that the typical time scale associated with heat waves is on the order of a
week while droughts may persist for months or even years. Although heat waves may
develop in the absence of droughts, the two conditions often occur simultaneously. This is
indeed the case for the two events that we will study.
The tendency for drought conditions to be accompanied by above normal temperatures
is reflected in statistical studies, which show negative correlations between
contemporaneous seasonal mean temperatures and precipitation over the US (e.g. Madden
and Williams, 1978). However, similar lag relations have also been observed, whereby
dry springs tend to be followed by hot summers (Namias, 1960; Chang and Wallace,
1987) and positive feedbacks from anomalous surface boundary conditions have long been
114
suggested as being important to the development and/or maintenance of droughts and
attendant heat waves (e.g. Namias, 1962, 1978, 1982, 1991).
Locally, the physical basis for such feedbacks involves changes in the surface energy
balance. During anomalously dry conditions, latent heat losses through evapotranspiration
(ET) from the surface are greatly reduced. This reduction is predominantly balanced by
increases in sensible heating. In addition, drought conditions are often associated locally
with an anomalous upper-level anticyclone, which favors subsidence, decreased cloudiness
and greater surface heating (e.g., see Chang and Wallace, 1987). In addition, local ET is
likely to be a significant source of moisture for summertime precipitation over continents
(e.g. Mintz, 1984; Lettau et al., 1979) ; a decrease in ET therefore further contributes
positively to the feedback between heat waves and droughts by reducing the likelihood of
precipitation in an already desiccated region1 . Sensitivity studies with numerical models
have identified many of these effects in runs with specified soil moisture anomalies (e.g.,
Shukla and Mintz, 1982; Rind, 1982; Oglesby and Erickson, 1989).
Not all summer heat wave/droughts in the US are associated with antecedent dry
conditions, however, and anomalous large-scale circulations have been observed to precede
their onset. This has led several investigators to suggest that an anomalous large-scale flow
may act to establish the initial drought conditions which are then maintained or enhanced
through anomalous local feedbacks (Namias, 1962, 1978, 1982; Oglesby and Erickson,
1989; Wolfson et al., 1987).
Previous diagnostic studies of the 1980 and 1988 heat wave/droughts have generally
considered separately the roles of anomalous boundary conditions within the drought
region and of remote effects associated with the large-scale circulation. For example,
Wolfson et al. (1987) and Hao and Bosart (1987) have emphasized the role of local
anomalous boundary conditions during the 1980 event, while remote influences relating to
1Some
(1989).
studies have suggested water vapor flux convergence can offset this effect, see Oglesby and Erickson
115
the development of the 1988 case have been investigated by Trenberth et al. (1988), Mo et
al. (1991) and Palmer and Brankovic (1989).
In this Chapter we will consider anomalies in both the large-scale circulation and local
boundary conditions during different stages of the evolution of the 1980 and 1988 heat
wave/droughts.
We will attempt to identify possible source regions for anomalous
stationary wave activity associated with these events, and will also consider changes in the
surface energy balance of the drought regions through evaluation of changes in ET.
C. Data
Our primary datasets in this portion of the study are NMC gridded analyses of
geopotential heights and temperatures for the 10 standard pressure levels from 1000 to
100mb, and radiosonde observations for stations in the Great Plains. In addition, our
water vapor budget analyses utilize monthly precipitation reports for roughly 1500 stations
in the US that were obtained from reports published by the US Environmental Data
Service.
D. Case Descriptions
1. The 1980 Case
The 1980 heat wave/drought had its largest impact on the central and southern Plains
(fig. 6.1). The development of this event followed near-normal conditions in the region
during spring (Wagner, 1981; Namias, 1982) and was associated with an anomalous largescale circulation which developed rapidly near the end of May and was identifiable in
seasonal (June, July and August) averages of upper-level heights (Dickson, 1980; Namias,
1982). This circulation was characterized by three anomalous ridges located over the
116
eastern North Pacific, the southern Plains and the North Atlantic (fig. 6.2). This general
pattern has been observed during previous droughts in the region, most notably during the
summers of 1952 to 1954 (e.g. Namias, 1982).
The largest temperature anomalies in 1980 occurred during late June and July, well
after the onset of the drought, with record temperatures in several southern Plains states.
For example, Dallas, Texas recorded maximum temperatures in excess of 100 *F every day
in July (Wagner, 1981). The drought and heat wave persisted in the area through August.
Further descriptions details of this event are given in Namias (1982), Dickson (1980),
Livezey (1980) and Wagner (1980, 1981).
2. The 1988 Case
The heat wave/drought of 1988 contrasted with the 1980 event in a number of respects.
First, the 1988 event was proceeded by a dry spring over large portions of the western and
midwestern US (Janowiak, 1988; Trenberth et al., 1988). In addition, a persistent and
anomalous large-scale flow pattern was observed during spring and early summer, as
indicated by the 300mb height anomaly pattern averaged for the April, May and June (Fig.
6.3). As indicated in the figure, the large positive height anomalies centered over the
northern Plains appeared to be part of a larger height anomaly pattern, which Trenberth et
al. (1988) suggested was part of a tropically forced wave train extending from the
southeastern North Pacific to over North America.
This anomalous flow pattern is
markedly different from the three-cell pattern observed during the 1980 event.
The largest temperature anomalies were observed during the month of June in the
central and northern Plains (Fig. 6.4), although monthly record-high temperatures were set
in several locations in July and August as well (Heim, 1988). For additional description of
the 1988 event, see Trenberth et al. (1988) and Namias (1991).
117
E. Sources of Anomalous Stationary Wave Activity
In their early stages the heat wave/droughts of 1980 and 1988 were both accompanied
by anomalous stationary wave patterns. As an initial step towards understanding the
mechanisms contributing to the onset of the heat wave/droughts, we will attempt to identify
potential source regions for the anomalous stationary wave activity.
Our basic
methodology is to apply the local conservation relation for quasi-geostrophic stationary
waves developed by Plumb (1985).
This diagnostic has been applied in previous
modelling (e.g. Mo et al. , 1987; Marks, 1988) and observational investigations of
anomalous stationary waves (Plumb, 1985; Karoly et al., 1989; Black, 1990).
For linear disturbances on an otherwise zonal flow this relation may be written as:
aAs + V - Fs = Cs
at
Where As is the stationary wave activity,
As=
2p
+p E
1 q2
a(Q sin$)
U
$ _
,a sin$
Fs is the three dimensional flux of stationary wave activity,
v*2
Fs = p cos$
a(v*I *)
1
-
20a sin2$
1
-u *v* +
2Qa sin2$
20 sin v*T*
S
-
Al
1
22a sin2$
ak
a(T*D*)1
A
I
(6.1)
118
and Cs is a non-conservative source/sink term which includes diabatic and frictional effects
and interactions with transient eddies. The quantities marked with asterisks, ( )*, in the
above expressions represent departures from a zonal average. For the variables shown p is
pressure, po is a reference pressure,
Q and q* are the mean and perturbations of the quasi-
geostrophic potential vorticity, U is the zonal mean flow, E the wave energy density, u and
v are the horizontal components of the geostrophic wind, a is the earth's radius,
CD
the
geopotential, T is temperature, Q the angular rotation rate of the earth and S an areaaveraged static stability. Plumb (1985) shows that for steady, conservative waves Fs is
non-divergent and, further, for slowly varying, almost plane waves, Fs is parallel to the
group velocity. The computer routine to compute Fs was kindly supplied by Dr. Robert
Black.
