MIDLATITUDE MESOSCALE CONVECTIVE COMPLEX
PRECIPITATION CYCLES AND STRUCTURES
by
ROBERT PATRICK CALLAHAN
B.S., Texas A&M University
(1979)
Submitted to the Department of
Meteorology and Physical Oceanography
in Partial Fulfillment of the
Requirements of the
Degree of
MASTER OF SCIENCE
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 1983
The author hereby grants to M.I.T. permission to reproduce and to
distribute copies of this thesis document in whole or in part.
Signature of Author:
I
___
Department of Meteorology and Physical Oceanography, 18 May 1983
~J
Certified by:
Frederick Sanders, Thesis Supervisor
Accepted by:
Ronald George Prinn, Cl
Departmental Committee
Isical Oceanography
MIDLATITUDE MESOSCALE CONVECTIVE COMPLEX
PRECIPITATION CYCLES AND STRUCTURE
by
ROBERT PATRICK CALLAHAN
Submitted to the Department of
Meteorology and Physical Oceanography
in Partial Fulfillment of the
Requirements of the
Degree of
Master of Science
ABSTRACT
The precipitation cycles and structures of sixteen Mesoscale Convective
Complexes (MCCs) from the warm seasons of 1978 and 1979, and August 1982
were studied.
Manually digitized radar data from the National Weather
Service 10 cm radar network was primarily used.
A large subclass of the
MCCs examined were found to have consistently observable precipitation
cycles and structures.
In the early phase, the precipitation of an MCC is
nearly identical to the structure of a tropical squall line, while in the
late phase, the active regions have characteristics of a weak midlatitude
squall line.
Meso-circulations, particularly a mesolow which forms in the
lower troposphere, appear responsible for this change in precipitation
structure.
The usefulness of classifying MCCs as unique organized
mesoscale convection was discussed.
Thesis Supervisor: Dr. Frederick Sanders, Professor of Meteorology
111i
CONTENTS
1.
Introduction............................................ ............ 1
2.
Radar data ............................................................2
3.
6
Selection of MCCs for study................................. .........
4.
Radar time series..................................................... 8
5.
Synoptic conditions for MCC of 19/20 May 1979(B).....................29
6.
Horizontal precipitation structures.................................36
7.
Evidence of the significance of mesolow aloft........................55
8.
MCC and tropical cloud cluster similarities..........................80
9.
Discussion........................................................... 85
10. Conclusions.........................................................103
Appendix....................................... ........................ 105
Acknowledgements........................................................106
References..............................................................107
FIGURES AND TABLES
Figures
........
10
........
12
1.
Total area of rain, MCC of 19/20 May 1979(B).......
2.
Total rain rate, MCC of 19/20 May 1979(B).......... .
3.
........... 14
Total intensity, MCC of 19/20 May 1979(B)..........
4.
Area of convective and stratiform rain, MCC
of 19/20 May 1979(B)............................... .
5.
.......
16
Convective and stratiform rain rates, MCC of
19/20 May 1979(B)..................................
........... 19
6.
Stratiform rain intensity, MCC of 19/20 May 1979(B) ........... 21
7.
Convective rain intensity, MCC of 19/20 May 1979(B) ........... 23
8.
Modified presentation of convective and stratiform
rain area, MCC of 19/20 May 1979(B)................ ........... 26
9.
Modified presentation of convective and stratiform
N
rainfall rates, MCC of 19/20 May 1979I h(B.
..28
.....
..31
10. Surface map for 20 May 1979, 0000 GMT
S..33
11. Upper air chart, 500 mb, 20 May 1979, 0000 GMT
0000
12. Stability analysis , lifted index, 20 May 1979, 0000 GMT
..35
13. Radar map, 20 May 1979, 0135 GMT.....
..38
... •
14. Radar Map, 20 May 1979, 0535 GMT.....
S........
......
...
..41
15. Infrared satellite picture, 20 May 1979, 0530 GMT
16. Radar map, 20 May 1979,0835 GMT......
17. Radar map, 20 May 1979, 0935 GMT.....
.. 0
........
....
...
18. Radar map, 20 May 1979, 1235 GMT.....
..43
..45
.47
..
..
. .49
.........
19. Radar map, 20 May 1979, 1335 GMT.....
20.
Radar map,
20 May 1979,
1535 GMT ..............................
..51
..
54
21.
Relative wind flow at 700 mb from Maddox'
composite model ..... 57
22. Surface map, 20 May 1979, 1200 GMT......... ................... 59
23.
Infrared satellite picture, 27 August 1982, 0600 GMT.. ........ 61
24. Radar map, 27 August 1982, 0635 GMT........
..
25. Surface map, 27 August 1982, 0600 GMT......
......
,..
........ 63
........ 65
26. Infrared satellite picture, 27 August 1982, 0900 GMT.. ........ 68
27. Radar map, 27 August 1982, 0935 GMT........
......
28. Surface map, 27 August 1982, 0900 GMT......
........ 70
........ 72
29. Infrared satellite picture, 27 August 1982, 1201 GMT.. ........ 74
30. Radar map, 27 August 1982, 1235 GMT........
........ 76
31. Surface map, 27 August 1982, 1200 GMT...... WMONEX
........ 78
32. Area of convective and stratiform rain for WMONEX clou d
cluster, 09-10 December 1978 ...............
........ 82
33. Total convective and stratiform rain rates for WMONEX
cloud cluster, 09-10 December 1978.........
........ 84
34. Convective rain from Fritsch and Chappell's mesoscale
numerical model ............................................... 87
35. Tropical squall line model from Leary and Houze ............... 89
36. Model of mature mesoscale convective area from Pedgley........ 92
37. Range height indicator view of a midlatitude mesoscale
anvil cloud by Leary .......................................... 94
38. Surface and precipitation analysis of mesoscale convection
by Fujita and Brown, 5 June 1953, 0400 GMT .................... 96
39. Surface and precipitation analysis of mesoscale convection
by Fujita and Brown, 5 June 1953, 0700 GMT.................... 98
40. Movement of meso-pressure areas in mesosystem 416 from Fujita
and Brown
........... ....................................... 100
vi
Tables
1.
Definition of intensity levels.................................3
2.
List of MCCs and types of data available in this study......... 7
1.
