Noteworthy Cool-Season Extreme Weather Events over Central and Eastern North
America Associated with Strong Extratropical Cyclones
Ph.D. Dissertation Prospectus
Alicia M. Bentley
Department of Atmospheric and Environmental Sciences
University at Albany, State University of New York
1. Introduction
a. Motivation and purpose
ECs are a central component maintaining the global atmospheric energy, moisture,
and momentum budgets (Neu at al. 2013).
b. Literature review
c. Research questions and hypotheses
2. Data and methodology
a. Datasets
The primary data source for this Ph.D. dissertation will be the 0.5° National Centers
for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR)
global gridded dataset (Saha et al. 2010, 2014), with data available at 64 vertical levels
every 6 h during the period 1979–present. The 0.5° NCEP CFSR dataset is the first
reanalysis dataset to be created using a global coupled atmosphere–ocean–land-surface–
sea-ice model and to assimilate satellite radiances over the entire period of its availability,
making it preferable to its predecessors, the 2.5° NCEP–National Center for Atmospheric
Research reanalysis dataset (Kalnay et al. 1996; Kistler et al. 2001) and the 1.125° 40-yr
European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA40) reanalysis dataset (Uppala et al. 2005).
Daily teleconnection indices, calculated from the 0.5° NCEP CFSR dataset using the
methodology of Archambault et al. (2008), will be used to determine how the
combinations of baroclinic, diabatic, and barotropic processes most likely to yield strong
ECs leading to noteworthy cool-season EWEs over central and eastern NA may be
influenced by teleconnection pattern. Daily teleconnection indices are utilized in order to
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capture rapid planetary-scale regime transitions, defined in Archambault et al. (2010) as a
North Atlantic Oscillation (NAO) or Pacific–North American pattern (PNA) index
change from at least a 1 standard deviation anomaly to at least a 1 standard deviation
anomaly of opposite sign within 7 days. Archambault et al. (2010) determined that
planetary-scale regime transitions, specifically positive NAO (NAO+) to negative NAO
(NAO−) and negative PNA (PNA−) to positive PNA (PNA+) transitions, are associated
with periods of enhance cool-season precipitation over the northeast United States. The
results of Archambault et al. (2010) suggest that NAO+ to NAO− and PNA− to PNA+
transitions may also be associated with the formation of strong ECs leading to
noteworthy cool-season EWE over central and eastern NA.
Version 2 of the Earth System Research Laboratory/Physical Sciences Division 1°
Global Ensemble Forecasting System (GEFS) Reforecast dataset (Hamill et al. 2013) will
be used in order to establish the relationship between the skill with which strong ECs
leading to noteworthy cool-season EWEs over central and eastern NA are forecast and
the combinations of baroclinic, diabatic, and barotropic processes associated with their
formation. The 1° GEFS Reforecast dataset, created using the 2012 version of the NCEP
GEFS, includes forecasts from an 11-member ensemble initiated once daily (0000 UTC)
during the period 1985–present. Strong ECs leading to noteworthy cool-season EWEs
over central and eastern NA associated with particularly low/high forecast skill will be
examined in greater detail using forecasts from the 51-member ECMWF ensemble
prediction system (EPS) (Buizza et al. 2007). ECMWF EPS forecasts are initiated twice
daily (0000 UTC and 1200 UTC) during the period 1 October 2006–present, and may be
obtained from The Observing System Research and Predictability Experiment
(THORPEX) Interactive Global Grand Ensemble (TIGGE) archive (Bougeault et al.
2010) (available online at http://apps.ecmwf.int/datasets/data/tigge/).
b. Selection of strong ECs leading to noteworthy cool-season EWEs over central and
eastern NA
In order to construct the 1979–present climatology of strong ECs leading to
noteworthy cool-season EWEs over central and eastern North America proposed in
section 1c, EC tracks will be obtained from the 0.5° NCEP CFSR dataset using the
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automated Hodges (1999) tracking algorithm. The Hodges (1999) tracking algorithm has
been extensively used to identify EC tracks within reanalysis datasets (e.g., Hodges et al.
2011; Dacre et al. 2012; Azad and Sorteberg 2014; Colle at al. 2015) and global climate
model output (e.g., Catto et al. 2010; Zappa et al. 2013; Chang 2013; Colle at al. 2015).
A strong EC is defined in the present study as an EC that 1) attains a minimum central
pressure of <980 hPa and 2) exhibits a deepening rate of ≥1 Bergeron (Sanders and
Gyakum 1980), normalized by latitude, along its track. The 980-hPa MSLP threshold
used to identify strong ECs in the present study is consistent with the recent work of
Colle et al. (2015), who also used a 980-hPa MSLP threshold to identify strong ECs
within the 0.5° NCEP CFSR dataset. Only strong ECs forming during the cool season,
defined as September–April, will be retained in the present study. This cool-season
definition is consistent with the cool-season definition in the climatology of explosive
cyclogenesis constructed by Sanders and Gyakum (1980) (Fig. 2).