Fig. 6.5 presents the Fs pattern for the climatological mean summertime stationary
waves at 300mb. For comparative purposes, we have also included an analogous plot for
the winter climatology. The 300mb level has been selected for presentation since Rossby
waves propagate quasi-horizontally along potential vorticity gradients near the tropopause.
A more complete interpretation of the patterns requires examination of the full threedimensional Fs field. (For greater detail on the wintertime pattern see Plumb (1985)).
We see that the summertime stationary waves are considerably weaker than their winter
counterparts. The overall wave patterns for the two seasons are markedly different, the
summer pattern being characterized by a relative weakening of the east-Asian trough and
North Atlantic ridge and the development of a ridge over the interior of the US, well
displaced from its mean winter position along the west coast of North America.
As might be anticipated, the corresponding Fs patterns are also quite different. During
summer the stationary wave fluxes are generally weaker. The wintertime divergence of
stationary wave fluxes from the vicinity of the Himalayas and east Asia is not observed
during summer, suggesting that in summer the Himalayas are a less prominent source of
119
stationary wave activity. During summer there is a pronounced tendency for Fs to be
directed towards the subtropics in the lower latitudes over the North Pacific. Over the
North Atlantic, the summertime pattern is somewhat similar to winter, with the fluxes
directed towards the subtropics from higher latitudes. A possible summertime source of
stationary waves is located over the eastern Mediterranean during summer, with the Fs
vectors diverging from that region.
We next examine the seasonal (June, July and August) height anomaly pattern for the
1980 case. In the following analyses, we have computed the stationary wave flux directly
from the time-mean height anomalies. We thus examine the flux of the anomalies rather
than the total anomalous fluxes. The implications of the two approaches are discussed by
Karoly et al. (1989).
Figure 6.6 displays the seasonal 300mb height anomalies and the horizontal component
of Fs for the 1980 case. The key features of the height fields are the anomalously high
heights centered to the south of the Gulf of Alaska and over the southern Plains, and the
anomalously low heights along the west coasts of the US and Canada. The corresponding
Fs vectors diverge from near the North Pacific positive anomaly center toward the anomaly
centers that appear as part of a downstream wave train. This suggests that a possible
source for the anomalous wave pattern is located over the North Pacific.
Fig. 6.7 displays similar monthly mean height anomaly and Fs patterns for June and
July 1980. The height anomaly pattern in June (fig 6.7a) is quite similar to the summer
mean pattern, but with the anomalies having somewhat larger magnitudes. The Fs pattern
for June is also similar to the summer mean conditions with a divergence of stationary
wave activity fluxes near the Aleutians.
By July (fig. 6.7b) height anomalies are
considerably weaker, with the corresponding Fs vectors not displaying any coherent
pattern. It is interesting that the largest surface temperature anomalies were observed in late
June and during July, at which time the anomalous large-scale flow pattern had weakened
considerably. This observation suggests that remote forcing alone may not account for the
120
later stages of the heat wave/drought. We will return to this point later in our water vapor
budget analyses.
Similar analyses have been performed for the 1988 event. As noted earlier, an
anomalous stationary wave pattern was first observed during the spring of 1988. Fig. 6.8
displays the 300mb height anomaly and Fs patterns for the months of April, May and
June. The height anomaly pattern, as discussed earlier, is suggestive of a wave train
emanating from the subtropical North Pacific, which Trenberth et al. (1988) have argued
was ultimately related to anomalous sea surface temperatures in the tropical eastern North
Pacific. Trenberth et al. based their conclusion on results from integrations of a global
numerical planetary wave model which was linearized about the observed (1988) June
conditions and forced by the May to June average of observed sea surface temperature
anomalies in the Pacific. They argue that the upper-level height anomaly pattern produced
by the integration of their model was similar to the observed pattern of the spring of 1988,
although the positive anomaly center over North America produced by the model was
weaker than observed by roughly a factor of two.
The Fs pattern in fig. 6.8, however, provides no clear indication of anomalous wave
propagation out of this region. Rather, there is a divergence in the wave activity flux from
the region just north of Hawaii, well northwest of the source region
proposed by
Trenberth et al. (1988). Another potential source region is located upstream in midlatitudes
along the east Asian coast. These regions may not be independent, however, since the
poleward directed component over the mid-Pacific may be at least in part a manifestation of
a low latitude reflection of wave activity emanating from the upstream source.
Some caveats should be noted on interpreting these results. First, since potential
vorticity gradients along the tropopause serve as a wave guide for Rossby waves, the
300mb level may be too low to detect a full tropical signal. However, similar calculations
at the 200mb level (not shown) yielded the same qualitative Fs pattern. Second, as
Sardeshmukh and Hoskins (1985) have noted, it is also possible that tropical forcing may
121
alter the local Hadley circulation which could move the effective stationary wave source
from the tropics to higher latitudes (Plumb, 1985). Our analyses do not exclude this
possibility. Overall, however, our results do not provide compelling evidence for a
dominant role of anomalous tropical forcing over the tropical eastern Pacific in producing
the observed spring height anomaly pattern.
Height anomalies and Fs fields for later months during the 1988 event were also
examined. Fig. 6.9 presents these fields for July 1988. By this time, the observed height
anomaly pattern has changed dramatically from spring, with the wave train discussed
previously now absent. The weakening of the large positive height anomalies over the
northern US is particularly interesting, since the intense heat wave and drought continued
into July in that region, just as in the 1980 case.
G. Water Vapor Budgets
Our analyses of the height anomaly patterns associated with summer heat
wave/droughts indicate that the largest anomaly magnitudes occurred during the early
stages of development. Nevertheless, in both cases despite a weakening of these upperlevel features the heat wave and drought persist. A possible explanation for this persistence
of the heat wave/drought is that anomalous surface boundary conditions may have acted to
enhance these events at later stages. This possibility can be discussed qualitatively in terms
of the surface energy balance, which may be written as:
RN = RL + SH + LH - Storage
(6.3)
where RN is the net incoming radiation (both short and long-wave), RL is the outgoing
long wave radiation from the surface, SH and LH are the sensible and latent heating and the
storage term includes temporal changes and the loss of heat to deeper soil layers. The
122
storage term in (6.3) is usually quite small, especially when considered over monthly or
seasonal time scales (Sellers, 1965). Neglecting this term, the net incoming solar and longwave radiation is balanced by heat losses due to upward long-wave radiation and to
sensible and latent heat transfers. Thus, assuming RN is approximately constant 2 ,
decreases in LH must be offset by increases in sensible heat transports from the surface and
upward long-wave radiation.
To test whether systematic decreases in latent heating occur during the evolution of the
1980 and 1988 events, we have computed water budgets over the drought regions,
estimating the evapotranspiration (ET) as a residual. Twice-daily radiosonde data and
monthly precipitation observations are used in this calculation.
Our approach is similar to that of Rasmusson (1968) and Hao and Bosart (1987). We
first consider the conservation of water vapor in a vertical column extending from the
surface to 300mb:
aW+V Q = E - P
-It
where W is the precipitable water,
Q is
(6.4)
the horizontal water vapor flux, E is
evapotranspiration, P precipitation, q the specific humidity, g is gravity, u and v are the
horizontal wind components, and pt and ps are the pressure at the top of the column
(300mb) and at the surface. Taking time and areal averages of (6.4):
(U)=(
2For
)+
n-ds+ (P)
(6.5)
the latitudes we are considering, the June to August change in insolation received at the top of the
atmosphere is roughly 10% (see Sellers, 1965).