Introduction
By documenting convective and stratiform precipitation cycles and
structures then integrating this knowledge into the structures and models
elucidated by other researchers, a better understanding of the Mesoscale
Convective Complex (MCC) is obtained.
National Weather Service network
radar data was used for the bulk of this study.
This operational data was
chosen because no research radar data covering the entire lifetime of the
MCC is known to exist.
Although the author could not control the quality
of each radar observation, the data was taken using mostly standardized
procedures, then put into numerical form.
Standardized procedures and
numerical data are conducive to scientific research.
2.
Radar data
Radar observation logs for each station of the National Weather
Service network of WSR-57 10 cm radars were obtained.
Observations on the
logs were taken hourly, 35 minutes past each hour, by the observer at each
station, in a digital form.
The digital information was obtained by laying
a grid over the plan position indicator (PPI) and noting the maximum
observed intensity in each box containing echoes of moderate or greater
intensity.
If light intensity was the greatest intensity observed in a
grid box, it was reported only if more that 20% of the box was covered.
Intensities were reported with code numbers 1 to 6 and correspond to the
dBZ levels in Table 1.
Two other numbers, 8 and 9, were sometimes
reported; they signify echoes of unknown intensities observed beyond the
maximum intensity measuring range, 232 km, of the radar.
In this study, 8
and 9 were always assigned the value 1 because they occurred on the
periphery of the MCC precipitation.
Overlapping radar coverage was
sufficient to rule out the possibility of higher intensities going
unobserved in most cases.
Hourly digital data from all stations was plotted on a subgrid of the
Limited Fine Mesh (LFM) grid.
Each subgrid has one-fourth the mesh length
2
of the LFM, a side of approximately 40 km, and an area of about 1600 km .
Whenever data from more than one station was entered at the same box, the
data of highest value was plotted.
Determining which data belonged to an
MCC was generally easy because the data would be closely spaced and under
the cloud shield observed in infrared (IR) satellite pictures.
In the
cases where closeby data was not considered MCC related, analysis including
these generally small amounts of data did not make significant changes in
Table 1. Definition of intensity levels.
Dept. of Commerce (1978).
Intervals of dBZ from U.S.
Rainfall rates derived from Z-R relationship
Z=200R1. 6 as modified in text
Intensity
Level
1
Echo
dBZ
Intensity
Rainfall Rate
(mm/h)
Extreme
>57
Intense
50-56
Very Strong
45-49
Strong
41-44
Moderate
30-40
Weak
<29
the results.
Once the MCC data was
isolated, each datum was designated as either
stratiform or convective rain.
By observing that most of the uniform,
stratiform-like areas were either
l's or 2's,
the clumpy, convection-like
areas, 3 and above, and with the knowledge that the highest intensities are
convective, a simple procedure was devised where boxes containing 1 or 2
were considered stratiform rain and 3 to 6 convective rain.
approach was used by Houze (1977)
A similar
in his study of a tropical squall line
system.
The weakness in using this digitized radar data is twofold.
the grid size is so large,
First,
it will tend to overestimate the area of rain.
This is especially true in the case of convection where a single convective
cell of, say 50 km 2 could cause a grid box of 1600 km 2 to be considered
convective.
Second, the intensity value reported for each grid -box is the
maximum observed intensity, and it is not known exactly what the
relationship is between the maximum value and the mean, especially with
respect to convection, except that this maximum observed intensity is
overestimating the mean intensity for the grid box.
Problems of overestimation were overcome in the following ways:
the
often used Z-R relationship,
Z=200R 1 .6 ,
(1)
where radar reflectivity factor Z(mm 6 /m 3 ) and rainfall rate R(mm/h), was
used, which gives realistic values for stratiform rain, but slightly
underestimates convective rain and takes into account that each grid value
is used as an estimate over an hour.
The original intensity observation
could have fallen anywhere within the range of dBZ for a particular
intensity level, but, again, to make a closer estimate to the mean
intensity, the rainfall rate for the lowest value of dBZ in an intensity
level was used.
Additionally, the rainfall rate for level 1 was
given a nominal value of 1 mm/h, and level 6 was limited to 75 mm/h because
of the likely presence of hail, which can add to the reflectivity without
adding much to the rainfall rate.
To check these correction factors, the hourly total rain rates (kg/h)
for the MCC of 19/20 June 1978 were painstakingly computed from
Environmental Data and Information Service Hourly Precipitation Data and
compared with the radar-derived rates.
Hourly radar-derived stratiform
rainfall rates can be considered a reasonable estimate for many reasons,
including : (1) the ratio of the area of a grid box to the area of the rain
in an MCC is approximately 1/100, (2) in widespread rain the difference
between the maximum observed value of reflectivity in a grid box and the
mean is small, and (3) on the edge of the rain the observing rules
eliminate light rain areas of less that 20% coverage.
Because these
radar-derived stratiform rain rates are a good estimate, overestimates of
total rain rates come from inaccuracies in the convective estimate, and a
final correction factor of .36 was computed.
This correction factor, when
multiplied times the convective rain rates, causes the sum of the
stratiform and convective rain rates to equal the total rain rates
observed on the ground.
Due to missing data in the early part of the June
1978 MCC, the final correction factor was derived only from the last eight
hours of precipitation data.
Since the correction factor was derived from
only a part of a single storm, and not the May 1979 storm presented in this
paper, plots of convective and stratiform rain rates should be viewed
carefully when their relative magnitudes are compared.
3.
Selection of MCCs for study
The MCCs for the warm seasons of
1978 and 1979
identified by Maddox
(1980,1981) and MCCs which occurred in August and September 1982 form the
basis of this
study.
This group was reduced to sixteen cases because of
data availability, and an attempt to sample storms of various size,
duration and months throughout
of the ten MCCs Maddox (1981)
the warm season
(Table 2).
It includes four
used in making his composite storm.
Although
the sixteen cases were examined to varying degrees, only the cases of 19/20
May 1979 and 26/27 August
1982 will be included in this
paper,
These two
cases do not represent all the possible MCC precipitation configurations,
but both cases are similar to those MCCs used to create Maddox' composite
storm.
The May 1979(B) case best exhibits
the most fundamental
characteristics of the sixteen storms, and the 26/27 August
1982 case
clearly demonstrates an important phase in the life of the MCC.
