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
Extratropical cyclones (ECs), which form in response to a combination of baroclinic, diabatic, and barotropic processes, play a major role in determining the day-to-day weather conditions in the midlatitudes during the cool season. Particularly strong ECs can have considerable socioeconomic impacts on the regions they traverse due to their frequent association with damaging winds and heavy precipitation (e.g., Mailier et al.
2006; Dacre et al. 2012; Martínez-Alvarado and Plant 2014), and can contribute disproportionately to precipitation anomaly statistics during a particular season.
Particularly strong ECs forming over and traversing the densely populated regions of central and eastern North America have the potential to lead to noteworthy cool-season extreme weather events (EWEs), defined here as a category of high-impact weather events that are societally disruptive, geographically widespread, exceptionally prolonged, and climatologically infrequent. The opportunity to investigate the frequency, evolution, and predictability of strong ECs leading to noteworthy cool-season EWEs over central and eastern North America motivates this Ph.D. dissertation.
The overarching goal of this Ph.D. dissertation is to increase our understanding of strong ECs leading to noteworthy cool-season EWEs over central and eastern North
America. Specifically, this Ph.D. dissertation seeks to increase our understanding of these strong ECs by documenting the interannual and intraseasonal variability associated with their formation, as well as by investigating the ways in which baroclinic, diabatic, and barotropic processes evolve and combine to produce them. The influence of geographical location, season, and teleconnection pattern on the combinations of baroclinic, diabatic, and barotropic processes most likely to yield these strong ECs will
1

be examined as part of this investigation. In addition, this Ph.D. dissertation seeks to increase our understanding of strong ECs leading to noteworthy cool-season EWEs over central and eastern North America by exploring the structure, motion, and evolution of the upper- and lower-tropospheric features associated with the formation of these strong
ECs, as well as by identifying the combinations of baroclinic, diabatic, and barotropic processes associated with the most/least predictable of these strong ECs. b.
Literature review
The meteorological community has vigorously debated the various processes associated with the formation of ECs since the mid-19th century (Uccellini 1990). The first scientific theory of extratropical cyclogenesis proposed by Epsy (1841) emphasized the importance of diabatic processes during the development of ECs, stating that the expansion of air resulting from condensation heating in a region of heavy precipitation can produce a warm-core EC (Kutzbach 1979). Farrell (1999) reveals that the diabatically-driven thermal theory of extratropical cyclogenesis proposed by Epsy (1841) was quickly contradicted by an observational case study of the Great Storm of 1836
(Loomis 1841), which indicated that heavy precipitation and a rise in temperature do not always accompany extratropical cyclogenesis. Despite its almost immediate contradiction, the diabatically-driven thermal theory of extratropical cyclogenesis proposed by Epsy (1841) persisted over the following century as a possible mechanism for EC formation (Farrell 1999).
By the early 20th century, the meteorological community had begun to explore the possibility that ECs predominantly form in associated with baroclinic processes. The theoretical work of Margules (1903, 1906a,b) suggested that the primary energy source for developing ECs is potential energy associated with horizontal temperature gradients, but did not suggest a mechanism for converting this potential energy into the kinetic energy of the EC. Members of the Bergen School of Meteorology (located in Bergen,
Norway) expanded on the theoretical work of Margules (1903, 1906,b), introducing a physical model of the processes by which the conversion of potential energy into the kinetic energy of the EC takes place. The Norwegian cyclone model proposed by
Bjerknes (1919) and the polar front theory of extratropical cyclogenesis proposed by
2