123
where overbars represent a time average (taken to be one month), bracketed terms represent
an average over the domain of area A, and n is a unit vector orthogonal to the boundary of
the domain. The first two terms on the right-hand side of (6.5) are determined from the
radiosonde data, while the third term is computed manually from archived data utilizing
roughly 800 monthly precipitation reports for the 1980 case and 700 reports for the 1988
case. Having obtained these terms, the ET rate is calculated as a residual quantity. As
controls for the two regions we have performed similar analyses for seasons with nearnormal precipitation. For this purpose ET rates for the summers of 1979 and 1985 are
compared with the 1980 and 1988 results, respectively. The domains used are displayed in
fig. 6.10 while the individual terms in (6.5) for the 1980 and 1988 cases are listed in Table
6.1. Control values of the terms in (6.5) are listed in Table 6.2. Further details of the
method used in computing the terms in (6.5) are given in Appendix F along with other
checks of the reliability of our results.
Fig. 6.11 displays the estimated monthly mean ET rates for both the 1980 and 1988
events along with their control values. A steady decrease in ET rates is observed during the
evolution of the two heat wave/drought cases, with the greatest (negative) departures from
the control cases observed in July and August. The decrease in the surface latent heat flux
2
from June to August in both the heat wave/drought cases is roughly 50 W/m . Tables of
net incoming radiation by Henning (1989) indicate mid-summer values in these regions are
on the order of 150 W/m 2 . Thus the observed perturbations to the surface energy balance
are nontrivial, apparently being most significant during the month of August in both cases.
Changes in the surface energy balance due to to decreases in ET may thus help to account
for the persistence of the heat wave conditions through the summer, despite the absence at
that time of notably anomalous stationary waves.
124
H. Summary
We have investigated the roles of large-scale circulation anomalies and anomalous
surface boundary conditions on the development and maintenance of two major heat
wave/drought events. We find that during the early stages of development, both events
were associated with anomalous stationary wave patterns. These patterns were markedly
different for the two events, with a three-cell pattern commonly associated with heat waves
and drought in the Plains observed during the incipient stages of the 1980 case, and a wave
train emanating from the subtropical North Pacific present during the early stages of the
1988 event.
To identify potential source locations for the anomalous stationary waves, diagnostic
analyses of stationary wave activity were performed following a technique developed by
Plumb (1985). A source of anomalous stationary wave activity in the vicinity of the Bering
Sea is strongly suggested during the 1980 case. In the 1988 case a more complicated
pattern is observed, with two possible source locations being in the region north of Hawaii
and at mid-latitudes upstream near the Asian coast. The analyses do not provide
compelling evidence that the 1988 anomalous stationary wave pattern was forced from a
tropical heating anomaly over the eastern Pacific, contrary to a suggestion by Trenberth et
al. (1988). Our results are consistent with those of a recent modelling study by Mo et al.
(1991) who found that the NMC Medium-Range Forecast model was able to produce a
height anomaly pattern qualitatively similar to that observed in the spring of 1988 using the
observed initial conditons but with climatological sea surface temperatures.
125
The vertical structures of area-averaged thermal anomalies 3 associated with the two
cases (not shown) were similar, with the largest anomalies located near the surface. This is
consistent with the findings of Chang and Wallace (1987), who indicate 500mb
temperature anomalies are often a factor of two smaller than surface values for heat
wave/drought events in the Plains. There were also changes in the vertical structure of the
area-averaged temperature anomalies during the evolution of these events, with the
anomalies increasing near the surface during latter stages of both cases as displayed in fig.
6.12. The changes in the vertical thermal structures and observed weakening in the largescale height anomaly patterns during mid to late stages of both these events suggest that
different processes may be of primary importance at the early and later stages of the heat
wave/droughts.
Given the near absence of anomalous stationary waves at the later stages of the events,
we considered the possibility that changes in the local surface energy balance were playing
a key role in perpetuating the heat wave/drought conditions. Results of moisture budget
analyses for the two cases are consistent with this idea and show that reductions of
evapotranspiration and consequent changes in the surface energy balance were likely to
assume increasingly important roles at later stages of the events.
Overall, then, the summer cases show some similarities but also significant differences
from the wintertime PTA events. For both winter and summer cases, characteristic
anomalous large-scale circulations occur. However, in the summer cases, these anomalous
circulations weaken in time, while anomalous surface boundary conditions appear to
become increasingly important. This contrasts with the behavior seen in the winter cases,
where dynamical processes appear to dominate the evolution of PTA's throughout the life
cycles of the events.
anomalies were obtained by averaging daily anomaly values identified in the NMC gridded analyses
over the given month and from 30N to 38N, 85W to 100W in the 1980 case, and from 38N to 46N, 90W
3These
to 105W in the 1988 case.
126
127
TABLE 6.1
Water vapor budget terms for 1980 and 1988 drought regions
Units: mm/day
1980 Region
IVI IILI
Precipitable
W1tIe Chla e
Water Vapor Flux
-
Di,
ke
ci
Pr
4zMr
Evanspmiratin
p'.1
itation
June
0.1
2.53
2.28
4.71
July
0.07
2.21
1.35
3.49
August
0.24
0.75
1.87
2.86
1988 Region
Precipitable
Water Change
Water Vapor Flux
June
0.21
1.65
1.46
3.32
July
0.25
1.03
1.56
2.84
0.59
2.55
1.51
Month
August
- 0.45
Divepr ence
gnriittn
-
pFn
itation
Preci
F~v~rnotr~rnsniration
rnprto
128
TABLE 6.2
Control water vapor budget terms for the 1980 and 1988 drought regions
Units: mm/day
1980 Control (1979 data)
Precipitable
Water Change
Water Vapor Flux
Divergence
June
0.05
1.27
July
0.27
August
0.0
Month
Precipitation
PreciDitation
3.20
EvaDotransDiration
Eva-D transr)iration
0.0
4.32
4.59
1.88
2.73
4.61
4.42
1988 Control (1985 data)
Month
Month
June
July
August
Precipitable
Water Change
Chanpre
Water
0.3
Water Vapor Flux
Divergence
Divemence
prpf-i itntinn
Evpnotrnspirati n
Precipitation
0.45
2.48
3.23
0.4
0.66
2.27
3.33
-0.25
0.08
2.89
2.72
129
(a)
PERCENTAGE
OF NORMAL PRECIPITATION
(b)
oo0
Ica
Fig. 6.1. Summer 1980 a) average temperature departure from normal (*F) and b) percent
of normal precipitation. From Wagner (1981).
130
Fig. 6.2. Average 300mb height anomlies (m) for June 1980. Contour interval 25, zero
contour omitted.
131
Fig. 6.3. Average 300mb height anomalies (m) for April, May and June 1988. Contour
interval 25, zero contour omitted.
132
2
-
B
-2
SHADED AREAS ABOIE NORMAt.
-.-
UEATWCR
FACILITY
AGRICULTURAL
MOAA/USDA
p
HAWAI
ALASKA
PERCENTAGE OF NORMAL PRECIPITATION
71f
9e
,1
\S IS /
VA
JUNE 1988
7S so\
/7-
2S
10SS
a77
I
57
'A25as
s--
T< s
-
7o
\7S
S
1ow
7
25
22
2S
7
soe
275
25
57
2S
.