7
Table 2.
List of MCCs and Types of Data Available in
the study
DATA
Date
Satellite
6/7 MAY 78
19/20 MAY 79(A)
19/20 MAY 79(B)
2/3 JUN 78
3/4 JUN 78
19/20 JUN 78
21/22 JUN 79
22/23 JUN 79(A)
22/23 JUN 79(B)
1/2 JUL 78
6 JUL 78
13 JUL 78
19/20 JUL 78
24/25 AUG 82
26/27 AUG 82
31 AUG/1 SEP 82
X
Facsimile Maps
Digital Radar
4.
Radar time series
Figs. 1-7 display the results of the analysis of the hourly radar maps
for the MCC of 19/20 May 1979(B).
In Fig. 1, the total area (convective
plus stratiform) covered by rain is plotted against time.
The times when
the storm met Maddox' (1980) criteria are plotted at the top of the graph.
Note the roughly four stages in the plotted data:
1800-0000 GMT, little
growth, 0000-1000 GMT, strong growth; and, after 1000GMT, slow decay.
Other storms exhibited similar behavior, with the differences occurring in
the slopes of the strong growth and decay, and in the height of the peaks.
Fig. 2 shows total area-integrated rainfall rate or rain rate (kg/h)
over the area and life of the MCC.
The distinguishing features are the
rapid growth, peak (occurring somewhat before maximum cloud extent), and
decay.
Examination of other storms reveals slight differences in the
placement of the peak before maximum cloud extent and slightly different
slopes for growth and decay.
The final graph in this subgroup, Fig. 3, is a plot of the
area-averaged rainfall rate or intensity, and is computed by averaging all
of the observed intensities. Most MCCs do not have this overall symmetrical
form; there is usually a rapid build up to a peak then a slower decay.
A trend is evident in this data which will become more apparent in the
next set of graphs, i.e., a peak in the intensity, followed by a peak in
the rain rate, and finally a peak in the area affected.
Processes affecting the storm's life become more evident once the data
is divided into convective and stratiform parts.
Fig. 4 shows the area
covered by convective rain along with a plot of stratiform rain area.
The
first significant period occurs between 0100-0800 GMT which shows a steady
Figure 1.
Total area of rain, MCC of 19/20 May 1979(B).
I, ME, and T mark
Maddox' times of initiation, maximum extent, and termination,
respectively.
91
I
t
I
(Iv
80
) 3 ll1
90
Ii
tV0
I
z0
00
zz 0
I
lI
I
81
I
0
Og
oo
009 m
00010
0
0081
0081i
S1
I
]IN
I
I
I
Figure 2.
Total rain rate, MCC of 19/20 May 1979(B).
9
--
~L
ME
8000
Jd
a0
6000
--o
w
4000
Iz-
Z
2000Ok
II
18
I
20
22
I
00
I
I
02
04
06
-TIME (GMT)
I
I
I
I
08
10
12
14
16
Figure 3.
Total intensityl, MCC of 19/20 May 1979(B).
1
lntensity is computed by averaging observed intensities, therefore plotted
intensity does not equal plotted rain rate divided by area unless
convective correction factor,.36, is taken into account.
I
T
I
I
ME
I
2O
E
E
I15-
z
0,,,
18
20
,I
22
I
I
II
O0
02
04
TIME (GMT)
06
I
08
I
10
12I
14
14
16
16
Figure 4.
Area of convective and stratiform rain, MCC of 19/20 May
1979(B).
Convective and stratiform rain areas are denoted by
dashed and solid lines, respectively.
I
SI
I
I
ME
T
1600
E 12000
0
800-
w 400
-
%.
O
%
-
0
-
I
18
20
"
.
-
m
I
I
22
00
-
I
i
I
I
04 06
02
TIME (GMT)
08
10
I
I
12
14
16
increase in both types of rain.
Small upward surges of the convective
total, along with a lessening of the increase in the stratiform rain,
indicates the growth of convection in stratiform areas.
At about 0800 GMT
a significant development occurs; the area of convection starts decreasing
and the stratiform rain continues to increase.
This change occurs before
the cloud top reaches maximum extent and from a convective viewpoint might
be considered the beginning of storm decay.
With respect to processes
creating stratiform rain, they do not peak until after the MCC reaches
maximum cloud extent.
Fig. 5 gives an hour by hour comparison of the relative mass of rain
falling in the convective and stratiform areas.
roughly interpreted as latent heat release.
The rain rate data can be
Note the substantial
domination of the convection in the early part of the MCC and the dominance
of stratiform rain later.
Convective then stratiform dominance is
reflected in the numerical model of Keitzberg and Perkey (1977), and
represents a system first dominated by convective updrafts then later by
stable updrafts.
Even though the stratifrom rain grows while the
convection decays, some amount of convection is probably needed to keep the
stratiform rain growing.
This is implied by the rapid drop in the
convective rain rate shown in Fig. 5 at 1200 GMT, and the peaking of the
stratiform rain area growth shown in Fig 4.
Some results in section 6 also
emphasize this fact, that an MCC is a convectively driven system even
though other mesoscale circulations become significant.
Changes in the intensity of the stratiform and convective rain is
illustrated in Figs. 6 and 7.
The stratiform rain undergoes a general rise
in intensity, while the convective intensity shows a sharp peak then a
gradual decline.
For the average MCC this peak occurs in the late
Figure 5.
Convective and stratiform rain rates, MCC of 19/20 May 1979(B).
Convective and stratiform rain rates are denoted by dashed and
solid lines, respectively.
I
I
600O
I
ME
s/
I
T
%
,
OD
\
/
w~o
-4000
2000
0
I
18
20
22
00
02
04 06
TIME (GMT)
I
08
I0
12
14
16
Figure 6.
Stratiform rain intensity, MCC of 19/20 May 1979(B).
91
I
I
I
01
31
V1
80
I
(IWO)
90
tV0
3011
O0
II
?
I
I
z
I
I
0
z
--4
m
Iz
-C
33
II
Figure 7.
Convective rain intensity, MCC of 19/20 May 1979(B).
I
I
I
I
ME
T
60
50
40
30
20
I0
OII
18
BI
II
20
I
22
00
aI
04 06
02
TIME (GMT)
A
08
I
10
I
I
12
14
16
afternoon.