Bjerknes and Solberg (1922) began “a new era [in meteorology]” (Volkert 1999), suggesting that the redistribution of air masses in the vertical (i.e., rising warm air and sinking cold air) is associated with the conversion of potential energy into the kinetic energy of the EC. The seminal works of Bjerknes (1919) and Bjerknes and Solberg
(1922) were followed by the introduction of baroclinic instability theory by Charney
(1947) and Eady (1949), who demonstrated mathematically that this instability, which occurs in the presence of an equatorward-directed tropospheric meridional temperature gradient and the associated vertical shear of the westerly wind, is the primary mechanism leading to extratropical cyclogenesis.
Despite an emphasis on baroclinic processes leading to extratropical cyclogenesis following the works of Bjerknes (1919), Bjerknes and Solberg (1922), Charney (1947), and Eady (1949), meteorologists of the mid-20th century continued to recognize the importance of diabatic processes during EC formation and development. Palmén (1951) discussed the importance of diabatic heating in focusing ascent in regions of heavy precipitation associated with ECs. Both Sutcliffe (1947) and Petterssen (1956) include a diabatic term in the EC development equations they derived that accounted for the effect of diabatic processes on vertical motion. Additional studies by Danard (1964, 1966) and
Lorenz (1955, 1960, 1965) illustrated the importance of diabatic processes in the generation of eddy available potential energy (A
E
), which can be subsequently converted to the eddy kinetic energy (K
E
) of the EC by baroclinic processes (C
E
) (Fig. 1).
The occasional failure of numerical models to simulate the rapid deepening of ECs in the mid-20th century resulted in a series of studies exploring the contributions of diabatic processes on extratropical cyclogenesis (e.g., Aubert 1957; Krishnamurti 1968; Tracton
1973; Johnson and Downey 1976). The most well-known study to explore rapid deepening ECs during the mid-20th century is arguably that of Sanders and Gyakum
(1980), who investigated rapidly deepening ECs (also known as “bombs”) across the
Northern Hemisphere (NH) during the cool seasons of 1976–1979.
In their study, Sanders and Gyakum (1980) _______________
3

c.
Research questions and hypotheses
2.
Data and methodology a.
Datasets
The primary data source used in 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–landsurface–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
4

1.125° 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-
Analysis (ERA-40) 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 North America may be influenced by teleconnection pattern. Daily teleconnection indices are utilized in order to capture rapid planetary-scale regime transitions, defined in Archambault et al.
(2010) as an 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 North Atlantic
Oscillation (NAO
+
) to negative NAO (NAO
−
) and negative Pacific–North American pattern (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 coolseason EWE over central and eastern North America, motivating the use of daily teleconnection indices in the present study.
Version 2 of the ESRL/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 coolseason EWEs over central and eastern North America 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
5

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 North America
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 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), as well as within 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 attains a minimum central pressure of <980 hPa and 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 North America, eastern North America, 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 North America, 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 extensive, spanning 40° of latitude (65° of longitude) at its tallest (widest) point (Fig. 4). Figure 5, which depicts cool-season
6

maximum PW values that occurred during 1979–2014, highlights the moisture variations that exist across the domain of interest. These moisture variations across the domain of interest suggest that geographical location may influence the combinations of baroclinic, diabatic, and barotropic processes most likely to yield strong ECs (research question 3), specifically proximity to a major moisture source (hypothesis 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 of a meteorological field from the long-term climatology ( N ) 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 of a meteorological field,
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 925hPa zonal winds and 925-hPa meridional winds, averaged within a 15° box centered over the EC, that are ≥1σ above climatology for ≥24 h. Strong ECs meeting these EWE criteria are believed to have a sensible impact that is geographically widespread, exceptionally prolonged, and climatologically infrequent, in keeping with the definition of an EWE given section 1a. All strong ECs meeting these EWE criteria within the domain of interest (Figs. 4 and 5) are considered to be societally disruptive due to their proximity to the densely populated regions of central and eastern North America, as well as the shipping lanes of the northwestern North Atlantic.
An example of the selection of a strong EC leading to a noteworthy cool-season EWE over central and eastern North America is shown in Figs. 6–8 for the case of SS93. As previously stated, an EC is required to attain a minimum central pressure of <980 hPa and exhibit a deepening rate of ≥1 Bergeron, normalized by latitude, along its track in order to qualify as a strong EC. 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 reveals that SS93 had a minimum central pressure of <980 hPa for ~60 h during its life cycle. Figure 6 also reveals that
7