52
W
A RIEA
EIN
JTCHT
A H0N
EWA1 8
BL
FACILITY
Fig. 6.4. June 1988 a) departure of average temperature (*F) from normal and b) percent
of normal precipitation. From Ludlum (1988).
133
77"~~y/~t+
(a)
7
+>
-
N
/ ~
-\~~Nf
xd /
%A
QQM 7ST
/%
R
STMMERM -
MQ10
m2/s2
I
2
10 m /S2
Fig. 6.5. Climatological 300mb stationary waves (departures from zonal symmetry) and
horizontal component of Fs for a) summer and b) winter. Contour interval 25m for
summer, 50m for winter, zero contour omitted. Fs vector scale shown on lower-right
of plots.
134
15 m2/s2
Fig. 6.6. Average 300mb height anomalies for June, July and August 1980 and
corresponding horizontal component of Fs. Contour interval 25m, zero contour
omitted, Fs vector scale inicated on lower-right of plot.
135
(a)
2
2
45 m /s
(b)
2 2
45 m /s
Fig. 6.7. As in fig. 6.6 but for the months of a) June and b) July 1980.
136
15 m2/s 2
Fig. 6.8. As in fig. 6.6 but for April, May and June 1988.
137
45 m2/s2
Fig. 6.9. As in fig.6.6 but for July 1988.
138
/-
4'6
-..
-!
7s---
e
, C
-
'49
220
(242
-
_
_*_
_-
..
Fig. 6.10. Water vapor budget domains for a) 1980 case and b) 1988 case. See text and
Appendix F for more details.
139
ET Rates for Summer 1980 and Control
5.0
4.5
4.0 -
(a)
3.53.0-
*
2.5 -
Control
1980 case
2.0 -
1.0 0.50.0June
July
August
ET Rates for Summer 1988 and Control
4.0
3.5 -
3.0 -
(b)
2.5 -
Control
1988 Case
2.0 -
1.51.00.5 -
0.0 -
June
July
August
Fig. 6.11. Estimated evapotranspiration rates (mm/day) and controls for June, July and
August of a) the 1980 case and b) the 1988 case. See Appendix F and Tables 6.1 and
6.2 for more details.
140
Temperature Anomaly Difference ("C)
August - June 1980
(a)
-3
-2
-1
0
1
2
Anomaly Difference (*C)
Temperature Anomaly Difference (*C)
August - July 1988 Northern Plains
(b)
-2
-1
0
1
2
3
4
Anomaly Difference (*C)
Fig. 6.12 The difference in area-averaged monthly mean temperature anomalies (*C) for a)
August-June 1980 and b) August-July 1988. Surface anomalies are computed as
departures from climatology of the 1000mb to 850mb layer temperatures.
141
VII.
Summary and Conclusions
A. Summary
We have conducted an observational study aimed at identifying the key physical and
dynamical processes contributing to the development and maintenance of persistent
temperature anomalies (PTA's). Detailed analyses have been performed for PTA events
occurring in both winter and summer, with our primary focus being on events occurring
over the US and Canada. Our main emphasis has been on identifying the key synoptic
features and mechanisms leading to the development of PTA's. We have also investigated
potential physical processes that may serve to enhance or maintain these anomalous events.
We began by examining the geographical distributions and regional persistence
characteristics of PTA's. Both winter and summer events were considered. Results for
winter indicate that there is a strong continentality bias for PTA's, with a local maxima in
the frequency of occurrence for both positive (POS) and negative (NEG) events located
over northwestern Canada to the lee side of the Rockies. This frequency distribution
stands in marked contrast to those obtained by Dole and Gordon (1983) for persistent flow
anomalies (PFA's), with the latter occurring most frequently over the central North Pacific,
eastern North Atlantic and the northern Soviet Union.
For the winter events removing high frequency transients from the temperature anomaly
data increased the number of events without substantially altering the overall geographical
distribution. In winter, there was no strong bias toward the recurrence of PTA events of
either sign; that is, successive wintertime PTA events are as likely to be of the opposite as
of the same sign. Some of the observed temperature anomaly distributions departed
significantly from normality, with stations to the lee of the Rockies sometimes displaying
clear bimodality.
142
Summertime PTA events in the US and Canada display several similarities but also
important differences from their wintertime counterparts. For example, during summertime
the numbers of PTA events of a given magnitude and duration are observed to vary with
the sign of the anomaly. This behavior was not seen during winter. In addition, in
summer there is no single, dominant maxima in the frequency of occurrence of PTA events
as is observed in winter over northwestern Canada. For portions of the Great Plains,
summertime temperature anomaly distributions are unimodal, but with a bias towards the
recurrence of POS events. This bias suggests that additional, longer time scale physical or
dynamical processes may play an important role in the development or maintenance of these
events.
We next performed more detailed synoptic and diagnostic studies of winter events
located in the region of most frequent PTA occurrence over northwest Canada (the "key"
region). Our investigation of the temporal behavior of composite 850mb temperature
anomalies revealed that for both the POS and NEG PTA cases, the thermal anomalies
developed essentially in situ over the key region. Additional analyses of corresponding
500mb height anomalies showed that the developments were frequently associated with
particular anomalous large-scale circulations. For the POS and NEG PTA cases, these
anomalous flows closely resembled, respectively, the "PAC NEG" and "PAC POS" height
anomaly patterns described by Dole (1986a). For the POS PTA cases the height anomaly
pattern develops nearly contemporaneously with the temperature anomalies, while in the
NEG PTA cases, the upstream height anomalies develop well prior to PTA onset. In both
cases, however, developments are clearly tied to events with origins well upstream of the
key region.
Analyses of the vertical thermal structure of wintertime PTA events indicated that the
largest temperature anomalies are typically located near the surface to the lee of the Rockies.
Although the dominant vertical structures for these cases extend through the troposphere,
some interesting departures from this typical structure are sometimes observed. For
143
example, the temperature anomalies of NEG events are occasionally confined to the lowest
150-200mb, and can reverse sign above that level. These are related to warm air intrusions
aloft which can result in an extremely stable boundary layer with temperature inversions of
up to 200 C. In addition, rapid transitions from one temperature anomaly regime to another
of opposite sign are sometimes observed (over a few days). This is consistent with the
observed bimodal behavior of some wintertime temperature anomaly distributions, since in
these locations the temperature is more frequently markedly anomalous than near normal.
Heat budget analyses were performed to assess the principal mechanisms responsible
for the development of wintertime PTA events. For the NEG cases, horizontal advection
and diabatic cooling are the largest contributors to the PTA developments, with the two
terms being of comparable magnitude. In these cases, the anomalous low level circulation
over and upstream from the key region reduces or eliminates the climatological mean warm
advection that normally helps balance radiative cooling in the region. For the POS cases,
both horizontal advection and adiabatic warming associated with downslope flow
contribute positively to the developments, and are partially offset by diabatic cooling. For
both the POS and NEG cases, orography plays a significant role, with shallow and
strongly stratified cold air trapped to the lee of the Rockies in the NEG cases and adiabatic
warming in the downslope flow contributing to the often rapid development of the POS
cases.
We then examined the processes contributing to the development and maintenance of
summertime PTA's, concentrating on the extreme heat wave/drought cases of 1980 and
1988. Examination of the synoptic features associated with these cases indicated that in
their early stages both events were associated with anomalous stationary wave patterns. In
order to identify potential source regions for these anomalous stationary waves, we applied
a diagnostic technique developed by Plumb (1985).