After examining the radar data for a number of MCCs, it became
apparent that a slightly different presentation of the convective and
stratiform amounts would highlight some of the more important events.
Data
from the May 1979(B) case was again labeled either stratiform or
convective, with only data of intensity level 2 plotted for stratiform and
levels 4 to 6 for convective rain.
These criteria were chosen because: a)
intensity level 2 usually appeared on the radar maps in large groups and
was the clearest indicator of widespread rain, and b) levels 4 to 6 would
give a better indication of the vigor of the convection because they are
the samples of more likely convective areas.
Examination of Figs. 8 and 9 suggest that the beginning of the
stratiform rain started with the original intense convective impulse with
fairly steady growth until the convection rapidly dropped off.
The steep
drops in the convective rain rates during the period 0800-0900 GMT and
1100 GMT with the resulting large increase in the stratiform area during
the same periods indicates that the rapid increase in stratiform rain area
as the convection drops off is a result of large amounts of convection
decaying into stratiform rain at that time.
The observations above give only a partial picture of the behavior of
precipitation in an MCC.
A fuller understanding can be obtained by also
observing the horizontal structure, and this will be done next.
Figure 8.
Modified presentation of convective and stratiform rain areas,
MCC of 19/20 May 1979(B).
Convective and stratiform rain areas
are denoted by dashed and solid lines, respectively.
I
I
I
I
ME
T
1000
800
E
N
0
600
400
w
"O
13:
'he
200
... ,..
m
OF
I
18
m
1
a
20
a
1
22
a
I
00
a
al
02TIME
I.
II.
04
06
(GMT)
I
JI
08
JI
,10
I
I
12
~
I-
14
Figure 9.
Modified presentation of convective and stratiform rain rates,
MCC of 19/20 May 1979(B).
Convective and stratiform rain rates
are denoted by dashed and solid lines, respectively.
I
I
5000
-~
I
c
I
T
ME
I
1
4000
I
-
I
3000
LLI
-I
I-
/
< 2000
z
.,,.<
1000
-
.-
-
0
I
18
I
II
20
22
00
I
III
04
06
02
TIME (GMT )
08
!
II
1.0
12
14
16
5.
Synoptic conditions for MCC of 19/20 May 1979(B)
At 0000 GMT, 20 May 1979 the synoptic conditions closely match the MCC
genesis conditions as represented by Maddox' composite storm.
The genesis
region for this MCC is approximated by the circle, centered in SW Kansas,
in Fig. 10.
Matching surface features included a low to the west and a
surface front within the genesis region.
displayed in Fig. 11.
Upper air similarities are
The 500 mb chart shows a trough approaching from the
west along with generally SW winds which closely approximate the mean
tropospheric wind.
A final likeness to Maddox' model is seen in the
stability analysis of Fig. 12.
A maximum in the analysis of lifted index
is found within the genesis region with generally unstable air to the south
and more stable air north.
Detailed synoptic or mesoscale analysis of the MCCs in this study was
not attempted because of the inferior quality of the available surface and
upper air data and the emphasis on the radar derived cycles and
structures.
By showing the similarities between the May 1979 MCC and
Maddox' composite model it is assumed other aspects of the composite model
can be used and its interpretation tested for compatability with the
results of this study.
Other observations and models of similar mesoscale
systems will be used when they appear to supply explanations of the radar
derived behavior which can not be found completely in the data available
for this study.
Figure 10.
Surface map for 20 May 1979, 0000 GMT. Solid lines are sea
level isobars. Circle approximates area of Maddox' genesis
region.
400
350
300
25
Figure 11.
Upper air chart,
500 mb, 20 May 1979,
0000 GMT.
Solid lines are
heights in 60 dam intervals and dashed lines are isotherms.
ita
loo
Figure 12.
Stability analysis,
lifted index,
20 May 1979,
0000 GMT.
95
90*
6.
Horizontal precipitation structures
The horizontal structures described in this section were generally
observed in most of the analyzed cases.
Radar data for each MCC was taken
by a number of stations; approximately ten stations were used during the
life of each storm.
Since observation data was collected in a PPI mode,
and not reconstructed into constant altitude displays, it was not possible
to determine at what altitude the data for a particular grid box was
taken.
Concern for this height variability is eased by the knowledge that
standardized observing practices include the use of low elevation angles.
With this restriction in mind, it is concluded that most data was taken
below the freezing level in rain.
Fig. 13 is the combined radar map for 20 May 1979, 0135 GMT.
The
skewing of the map towards the northeast is caused by the application of
the LFM grid to the curved globe.
Numbers 1 to 6 represent intensity
levels and rainfall rates as described in section 2.
or M indicate radar station location.
Either the symbol 7
The symbol 7 indicates available
When data is
station reports; the symbol M indicates missing reports.
available at a station location grid, the data is plotted.
Dots are grid
box locations while stars indicate both grid boxes and state boundaries.
Since there is another MCC and other rain areas on this map, the data
included in the storm of interest is heavily outlined.
All data was
available along the track of the storm.
The time 0135 GMT corresponds with the peak of convective intensity
(See Fig 7).
storm.
Notice the clump of convection in the SW corner of the
This is the side from which the mean tropospheric flow is coming.
Another area of convection is downwind of the first, and located near
Figure 13.
Radar map, 20 May 1979, 0135 GMT. Numbers 1 to 6 indicate
intensity as described in Table 1, seven means data was
available from the station and plotted,
while M indicates
missing data. Plotted data overides 7s. Data considered part of
the MCC of interest is enclosed by a heavy outline. The area
within the heavy outline roughly corresponds to the area of
cloud top with temperatures less than -32
°
C for this MCC.
Areas of intensity level 2 and above are lightly outlined, and
areas of 3 and above are outlined and shaded.
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t********
... *...'
..
9**...... ..,..4..
....
'...4
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t*
4+
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4 ..
4
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39
the right edge of the cirrus cloud which is streaming NE from the
convection to the southwest.
This dual-convection structure was
observed in other MCCs and tends to persist. As
stage continues,
the MCC begins
the convective growing
to take on characteristics of a tropical
squall line as observed by Houze (1977),
trailing cloud shield.
often
i.e.,
a line of convection with a
In the case of the MCC, the main convection area is
trying to propagate into the mean flow
(Figs. 14 and 15).