Only strong ECs forming over central NA, eastern NA, and the northwestern North
Atlantic during the cool season are of interest in the present study. The domain within
which strong ECs will be identified is outlined using a thick black line in Fig. 4. This
domain was selected based on 1) the location of cool-season minimum MSLP values that
occurred during 1979–2014 (Fig. 4), 2) the location of the eastern boundary of the Rocky
Mountains and the eastern boundary of NA, and 3) the location of the most frequent
occurrence of northwestern North Atlantic “bombs” identified in Sanders and Gyakum
(1980) (their Fig. 3). The domain within which strong ECs will be identified is
expansive, spanning 40° of latitude and 65° of longitude (Fig. 4). Figure 5, which depicts
cool-season maximum PW values that occurred during 1979–2014, highlights the
variations in moisture that exist across the specified domain. The cool-season maximum
PW values depicted in Fig. 5 suggests that geographical location may influence the
combinations of baroclinic, diabatic, and barotropic processes most likely to yield strong
ECs (research question 3).
The normalized departure of select lower-tropospheric meteorological fields from a
long-term (1979–2009) climatology derived from the 0.5° NCEP CFSR dataset will be
used to determining whether a strong EC qualifies as an EWE. The normalized departure
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from the long-term climatology (N) of a select lower-tropospheric meteorological field
can be calculated using the methodology of Hart and Grumm (2001), who state that
N = (X − μ)/σ,
(1)
where X is a gridpoint value, μ is the gridpoint 21-day running mean, and σ is the
gridpoint 21-day running standard deviation. In order to qualify as an EWE, a strong EC
must attain MSLP values, averaged within a 15° box centered over the EC, that is ≥1σ
below climatology for ≥24 h. In addition, a strong EC must attain 925-hPa zonal winds
and 925-hPa meridional winds, averaged within a 15° box centered over the EC, that are
≥1σ above climatology for ≥24 h. These criteria ensure that the strong EC is
geographically widespread, exceptionally prolonged, and climatologically infrequent, in
keeping with the definition of an EWE given section 1a. All strong ECs meeting the
EWE criteria within the specified domain (Figs. 4 and 5) are considered to be societally
disruptive due to their proximity to the densely populated regions of central and eastern
NA.
An example of the selection of a strong EC leading to a noteworthy cool-season EWE
over central and eastern NA is shown in Figs. 6–8 for the case of SS93. In order to
qualify as a strong EC, SS93 is required to 1) attain a minimum central pressure of <980
hPa and 2) exhibits a deepening rate of ≥1 Bergeron, normalized by latitude, along its
track. A track map indicating the position and minimum central pressure associated with
the center of SS93 every 6 h between 1800 UTC 12 March and 0000 UTC 16 March
1993 is shown in Fig. 6. Figure 6 indicates that SS93 had a minimum central pressure of
<980 hPa for ~60 h during its life cycle. Figure 6 also indicates that SS93 deepened 28
hPa during the 24 h between 1800 UTC 12 March and 1800 UTC 13 March 1993. This
24-h deepening rate is equal to ~2 Bergeron when normalized by the latitude, or
approximately double the deepening rate required for an EC to qualify as a strong EC.
In order to qualify as an EWE, a strong EC must attain 1) MSLP values, 2) 925-hPa
zonal winds, and 3) 925-hPa meridional winds, averaged within a 15° box centered over
the EC, that are ≥1σ below climatology for ≥24 h. Figure 7 depicts MSLP values and
standardized anomalies associated with SS93 every 6 h between 0600 UTC 13 March and
0600 UTC 14 March 1993. During the 24-h period shown in Fig. 7, SS93 attained MSLP
values, averaged within a 15° box centered over the EC, that were ≥2σ below
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climatology. Figure 8 depicts 925-hPa wind, as well as standardized 925-hPa zonal and
meridional wind anomalies, associated with SS93 every 6 h between 0600 UTC 13
March and 0600 UTC 14 March 1993. During the 24-h period shown in Fig. 8, SS93
attained 925-hPa zonal winds (925-hPa meridional winds), averaged within a 15° box
centered over the EC, that were ≥1.7σ (≥1.9σ) above climatology. These statistics easily
qualify SS93 as a strong EC that led to a noteworthy cool-season EWE eastern NA.
c. Candidate metrics for evaluating baroclinic, diabatic, and barotropic processes
during the evolution of strong ECs
3. Dissertation plan
a. Climatology of strong ECs leading to noteworthy cool-season EWEs over central and
eastern North America (1979–present)
b. Cyclone-relative composite analysis
c. Multiscale case studies
d. Assessment of predictability of strong ECs dominated by various combinations of
baroclinic, diabatic, and barotropic processes
Dissertation outline
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