For the 1980 case, the results
suggested that an anomalous mid-latitude wave source was located over the central North
Pacific ocean. For the 1988 case, anomalous wave activity at early stages appeared to
144
emanate from upstream over the western North Pacific; however there was little evidence
for poleward propagation from an anomalous tropical source over the eastern Pacific at this
time, contrary to the suggestion by Trenberth et al. (1988). For both the 1980 and 1988
cases, the magnitude of the anomalous waves decreased markedly during the summer,
although the heat wave drought conditions continued.
This lead us to examine the possibility that these summertime events were enhanced or
prolonged by changes in the surface energy budget associated with reduced
evapotranspiration over the drought regions. Results of water vapor budget calculations
supported this view, showing that there was a marked decrease in monthly-mean
evapotranspiration and associated latent heat fluxes from the surface between June and
August in both 1980 and 1988. The implied increase in sensible and radiative heat losses
from the surface was on the order of 50 W/m 2 in both cases, suggesting that this effect can
be significant.
B. Conclusions
Overall, the results suggest that the behavior of wintertime PTA events over the US and
Canada is predominantly determined by large-scale dynamical processes, with orographic
influences having a significant modifying effect.
In contrast, in the summer heat
wave/drought cases both dynamical forcing from a remote source and anomalous boundary
conditions in the drought regions appeared to contribute to the development and
maintenance of the two major heat wave/drought events, with the former being particularly
prominent at early stages and the latter assuming increased importance at later stages.
The wintertime results also suggested a possible twofold explanation as to why PTA's
occur most frequently in the CAN key region. First, persistent and anomalous large-scale
flow patterns often develop upstream over the central North Pacific, with the persistent
flow anomalies having associated circulations and thermal advection patterns that are
145
conducive to the subsequent development of PTA events downstream. In addition, the local
topography in the CAN region enhances the effects of the anomalous PFA circulations by
producing anomalous downslope flow and enhanced warming in the POS cases and
reduced warm advection and cold air damming in the NEG cases.
146
147
Appendix A.
Datasets
1. The NMC GriddedAnalyses
These grids consist of the twice daily (0000 UTC and 1200 UTC) final analyses of
temperature, wind and geopotential height at the 10 mandatory pressure levels from
1000mb to 100mb for the period 1 December 1963 to 31 December 1990. Spatial
interpolation was used to convert these data from the NMC octagonal grid to a 20 latitude by
50 longitude grid extending from 20N to 90N. Missing or obviously erroneous grids in the
time series were replaced by linearly interpolated values. Less than three percent of the
grids were interpolated in this fashion (Black, 1990). For additional details of the NMC
dataset see Trenberth and Olson (1988a).
2. The ECMWF GriddedAnalyses
These grids include twice daily (0000 UTC and 1200 UTC) fields of temperature,
horizontal wind, vertical velocity (in pressure coordinates), geopotential height and specific
humidity at the seven pressure levels 1000, 850, 700, 500, 300, 200 and 100mb levels for
the period 1 December 1978 to December 31 1990. Data for the period 1 December to 30
November 1978 are from the First GARP Global Experiment (FGGE) level II1b analyses;
the remainder of the data are initialized fields from the ECMWF 4-dimensional data
assimilation procedure (Trenberth and Olson, 1988b; Bengtsson et al., 1982). The FGGE
and ECMWF data, originally on 1.8750 and 2.5' latitude-longitude grids, respectively,
where interpolated onto an approximately 4.54 latitude by 7.5' longitude grid using spectral
analysis and rhomboidal truncation at wavenumber 15 (R15) . Further details of this
dataset are given in Trenberth and Olson (1988b).
148
3. Climate Station Data
These data consist of daily values of maximum and minimum temperatures,
precipitation and snow cover for approximately 400 US and 175 Canadian stations. Data
for the US stations were obtained from the National Center for Atmospheric Research
(NCAR), while the Canadian data was obtained from the Atmospheric Environment
Service (AES) of Canada. Time series typically extend back 40-50 years ending in 1986
for US, and 1988 for Canadian stations.
When creating these datasets, missing or obviously erroneous temperature observations
were replaced with linearly interpolated values, except in the case where data were missing
for the first day of a given month. In this latter case, the missing days were all assigned
the value of the first reliable observation. A bias was added to all of the interpolated (or
assigned) temperatures so they could later be identified.
Stations having more than 5
consecutive interpolated values during a given study period were excluded from the
analysis. Missing snow cover or precipitation data were not replaced with interpolated
values.
149
Appendix B.
Temporal Filtering
In several of our analyses we have temporally filtered the data in order to remove high
frequency transients. For this purpose we have applied a simple, symmetric four-point
filter which has the form:
3
t
= wo Tt +
Wk
(It-k + Tt+k)
k=1
where Tt is the filtered value of variable T at time (day), t and w0, wi, w2 and w3 are the
weights applied at day t, t±1, t±2, and t±3, respectively. The values of these weights are
shown below along with the response function of the filter.
wo = 0.3125
wi = 0.234375
w2 = 0.09375
w3 = 0.015625
RESPONSE FUNC 4 POINT FILTER
.58-
.3
.2
.1-
i
PERI80
5
6
ORTS)
7
150
151
Appendix C.
Winter Case Dates
The following PTA events were used in the construction of the wintertime composites.
They were identified in filtered daily mean temperature anomaly data for Mayo, Yukon
Territory meeting the criteria (±50 C, 10 days) for the 24 winters from 1962-63 to 1985-86.
Winter is defined as the 90-day period from 1 December to 28 February. Listed are the
onset dates and durations for the POS and NEG events.
POS Cases
Onset Date
1-13-65
Duration (days)
11
12-15-66
21
12-7-67
12-23-69
2-4-70
11
11
10
2-7-71
17
12-5-74
12-18-74
10
1-17-75
2-17-75
12-17-75
11
12+
1-18-76
1-14-77
1-18-78
2-5-78
12-6-78
1-17-79
16
46+
10
19
10
10
2-1-80
12
10
18
1Numbers marked with a ()+ are for events which began before 1 Decemeber or ended after 28 February and
have durations greater than indicated (the indicated duration is the number of days of the event occurring after
1 December or before 28 February).
152
POS Cases
Onset Date
12-31-80
Duration (days)
40
11
12-24-82
1-27-83
2-11-84
15
10
1-1-85
12-5-85
36
55
NEG Cases
Onset Date
12-10-64
12-24-65
1-20-67
12-23-68
1-9-71
1-9-72
2-16-72
1-13-73
1-8-74
1-20-74
1-3-75
1-30-75
1-6-76
2-16-76
1-1-79
2-3-79
12-7-79
12-16-80
Duration (days)
33
39
11
47
28
19
13+
10
11
17
11
16
10
13+
15
26+
10
14
153
NEG Cases
Onset Date
12-26-81
2-13-82
12-6-83
12-21-84
2-8-85
2-15-86
Duration (days)
35
16+
25
154
155
Appendix D.
EOF Analyses of Vertical Thermal Structure
The main goal in using EOF analysis is to extract from the observed data the structure
that best describes the vertical profile of the thermal anomalies associated with PTA's. Our
basic approach is to first obtain for each of the 27 POS and 29 NEG cases the temperature
anomalies at the 10 vertical levels from the surface to 100mb as described in Chapter V.
The 56 pooled cases are used to create a matrix T, where element Tij in T is the
temperature anomaly at level
j for
the ith case (i=1,2,3...,56 and j=1,2,3,...,10).