More
similarities between the tropical and mid-latitude mesoscale systems will
be presented in later sections.
Shortly
after
the maximum convective rain rate was reached
5) drastic structural changes began.
convective freeregion in the
convection,
happened.
First, as
16,
Fig.
the
center of the MCC appeared to fill with
but because of the
large grid size,
Other cases, however, showed
Fig.17) a line of convection
shown in Fig.
(see
it
is
not
clear
this same behavior.
(marked by the dashed line)
how this
Next (see
formed on the SE
side of the storm and the convection on the W side was significantly
reduced.
with
The convective
the mean wind
line
on the SE side of the MCC was roughly
aligned
and is probably the same as the linear features
observed by Maddox (1981).
By 1235 GMT,
as
seen in
the west side of the MCC has
(Fig.
4)
Fig.
18,
ceased.
it
appears
that
all
convection
along
The stratiform rain reached its
and the line on the south side
of the storm grew southward
peak
and
moved east.
On the next map, (Fig. 19)
1335 GMT, it becomes apparent how important
the convection on the west side of the storm was in the maintenance of the
stratiform rain.
With the convection gone, the
pushed to the NE by the wind, where it decayed.
stratiform rain was quickly
Figure 14.
Radar map, 20 May 1979, 0535 GMT. Details similar to fig. 13.
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Figure. 15.
Infrared satellite picture, 20 May 1979, 0530 GMT. Enhanced
areas indicate cloud top temperatures less than -3 2 ° C. States
overlay is slightly offset.
43
052O2MY79 12E-21B 00872 13231 KB8
Figure 16.
Radar map, 20 May 1979, 0835 GMT. Details similar to fig. 13.
45
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Radar map,
20 May 1979,
0935 GMT.
Details similar to fig.
13.
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48
Figure 18.
Radar map, 20 May
1979, 1235 GMT. Details similar to fig.
13.
*~~~~~~~~~~~~~
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Figure 19.
Radar map, 20 May 1979, 1335 GMT. Details similar to fig. 13.
lip
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character of a weak midlatitude squall line.
The line is roughly aligned
with the mean wind, slightly bulged, and moving approximately perpendicular
to its long axis (Fig. 20).
This final stage was frequently observed, with
the convection in each case developing to different degrees of vigor.
Ik
Figure 20.
Radar map, 20 May 1979,
1535 GMT. Details similar to fig. 13.
* **
4 4
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4 4 4+
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+ 1 1441 . . .
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7.
Evidence of the significance of mesolow aloft
One simple model of an MCC would consist of three parts-- early phase
(tropical squall line-like), transition phase, and late phase (midlatitude
squall line-like).
The question arises -- why does an MCC change from one
type of system to another?
Part of the answer can be found in the
composite model of an MCC constructed by Maddox (1981).
In the composite model of a mature MCC, Maddox diagnosed a strong
relative mesolow which forms above the rain-induced surface mesohigh (Fig.
21).
Evidence for this low can not usually be seen in the upper air
patterns, because it is too small; nor in the surface fields, because these
fields are dominated by the mesohighs.
Examination of the surface pressure
and wind for the case presented in this paper, however, does suggest the
presence of a surface low in approximately a position that could help
explain the late phase line, and supplies indirect evidence for the mesolow
aloft (see fig. 22).
The surface low's position corresponds to the
northeast edge of the late phase convective line. A better example of the
relationship between the surface mesolow and the late phase convective line
was found in the MCC of 26/27 August 1982.
Figs. 23, 24, and 25 show the
satellite picture, radar map, and surface chart, respectively, for times at
or near 0600 GMT.
The large cloud area resulted from the nearly simultaneous growth of
three MCCs, with the one on the right being of most interest because it
became very large and clearly exhibited the formation of a mesolow.
The
radar map shows a transitional signature for the large area of convection
on the rightmost MCC.
The surface map indicates a low pressure trough
lying across southern Indiana and Illinois.
Figure 21.
Relative wind flow at 700 mb from Maddox' (1981) composite
model.
700 mb relative flow at the time of the MCC. Streamlines are
shown with relative winds (full barb = 10 kt or about 5 m/s) plotted at
every other grid point. Contours of omega in microbar/s (light solid and
dashed lines) are shown for the total flow.
Figure 22.
Surface map, 20 May 1979, 1200 GMT from Maddox (1981) except
for low which was added.
Surface analysis for 1200 GMT 20 May 1979. Surface features
are indicated, along with 2 mb isobars. Winds are in kt (full
barb = 10 kt or A5 m s-1) and squall symbols with frontal
barbs indicate positions and movements of cold air outflow
boundaries. Six hour precipitation amounts, in inches, are
also shown.
Figure 23.
Infrared satellite picture, 27 August 1982, 0600 GMT. Enhanced
areas indicate cloud top temperatures less than -32 ° C.
'~0~'
V-4 ,
%
IN
cc'
0i
cI
'W
II
4'
?~'
Figure 24.
Radar map, 27 August 1982, 0635 GMT. Details similar to fig.
13.
44**444
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Figure 25.
Surface map, 27 August 1982, 0600 GMT. Solid lines are sea
level isobars.
65
85 °
900
950
L
H
1000
1012
123
119
40*
r 125
132
117
24
126
12 3
104*
IO
10 7
104
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o
121
1012
1 15
113
123
112
13
04
129
1 1
-15635
105
350-12
117
139
170
H
95*
900
850
By 0900 GMT the MCC has passed maximum cloud extent and is
significant changes of structure.
showing
A bulge is appearing in the cloud shield
on the southern side of the MCC (Fig. 26) which corresponds with the
convective line on the radar map (Fig. 27),
and a large mesolow has
developed on the border between Indiana and Kentucky (Fig. 28).
The
position of the low appears to be on the north edge of the convective line,
as it was in
the previous case.
A final glimpse at the behavior of this MCC is given for 1200 GMT in
Figs. 29, 30, and 31.
At this time the convective line was weakened
substantially, but a new center of organized cloud and rain area has
appeared which grew through the eastern third of the MCC cloud shield.
This rain is
located ahead of the low in
an area of expected uplift.
tracking movement of the center of the new cloud mass it
By
was revealed that
the associated
low was moving east at a speed of 45 kt during the period
1000-1400 GMT,
a direction and speed which was observed in
the atmospheric layer from 500-300 mb.
the windfield
of
This same behavior, the appearance
of a fast moving cloud mass and rain area on the lee side of a decaying
MCC, was also observed in an MCC two days before.
The MCC occurring on 25
August 1982 was in the same geographic location with similar environmental
flow.
An explanation of the latter August 1982 MCC behavior can only be
speculated at this time.
Area measurements were made of the cloud top
colder than approximately -65°C.
At 0800 GMT, the area measured 100,000
kin2 , and by 1000 GMT, it had shrunk to nearly zero.
The rapid warming of
the cloud top is probably indicative of a rapid collapse of the cloud air.
Widespread warming of the cloud top continued after 1000 GMT, and this
warming period corresponds to the appearance of the separate cloud mass on
Figure 26.
Infrared satellite picture, 27 August 1982, 0900 GMT. Enhanced
areas represent cloud top temperatures less than -32
°
C.
X *C I
, ks
ilk
j, 1:
it
1&4
tv
, # '' I
IIr
:7
it
tiw
lilt
41
Figure 27.
Radar map, 27 August 1982, 0935 GMT. Details similar to fig.
13.
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25.
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350
90
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Figure 29.
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areas indicate cloud top temperatures less than -32
°
C.
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iLl-u2 17 F -2MB
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25.
450
40 °
350
90"
850
the lee side of the MCC,
and the beginning of its fast movement.
appears that these events are connected;
It
possibly the collapse of air is
creating a gravity wave or a perturbation that is transported by the wind.
8.
MCC and tropical cloud cluster similarities
Similarities between MCCs and tropical cloud systems have been
observed in this study.
To make this connection stronger without trying to
obscure what appears to be significant differences, Figs. 32 and 33 are
presented from Churchill (1982).
Data for these figures was derived from
the observation of a stationary cloud cluster off the coast of Borneo
during the Winter Monsoon Experiment (WMONEX), and favorably agree with the
studies of tropical squall lines with large cloud shields.
Observations
were taken with a single 5.3cm radar, and radar maps of echo intensity were
2
generated at constant altitudes with 16 km resolution.
Precipitation was
partitioned into convective and stratiform components by an objective
technique which compared the reflectivity observed in a grid box with the
surrounding data.
Generally, a grid box, and a few adjacent boxes, were
considered convective if there existed a gradient of reflectivity between a
box and nearby boxes which was above a certain threshold.
The data for
Churchill's study was available to the author of this paper, so a
comparison was made between Churchill's results and data taken by the
techniques of this study.
Qualitatively similar results were obtained,
with the differences being a shift of the data to the left by an hour, and
a proportionate overestimation of area and rain rates.
Both of these
changes were caused by the large grid size, which is also more sensitive to
broad horizontal trends.
The fundamental results, however, and the
similarities between these figures and Figs. 5 and 6 are not changed.
MCCs
and tropical cloud systems do exhibit similar radar time series behavior
with the implication that similar processes are occurring.
Figure 32.
Area of convective and stratiform rain for WMONEX cloud
cluster, 09-10 December 1978 from Churchill
(1982).
22
20
18
16
tJ
-E
0"'
14
12
I0
8
6
4
2
1200
1400
1600
1800
2000 2200 0000 0200
TIME (GMT)
09-10 DECEMBER 1978
Area covered by stratiform precipitation (dashed curve) and
convective precipitation (solid curve) at (a) 3 km, (b) 7 kmn, (c) 9 km
and (d) 11 km above sea level.
Objectively determined from the MIT
land-based radar data.
Figure 33.
Total convective and stratiform rain rates for WMONEX cloud
cluster, 09-10 December 1978 from Churchill
(1982).
o
7-
3km
0
CL
00
54=
"-
----\
1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 0000 0100 0200 0300
TIME (GMT)
09-10 DECEMBER 1978
Area-integrated precipitation rate for cloud cluster B at 3 km. The top (dot-dashed)
line represents the sum of the convective (solid line) and stratiform (dashed line) components
of precipitation.
9.
Discussion
Considering the results of the previous sections, it is now possible
to derive a more complete model of an MCC.
This model does not reflect the
behavior of all mesoscale convection that is classified MCC, but it does
represent a large subclass of storms whose characteristics are identifiable
and repeatedly observed.
Individual storms will slightly deviate from the
model, and larger deviation within a storm whose basic character fits the
model are probably related to atmospheric processes acting independent of
the MCC.
An example of this is the case of a cold front moving into the
late phase stratiform rain, resulting in convection occurring when the
model predicts none in this area.
A description of the model MCC follows; the setting is the overall
environment described by Maddox (1981):
a.
Early Phase
1.)
In a favorable environment a small group of very intense cells begins
Within a short time an anvil forms, streaming off to the NE, the
growing.
direction of the mean tropospheric flow.
Either simultaneously or slightly
later, a secondary convective area forms downwind and to the right of the
first.
The meso-B scale numerical model of Fritsch and Chappell also
suggests the secondary convective area (Fig. 34).
2.)
Convection continues to expand.
Most of this growth occurs on the
windward side of the storm in a loosely defined line which is expanding
northward.
This growing line is the source of most of the anvil cloud.
These characteristics are clearly the same as the model of a tropical
squall line proposed by Leary and Houze (1979). See Fig. 35.
Additional
Figure 34.
Convective rain from Fritsch and Chappell's (1980) mesoscale
numerical model.
PRECIPITATION( C)
TIME(SEC)-18ee.
329.
249
N-5(KM.
16e.
go.
.
80.
160.
240.
329.
400.
W-E(KM)
Convective precipitation (cm). Amounts shown are the
total for the 2 h period of convection.
Figure 35.
Tropical squall line model from Leary and Houze (1979).
HORIZONTAL DISTANCE (KM)
50
16 PROFILE A-A
100
150
200
A
1412-
CLOUD PARTICLES ONLY
T
10
SINGLE ICE
S
I
CRYSTALS
8--
-r"
W
-CONVECTIVE
6-6.T.