This
matrix is input into a routine which calculates ten eigenvectors each in the form of a
column matrix E, where Ej,k is element j of eigenvector k (j=1,2,3,...,10 representing the
10 pressure levels and k=1,2,3,...,10 indicating the 10 eigenvectors).
Time coefficients ak, associated with the k eigenvectors were computed for each of the
56 cases by taking the inner product of the eigenvectors E and the rows of matrix T. Thus
for case i and eigenvector k this time coefficient is given as:
10
ai,k=
Tij Ej,k
j=1
The percent of variance explained by eigenvector k for case i is determined as:
a,,k
10
k=1
and are displayed in Tables 5.1 and 5.2.
156
157
Appendix E.
Heat Budget Cases
The following POS and NEG PTA cases were used in the wintertime heat budget
analyses in Chapter V. These cases were selected from light low-pass filtered data at
Mayo, Yukon Territory during the 10 winters (1 December to 28 February) from 1978-79
to 1987-88. Indicated are the dates for which terms in the heat budgets were computed.
POS Cases
Ending date
1-19-79
1-2-81
2-1-82
Starting Date
1-13-79
12-27-80
1-27-82
1-30-83
1-3-85
1-23-83
12-28-84
NEG Cases
Starting Date
2-1-79
Ending date
2-7-79
12-18-81
2-15-82
12-4-83
2-7-85
12-27-81
2-24-82
12-10-83
2-13-85
158
159
Appendix F. Water Vapor Budget Calculations
The horizontal domains for the water vapor budget studies of the 1980 and 1988 heat
wave/drought cases consist of polygons defined by the radiosonde stations indicated in fig.
6.10 of Chapter VI. The names and locations of the stations used for the two cases are
listed in Table F. 1. We have chosen these regions for study since they are the areas where
the largest seasonal temperature anomalies were observed during the two events. Our
objective is to estimate monthly evapotranspiration (ET) rates for June, July and August of
1980 and 1988 from the relation shown in (6.6) of Chapter VI, namely:
Q - n ds +(P
+A
I
II
(F.1)
III
Where overbars in (F.1) indicate a monthly mean, brackets indicate an average over the
domain of area A, W is the precipitable water, Q is the horizontal water vapor flux, n a
unit vector orthogonal to the boundary of the domain, E is the evapotranspiration and P is
precipitation. The precipitable water, W is defined as
W
=
q dp
where g is gravity, q the specific humidity, ps is the surface pressure and pt is the pressure
at the top of the domain, set at 300mb. The horizontal water vapor flux term,
Q is defined
as:
Q =(QXQ) where QX=
qudp and Q,=
qv dp
160
where u and v are the horizontal wind components. Before computing the terms on the
right hand side of (F. 1), linear interpolation was used to construct twice daily (0000 UTC
and 1200 UTC) soundings with values of specific humidity and wind specified every 25mb
from the surface to 700mb and every 50mb from 700mb to 300mb for each radiosonde
observation in our domain. Significant level temperature and humidity data was employed
in the construction of the interpolated soundings. Missing days at a given station were
ignored.
The area averaged monthly mean precipitable water change (term I in F. 1) was
calculated as the average daily change (0000 UTC to 0000 UTC) of all the stations in the
domain for a given month. The water vapor flux divergence (term II) was evaluated by
first computing the monthly mean value of Q at each radiosonde station and then computing
the line integral around the periphery of the domain, using linear interpolation to obtain
mean
Q values
between stations. The mean precipitation (term III) was determined from
monthly precipitation data at roughly 800 stations for the 1980 case and 700 for the 1988
case. Term III was determined as the average of all these reports for a given month, each
report having equal weight.
Control values of ET were estimated in a similar fashion for seasons having nearnormal precipitation. The summer of 1979 was used as a control for the 1980 case, while
the summer of 1985 was used as a control for the 1988 case. The control values of the
terms in (F. 1) are listed in Table 6.2.
We have made some additional checks on the reliability of our ET rates. First,
evaporation data was obtained for "pan" stations located within our two drought regions.
A 12-year (1977 to 1988) climatology of monthly mean evaporation rates for May to
September was constructed at 6 stations, 3 in each the 1980 and 1988 drought domains.
These pan measurements represent a potential (or maximum) evapotranspiration rate since
an unlimited amount of water is available to evaporate. Month-by-month comparisons of
our drought ET rates with the climatological pan evaporation rates are shown in fig. F.1
161
and indicate our estimated drought ET rates are all considerably less than their
corresponding pan values, as would be expected. Additionally, we have compared our
drought ET rates with the climatological ET rates over North America presented by
Henning (1989). Henning's climatological values are for the period 1930 to 1960 and
were computed using the surface energy balance method of Albrecht (1965) and are
presented in fig. F.2. We find our seasonal (June, July and August) ET rates for both the
1980 and 1988 cases to be comparable in magnitude to this climatology, if not slightly
lower. We note, however, that the period 1930 to 1960 used in computing Henning's
climatology contains some of the most intense recorded droughts in the Great Plains, which
may bias his rates towards the low side.
Finally, the decline in our ET rates during the 1980 case is consistent with the results of
Hao and Bosart (1987) who indicated a similar decline in ET rates for a slightly different
region in the southern Plains during the 1980 event.
162
163
TABLE F.1
Radiosonde stations used in water vapor budget studies
1980
Station Number
72562
72553
72532
72433
72327
72229
72235
72240
72555
72260
72363
72451
72456
72353
72349
72247
72340
Station Name
North Platte, NE
Omaha, NE
Peoria, IL
Salem, IL
Nashville, TN
Centerville, AL
Jackson, MS
Lake Charles, LA
Victoria, TX
Stephenville, TX
Amarillo, TX
Dodge City, KS
Topeka, KS
Oklahoma City, OK
Monet, MO
Shreveport. LA
Little Rock, AR
Latitude
Longitude
41 08
4122
4040
3839
3607
32 54
32 19
3007
2851
32 13
3414
3746
3904
35 24
3653
3231
3444
10041
9601
8941
8858
8641
87 15
9005
93 13
9655
98 11
10142
9958
95 38
97 36
93 54
9345
92 14
Latitude
Longitude
48 13
4646
48 34
4429
4040
3839
3904
4108
4249
4403
4423
4122
45 33
106 37
10045
93 23
8808
8941
8858
95 38
10041
10844
103 04
98 13
9601
9404
1988
Station Number
72768
72764
72747
72645
72532
72433
72456
72562
72576
72662
72654
72553
72655
Station Name
Glasgow, MT
Bismark, ND
Internationa Falls, MN
Green Bay, WI
Peoria, IL
Salem, IL
Topeka, KS
North Platte, NE
Lander, WY
Rapid City, SD
Huron, SD
Omaha, NE
Saint Cloud, MN
164
165
Climatological PAN Evaporation
and Estimated ET for 1980 Case
(a)
0
PAN OK 1
*
0
PAN OK 2
PAN OK 3
Estimated ET
1
June
July
August
Month
Climatological PAN Evaporation
and Estimated ET for 1988 Case
(b)
SPAN ND 1
*
*
E
June
July
PAN ND 2
PAN ND 3
Estimated ET
August
Month
Fig. F. 1. Climatological pan evaporation rates a) for three stations in Oklahoma
along
estimated evapotranspiration for 1980 case and b) three North Dakota stations and with
estimated evapotranspiration during the 1988 case. Units are mm/day.