I
44
2
CELLS
T
AGGREGATION
0C
±
AND
COOLING
-
~~MELTING
c
TT Y
EVAPORATION
AND COOLING
35 45
REFLECTIVITY (dBZ)
0 25
SLOPING ECHO BOUNDARY
Is 0.5 gm
MESOSCALE ASCENT
I
I, -A/AN s 7"C h"
NO HAIL
HEAVILY RIMED AGGREGATES AND GRAUPEL
BIH AD
BRIGHT
BAND. MAXIMM REFLECTIVITY -40 dBZ
m3
11 3
MIg
4,MESOSCALE SUBSIDENCE
'.
".2 i-AT/A1
6°
I LOPING ECHO BOUNDARY
.
h
A
10-100
1-10
SURFACE RAINFALL RATE
(mm h- )
Schematic vertical cross section and vertical profile of radar reflectivity (along dashed line A-A' in the cross section) in horizontally uniform precipitation associated with an anvil cloud. The anvil cloud occurs to the rear of intense convective cells propagating
in the direction from right to left in the figure. The dark solid line is the contour of minimum detectable radar echo, lighter solid lines
and shading indicate contours of higher reflectivity, and the scalloped line indicates the cloud boundary.
-
evidence for
the existence of this type of structure in midlatitudes can be
found in the model from the observations of Pedgley (1962)
Range height indicator displays of the large
midlatitudes
(Fig. 36).
stratiform rain areas in
and their similarity to tropical mesoscale structures
found in Leary (1980) and Lepage and Leary
(1981).
Fig. 37
can be
is an example
from Leary (1980).
The secondary convective area also continues to grow, with lesser
convection forming along the SE side of the MCC. Another example of these
features can be
found in the MCC-like mesoscale
and Brown (1958).
Mesosystem 416
system studied by Fujita
in Fig. 38 shows the structure just
discussed -- basically a two-rain area structure, with the secondary rain
area to the NE
(outside the dashed line, also refer to Fig. 36 and note
cumulus clouds forming at the edge of
lesser convection to the SE,
the west
the high stratiform clouds),
and
and the implied convection in the trough on
side of the large rain area.
The end of this phase is marked by
the peak in the convective rain rate.
b. Transition Phase
1.)
A mesolow has
formed aloft and is best indicated by the surface low
which becomes preferentially
secondary convection.
located between the large rain area and the
The analysis of Fujita and Brown indicates
that
the
surface low moves from the west side of the large rain area to a position
between the large area and the secondary convection.
40.
It
is not clear what the mesolow aloft
probably its
release.
See Figs.
38 through
is doing at this time;
formation and movement are regulated by the latent heat
From the movement of the surface low it seems likely
the low aloft is
that
first strongest near the convection on the west side of
Figure 36.
Model of mature mesoscale convective area from Pedgley (1962).
Z
=C4
and S
cUamus clouds fcrmang
in area
of httle or no
medium-level clouds
cold aw ac
stratus do
Formed
by turbuierce :n
rain -coo-ed air
axIs oF wake/ l0w
'-VERTICAL CROSS-SECTION
thunderstorm
high
Figure 37.
Range height indicator view of a midlatitude mesoscale anvil
cloud by Leary (1980).
2
40
25
(a)
60
DISTANCE FROM SNYDER RADAR (kni
N
t
F
RHI (a) and low-level
(1)
PPI (b) displays derived
from Snyder digital radar
data for 2104 OrT, 8 July
1977. XI1 lies along line
A-A' shown in PPI.
Out-
side councour on RHI is
-
--
/
POST
boundary of weakest detectable echo, and inside
contours are for 25, 30,
and 35 d1Z.
1
A
Echo bomn-
dary on PPI is 25 dBZ.
Range marks on PPI are at
25, 40, 60, 80, 100, 120,
and 130 km. The heavy dashed
line on PPI outlines the
xLm an horizontal extent
of precipitation particles
detectable by radar in the
anvil cloud aloft.
+
-
aL'-&
lose
Figure 38.
Surface and precipitation analysis of mesoscale convection by
Fujita and Brown (1958), 5 June 1953, 0400 GMT.
Surface chart for 2200 CST 4 June 1953.
Figure 39.
Surface and precipitation analysis of mesoscale convection by
Fujita and Brown (1958),
5 June 1953, 0700 GMT.
0-
Surface chart for 0100 CST 5 June 1953.
Figure 40.
Movement of meso-pressure areas in mesosystem 416 from Fujita
and Brown (1958).
100
INDIANA
)URI
/COLD
~.
U0 ,
'LLINO'GI
0-0
I-O
/
FRONT BOUNDARY
MESOSYSTEM
BOUNDARY
PRESSURE EXCESS PATH
PRESSURE OEFICIT PATH
Isochrones of cold front and Mesosystem 416 and paths of excess and deficit pressure.
101
the storm then moves east as the center of maximum latent heat release
moves east because of the increase in stratiform rain.
Transport by the
wind may also play a part in moving the low aloft.
At the beginning of transition, the low and its circulation are
significant enough to begin altering the environment which is supporting
the convection.
decay,
The convection on the west side of the MCC begins to
and a convective
line forms on the SE side.
Both of these
occurrences can be explained by the circulation around the low, i.e., more
stable air is drawn in from the north, killing the west convection, and the
cold air under the stratiform rain is pushed into warm air to the
southeast.
created.
Thus, a strong discontinuity along which convection forms is
During this time the total rain and cloud area have reached their
maximum extent.
The end of this period occurs when the stratiform rain
area stops growing.
Although the stratiform rain can continue growing
after the convection begins to decrease, the upwind source of convection
supplied moisture seems to influence the time span in which the stratiform
rain will grow.
When the west side convection begins to decay, the overall
growth of the rest of the storm soon stops.
The largest and longest
lasting storms are those where the west side convection is regenerated,
usually, by a feature independent of the MCC.
c.
Late Phase
Decaying stratiform rain areas and squall line characteristics of the
remaining convection are the most distinguishing feature of the MCC at this
time.
Widespread stratiform rain can not be supported when the convection
is oriented parallel with the mean flow of the MCC environment.
The line propagation is roughly perpendicular to its axis and may persist
102
for many hours, occasionally producing severe weather (the maximum number
of severe events, however, occur in the earlier phases of the storm).
As
seen in the previous section, a large cloud mass may form at the northern
edge of the line and slightly ahead of the surface mesolow, but this
phenomenon, and other characteristics of the MCC, appear with varying
intensities related only in a general way to overall storm size.