166
Fig. F.2. Climatological evapotranspiration (cm) for June, July and August based on data
for the 30-year period 1931 to 1960 from Henning (1989). Approximate domains used
in the water vapor budget studies for the 1980 and 1988 cases are also indicated.
167
References
Albrecht, F., 1965: Untersuchungen des Warme und Wasserhaushaltes der Sudlichen
Kontinente. Berichte des Deutschen Wetterdienstes, 14 (99): 54 pp.
Bengtsson, L., M. Kanamitsu, P. Kallberg, and S. Uppala, 1982: FGGE 4-dimensional
data assimilation at ECMWF. Bull. Amer. Meteorol. Soc. 63, 29-43.
Black, R. X., 1990: A Dia'gnostic Study of the Life Cycles of Persistent Flow Anomalies.
MIT Dept. of Meteorology and Physical Oceanography, Ph.D. Thesis.
Blackmon, M. L., 1976: A climatological spectral study of the 500mb geopotential height
of the Northern Hemisphere. J. Atmos. Sci. 33, 1607-1623.
, Y.-H. Lee and J. M. Wallace, 1984a: Horizontal structure of 500mb height
fluctuations with long, intermediate and short time scales. J.Atmos. Sci. 41, 961979.
, M. L., Y.-H. Lee, J. M. Wallace and H. H. Hsu, 1984b: Time variation of
500mb height fluctuations with long, intermediate and short time scales as deduced
from lag correlation statistics. J. Atmos. Sci. 41, 981-991.
Brooks, C. E. P., and N. Carruthers, 1953: Handbook of Statistical Methods in
Meteorology. Her Majesty's Stationery Office. 412 pp.
Chang, F. C., and J. M. Wallace, 1987: Meteorological conditions during heat waves and
droughts in the United States Great Plains. Mon. Wea. Rev. 115, 1253-1269.
Chen, T.-C., C. -B. Chang and D. J. Perkey, 1985: Synoptic study of a medium-scale
oceanic cyclone during AMTEX '75. Mon. Wea. Rev. 113, p. 359-360.
Cohen, J., and D. Rind, 1991: The effect of snow cover on the climate. J. Climate 4,
689-706.
Dewey, K. F., 1977: Daily maximum and minimum temperature forecasts and the
influence of snow cover. Mon. Wea. Rev. 105, 1594-1597.
Dickson, R. R., 1967: The climatological relationship between temperatures of successive
months in the United States. J. Appl. Meteorol. 6, 31-38.
, 1980: Weather and circulation of June 1980 - Inception of a heat wave and
drought over the central and southern Great Plains. Mon. Wea. Rev. 108, 14691474.
Dole, R. M., 1982: Persistent Anomalies of the Extratropical Northern Hemisphere
Wintertime Circulation. Ph.D. Thesis, Massachusetts Institute of Technology.
168
Dole, R. M., and N.D. Gordon, 1983: Persistent anomalies of the extratropical Northern
Hemisphere wintertime circulation: Geographical distribution and regional
persistence characteristics. Mon. Wea. Rev. 111, 1567-1586.
Dole, R. M., 1983: Persistent anomalies of the extratropical Northern Hemisphere
wintertime circulation. Large Scale DynamicalProcesses in the Atmosphere, B. J.
Hoskins and R. P. Pearce, eds., Academic Press, New York, NY, pp. 95-110.
, 1986a: Persistent anomalies of the extratropical Northern Hemisphere wintertime
circulation: Structure. Mon. Wea. Rev. 114, 178-207.
, 1986b: The life cycles of persistent anomalies and blocking over the central North
Pacific. Advances in Geophysics, B. Saltzman, ed., Academic Press, New York,
NY pp. 31-69.
, 1989: Life cycles of persistent anomalies. Part I: Evolution of 500mb height
anomalies. Mon. Wea. Rev. 117, 177-211.
, and R. X. Black, 1990: Life cycles of persistent anomalies. Part II: The
development of persistent negative height anomalies over the North Pacific Ocean.
Mon. Wea. Rev. 118, 824-846.
Flocas, A. A., 1981: Persistence of cold and hot spells at Thessaloniki. Archivesfor
meteorology, Geophysics and Bioclimatology, Ser. A. pp 135-144.
Freund, J. E., and R. E. Walpole, 1980: Mathematical Statistics, third edition. PrenticeHall. p. 412.
Hao, W., and L. F. Bosart, 1987: A moisture budget analysis of the protracted heatwave
in the southern Plains during the summer of 1980. Wea. Forecasting 2, 269-288.
Heim, R. R. and K. F. Dewey, 1984: Circulation patterns and temperature fields
associated with extensive snow cover on the North American continent. Physical
Geography 4, 66-85.
Heim, R. R., 1988: About that drought. Weatherwise 41, 266-271.
, B. M. Lewis and I. Enger, 1959: Objective prediction of five-day mean
temperatures during winter. J.Meteorol. 16, 672-682.
Henning, D., 1989: Atlas of the Surface Heat Balance of the Continents. Gebrueder
Borntraeger. 402 pp.
Horel, J. D., 1981: A rotated principle component analysis of the interannual variability
of the Northern Hemisphere 500mb height field. Mon. Wea. Rev. 109, 20802092.
Hoskins, B. J., and D. Karoly, 1981: The steady linear response of a spherical
atmosphere to thermal and orographic forcing. J. Atmos. Sci. 38, 1179-1196.
Hughes, P., 1982: Weather, climate and the economy. Weatherwise 35, 60-63.
169
Janowiak, J. E., 1988: The global climate for March - May 1988: The end of the 198687 Pacific warm episode and the onset of widespread drought in the United States.
J. Climate 1, 1019-1040.
Karoly, D. J., R. A. Plumb and M. Ting, 1989: Examples of the horizontal propagation
of quasi-stationary waves. J. Atmos. Sci. 46, 2802-2811.
Klein, W. H., 1983: Objective specification of monthly mean surface temperature from
mean 700mb heights in winter. Mon. Wea. Rev. 111, 674-691.
Klein, W. H., 1985: Space and time variations in specifying monthly mean surface
temperature from the 700mb height field. Mon. Wea. Rev. 113, 277-290.
Kline, J. M. and W. H. Klein, 1986: Synoptic climatology of monthly mean surface
temperature in the United States during summer in relation to the surrounding
700mb height field. Mon. Wea. Rev. 114, 1231-1250.
Knox, J. L. and J. E. Hay, 1985: Blocking signatures in the Northern Hemisphere:
Frequency distribution and interpretation. J. Climatology 5, 1-16.
Lamb, H. H., 1954: Two-way relationship between the snow or ice limit and 1000500mb thickness in the overlying atmosphere. Quart.J. Roy. Meteorol. Soc. 81,
172-189.
Lau, N.-C., 1979: The observed structure of tropospheric stationary waves and the local
balances of vorticity and heat. J. Atmos. Sci. 36, 996-1016.
Leith, C. E., 1975: The design of a statistical-dynamical climate model and statistical
constraints on the predictability of climate. The PhysicalBasis of Climate and
Climate Modelling. GARP Publ. Ser. No. 16, Appendix 2.2.
Lettau, H., K. Lettau, and L.C.B. Molion, 1979: Amazonia's hydrologic cycle and the
role of atmospheric recycling in assessing deforestation effects. Mon. Wea. Rev.
107, 227-238.
Livezey, R. E., 1980: Weather and circulation of July 1980 - Climax of historic heat
wave and drought over the United States. Mon.Wea. Rev. 108, 1708-1716.
Ludlum, D. M., 1988: Weatherwatch. Weatherwise 41, pg. 246.
Madden, R. A. and D. J. Shea, 1978: Estimates of the natural variability of time-averaged
temperatures over the United States. Mon. Wea. Rev. 106, 1695-1703.
Madden, R. A., and J. Williams, 1978: The correlation between temperature and
precipitation in the United States and Europe. Mon. Wea. Rev. 106, 142-147.
Martin, D. E. and W. G. Leight, 1949: Objective temperature estimates from mean
circulation patterns. Mon. Wea. Rev. 77, 275-283.
170
McIntyre, D.P., 1950: On the air mass temperature distribution in the middle and high
troposphere in winter. J. Meteorol. 7, 101-107.
Mintz, Y., 1984: The sensitivity of numerically simmulated climates to land-surface
boundary conditions. The Global Climate, J. T. Houghton, ed., Cambridge
University Press, 79-106.
Mintz, Y. and J. Shukla, 1982: Influence of land-surface evapotranspiration on the
earth's climate. Science 215, 1498-1501.
Mo, K.C., J. R. Zimmerman, E. Kalnay and M. Kanamitsu, 1991: A GCM study of the
1988 United States drought. Mon. Wea. Rev. 119, 1512-1532.
Mo, K. C., J. Pfaendtner and E. Kalnay, 1987: A GCM study of the maintenance of the
June 1982 block in the Southern Hemisphere. J. Atmos. Sci. 44, 1123-1142.
Namias, J., 1952: The annual course of month-to-month persistence in climatic
anomalies. Bull. Amer. Meteorol. Soc. 33, 279-285.
, J., 1953: 30-day forecasting: A review of a ten-year experiment. Meteorological
Monographs, Vol. 2, No. 6, pp 1-81.
__
, 1960: Factors in the initiation, perpetuation and termination of drought. Extract
of Publ. No. 51, IASH Commision of Surface Waters, 81-94.
, 1962: Influences of abnormal heat sources and sinks on atmospheric behavior.
Proceedingsof the InternationalSymposium on Numerical Weather Prediction,
Tokyo. Meteorological Society of Japan, 615-627.
, 1978: Persistence of U.S. seasonal temperatures up to one year. Mon.Wea.
Rev. 106, 1557-1567.
__
, 1982: Anatomy of Great Plains protracted heat waves (especially the 1980 U.S.
summer drought). Mon. Wea. Rev. 110, 824-838.
__
, 1985: Some empirical evidence for the influence of snow cover on temperature
and precipitation. Mon. Wea. Rev. 113, 1542-1553.
__
, 1991: Spring and summer 1988 drought over the contiguous United States -
Causes and prediction. J. Climate 4, 54-65.
Oglesby, R. J., and D. J. Erickson, 1989: Soil moisture and the persistence of North
American drought. J. Climate 2, 1362-1380.
Palmer, T. N., and C. Brankovic, 1989: The 1988 US drought linked to anomlaous sea
surface temperature. Nature 338, 54-57.
Petterssen, S., 1956: Weather Analysis and Forecasting, Volume II: Weather and
Weather Systems. McGraw-Hill. 266 pp.
171
Plumb, R. A., 1985: On the three-dimensional propagation of stationary waves. J.
Atmos. Sci. 42, 217-229.
Rasmusson, E. M., 1968: Atmospheric water vapor transport and the water balance of
North America. Part II: Large-scale water balance investigations. Mon. Wea.
Rev. 96, 720-734.
Reed, C. D., 1933: Persistent weather abnormality. Mon. Wea. Rev. 61, 109-112.
Rex, D. F., 1950a: Blocking action in the middle troposphere and its effects on regional
climate. I. An aerological study of blocking. Tellus 2, 275-301.
Rex, D. F., 1950b: Blocking action in the middle troposphere and its effect on regional
climate. II. The climatology of blocking action. Tellus 2, 275-301.
Rind, D., 1982: The influence of ground moisture conditions in North America on
summer climate as modelled in the GISS GCM. Mon. Wea. Rev. 110, 14871494.
Sardeshmukh, P. D. and B. J. Hoskins, 1985: Vorticity balances in the tropics during the
1982-83 El Nino-Southern Oscillation event. Quart.J.Roy. Meteorol. Soc. 111,
261-278.
Sawyer, J. S., 1970: Observational characteristics of atmospheric fluctuations with a
time scale of a month. Quart.J. Roy. Meteorol. Soc. 96, 610-625.
Sellers, W. D., 1965: Physical Climatology. The University of Chicago Press. 272 pp.
Shukla, J., 1984: Predictability of time averages: Part II: The influence of boundary
forcings. Problems and Prospectsin Long and Medium Range Weather
Forecasting. D. M. Burridge and E. Kallen, eds. Springer-Verlag, pp 155206.
Shukla, J. and K. C. Mo, 1983: Seasonal and geographical variation of blocking. Mon.
Wea. Rev. 111, 388-402.
Straus, D. M., and M. Halem, 1981: A stochastic-dynamical approach to the study of the
natural variability of the climate. Mon. Wea. Rev. 109, 407-421.
Trenberth, K. E., G. W. Branstator, and P. A. Arkin, 1988: Origins of the 1988 North
American drought. Science 242, 1640-1645.
Trenberth, K. E., and J. G. Olson, 1988a: Evaluation of NMC global analyses. NCAR
technical note NCAR/TN-299+STR. 82pp.
Trenberth, K. E., and J. G. Olson, 1988b: ECMWF Global Analyses 1979-1986:
Circulation statistics and data evaluation. NCAR technical note NCAR/rN300+STR. 94 pp.
Van Den Dool, H. M., 1984: Long-lived air temperature anomalies in midlatitudes forced
by the surface. Mon. Wea. Rev. 112, 555-562.
172
Van Den Dool, H. M., W. H. Klein and J. E. Walsh, 1986: The geographical distribution
and seasonality of persistence in monthly mean air temperatures over the United
States. Mon. Wea. Rev. 114, 546-560.
Wagner, A. J., 1973: The influence of average snow depth on monthly mean temperature
anomaly. Mon. Wea. Rev. 101, 624-626.
, 1980: Weather and circulation of August 1980 - Eastward spread of the heat
wave and drought. Mon. Wea. Rev. 108, 1924-1932.
, 1981: The weather and circulation of 1980. Weatherwise 34, 5-12.
Wallace, J. M., 1987: Low-Frequency Dynamics - Observations. Dynamics of Low
FrequencyPhenomena in the Atmosphere. Vol. I: Observations. NCAR,
Boulder, CO, 1-75.
Wallace, J. M. and D. S. Gutzler, 1981: Teleconnections in the geopotential height field
during Northern Hemisphere winter. Mon. Wea. Rev. 109, 784-812.
Walsh, J. E., W. H. Jasperson and B. Ross, 1985: Influences of snow cover and soil
moisture on monthly air temperature. Mon. Wea. Rev. 113, 756-768.
Walsh, J. E., and B. Ross, 1988: Sensitivity of 30-day dynamical forecasts to continental
snow cover. J. Climate 1, 739-754.
Wolfson, N., R. Atlas, amd Y. C. Sud, 1987: Numerical experiments related to the
summer 1980 U.S. heat wave. Mon. Wea. Rev. 115, 1345-1357.
3246 031