103
Summary and conclusions
10.
A large subclass of storms which have been identified as MCCs by the
criteria of Maddox do have identifiable and repeatedly observed convective
and stratiform precipitation cycles and structures.
The early phase of
these storms appear identical to tropical squall lines, or cloud clusters
with convection mainly on one side.
While, in the late phase, the active
region of the precipitation takes on the character of a weak midlatitude
squall line. Mesoscale circulations, dominated by a mesolow in the lower
troposphere, appear mostly responsible for the transformation of the MCC
from a tropical equivalent storm to a storm resembling a midlatitude squall
line.
The usefulness of Maddox'(1980) MCC criteria, other than its success
at identifying the largest mesoscale convective systems when the criteria
is selectively applied to cloud area in the central United States, must be
questioned.
°
First, the area of cloud top with temperature -32 C or less
always overestimates the total rain area, with the greatest overestimation
occuring in storms where the late phase convective line is in the center of
the MCC.
In these and similar cases the large area of cloud SE of the line
has little, if any, stratiform or convective rain.
Second, at Maddox'
initiation and termination times, large horizontal areas are already, or
are still, being affected by these storms.
Initiation does not correspond
to the start of an identifiable process or change in character of the
storm, and occasionally occurs after the convection has begun to decrease.
Similar statements can be made about termination with, again, the
observation that at termination a significant meteorological phenomenon is
still occurring and may continue for some length of time.
Furthermore,
104
this misleading labeling of start and stop time obscures the facts that
these systems generally grow in a preexisting circulation, a circulation is
left after Maddox' cloud characteristics are gone, and the remaining
circulation can be the basis for later concentration of convection (Bosart
and Sanders, 1981).
Third, there is no reason to believe that mesoscale
convective systems which meet Maddox' MCC criteria are fundamentally
different in structure and processes from other systems which do
not. Detailed examination of non-MCC cases was not done in this study, but
the apparent applicability of many results from mesoscale studies of all
scales suggests that less emphasis should be placed on studying mesoscale
convection by size and more emphasis put on understanding fundamental
convective structures and processes.
105
APPENDIX
Many other MCC features were observed in this work, but there was not
sufficient time or data to thoroughly document them.
A short list of these
observations is written below with the hope that other investigators might
find them useful.
a.
The first strong convective areas tended to occur in, or on the border
of, a preexisting low level cloudy area which was usually trackable for
many hours before the first storms.
b.
Small linear cloud lines were sometimes observed to move into the west
side of an MCC and trigger new convection, thus enlarging the MCC and
making it last longer.
The best example of this was the case of 6/7 May
78.
c.
It has been stated that the low level jet is somehow tied into the
maximum convection of an MCC, but this connection becomes questionable when
it is noticed that the peak in convection in the average storm occurs
before the climatological peak of the low level jet.
d.
In the cases where much of the MCC was aloft above a cold front, the
late phase convective line would only be well organized to the south of the
surface cold front.
This suggests surface friction is responsible for
keeping the line well organized.
e.
The likeness between the early phase of the MCC and tropical squall
lines has now been well documented, but it is not clear if the
distinguishing feature of the MCC, the mesolow aloft, also develops in some
tropical cases.
It is suspected that lows do sometimes form from tropical
mesosystems, perhaps the stronger of these cases become tropical storms.
106
ACKNOWLEDGMENTS
I sincerely want to thank Dr. Sanders for introducing me to the topic
of mesoscale convection in his synoptic meteorology class, and supporting
my unorthodox study of the subject.
Dr. A. Thompson of Texas A&M
University and Mr. Walsh from the National Climatic Center were
instrumental in supplying data for this study and helped me many times with
"final" requests. Isabelle Kole's drafting of my figures was greatly
appreciated.
Thanks, also, to the taxpayers of the United States who paid
my salary and tuition through the offices of the U.S. Air Force, but I
would never have completed any of this without the love and sacrifice of my
wife, Joae, and my family.
107
REFERENCES
Bosart, L.F. and F. Sanders, 1981: The Johnstown flood of July 1977: A
long-lived convective storm. J. Atmos. Sci.,
38, 1616-1642.
Churchill, D.D., 1982: Development and structure of winter monsoon cloud
clusters. University of Washington Master of Science Thesis. 200pp.
Fritsch, J.M.,
and C.F. Chappell, 1980: Numerical prediction of
convectively driven mesoscale pressure systems, Part II: Mesoscale model.
J. of Atmos. Sci.,37, 1734-1762.
Fujita, T.,
and H.A. Brown, 1958: A study of mesosystems and their radar
echoes. Bull. Amer. Meteor. Soc.,39,538-554.
Houze, R.A., Jr.,
1977: Structure and dynamics of a tropical squall line
system. Mon. Wea. Re.,105,1541-1567.
Kreitzberg, C.W. and D.J. Perkey, 1977: Release of potential instability:
Part II.
The mechanism of convective/mesoscale interaction. J. Atmos.
Sci.,34, 1569-1595.
Leary, C.A.,
1980: Structural Similarities between mesoscale convective
systems observed over the Texas south plains during HIPLEX and over the
tropical ocean during Gate. Preprints Nineteenth Conf. on Radar
Meteorology,AMS,384-389.
108
Leary, C.A.,
and R.A. Houze,
1979: Melting and evaporation of hydrometeors
in precipitation
from the anvil clouds of deep tropical convection. J.
Atmos. Sci.,
669-679.
36,
Lepage, M.F.,
and C.A. Leary, 1981:
convective system.
Evolution of a midlatitude mesoscale
Preprints Twenthieth Conf. on Radar Meteorology, AMS,
243-250.
Maddox, R.A.,
1980:
Mesoscale convective complexes.
Bulletin of the
American Meteorological Society,61, 1374-1387.
Maddox, R.A.,
1981:
Structure and life-cycle of midlatitude mesoscale
convective complexes. Colorado State University Atmospheric
Science Paper
No.336.311.
Pedgley, D.E.,
August
1962: A meso-synoptic analysis of the
1958. Geophysical Memoir No.
U.S. Dept. of Commerce,
Guide, 24pp.
1978:
thunderstorms on 28
106, Meteorological Office 711,
74pp.
National Weather Service Radar Code User's