SS93 deepened 28 hPa during the 24-h period from 1800 UTC 12 March to 1800 UTC 13
March 1993. This 24-h deepening rate is equal to ~2 Bergeron when normalized by the latitude over which it occurred, or approximately double the deepening rate required for an EC to qualify as a strong EC.
As previously stated, a strong EC must attain MSLP values (925-hPa zonal winds and
925-hPa meridional winds), averaged within a 15° box centered over the EC, that is ≥1σ below (above) climatology for ≥24 h in order to qualify as an EWE. 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
2.3σ, 2.8σ, 3.2σ, 3.1σ, and 3.1σ below 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-meridional winds), averaged within a 15° box centered over the EC, that were 1.9σ, 2.1σ, 2.1σ, 1.9σ, and
1.7σ (2.5σ, 2.5σ, 2.3σ, 2.4σ, and 1.9σ) above climatology. Figures 6–8 reveal that SS93 would easily be selected as a strong EC that led to a noteworthy cool-season EWE over central and eastern North America and included in the proposed 1979–present climatology. 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)
Using the datasets and methodology outlined in sections 2a–b, a 1979–present climatology of strong ECs leading to noteworthy cool-season EWEs over central and eastern North America will be constructed. The 1979–present climatology will be used to determine when strong ECs leading to noteworthy cool-season EWEs over central and eastern North America most frequently occur (hypothesis 1). In addition, the interannual
8

and intraseasonal variability associated with the location and frequency of strong ECs included in the 1979–present climatology will be examined and compared to that of strong ECs included in previous EC climatologies (e.g., Eichler and Higgins 2006; Wang et al. 2006) in order to put the results of the present climatology in greater context.
The relative contributions of baroclinic, diabatic, and barotropic processes will be quantified during the evolution of strong ECs included in the 1979–present climatology using the candidate metrics discussed in section 2c. Quantification of the relative contributions of baroclinic, diabatic, and barotropic processes during the evolution of strong ECs included in the 1979–present climatology will be used to 1) identify the combinations of these processes most likely to yield strong ECs leading to noteworthy cool-season EWEs over central and eastern North America (hypothesis 2), as well as 2) establish the importance of geographical location, season, and teleconnection pattern in determining the combinations of these processes most likely to yield strong ECs leading to noteworthy cool-season EWEs over central and eastern North America (hypothesis 3).
Interannual and intraseasonal variability associated with the location and frequency of strong ECs included in the 1979–present climatology that are governed by similar combinations of baroclinic, diabatic, and barotropic processes will also be examined. b.
Cyclone-relative composite analysis
A cyclone-relative composite analysis will be performed on clusters of strong ECs identified in the 1979–present climatology that are governed by similar combinations of baroclinic, diabatic, and barotropic processes. This cyclone-relative composite analysis will be used to document the structure, motion, and evolution of the most prominent upper- and lower-tropospheric features associated with the formation and motion of strong ECs included in each cluster (hypothesis 4).
As stated in section 1c, the author hypothesizes that the majority of strong ECs identified in the 1979–present climatology will form downstream of an uppertropospheric disturbance, with strong ECs associated with dominant contributions from diabatic processes forming upstream of a well-defined lower-tropospheric corridor of meridional water vapor transport (hypothesis 4). The author also hypothesizes that strong
ECs governed by similar combinations of baroclinic, diabatic, and barotropic processes
9

may be associated with similar upper-tropospheric jet configurations. Anecdotal evidence suggests that strong ECs associated with dominant contributions from diabatic processes may typically form in equatorward jet-entrance regions, while strong ECs associated with dominant contributions from baroclinic (barotropic) processes may typically form in conjunction with coupled jets (in poleward jet-exit regions). This additional hypothesis will also be tested using the cyclone-relative composite analysis. c.
Multiscale case studies
Multiscale case studies of individual strong ECs leading to noteworthy cool-season
EWEs that are representative of each cluster of strong ECs will be utilized to illustrate the structure, motion, and evolution of the upper- and lower-tropospheric features associated with the formation and motion of each strong EC (hypothesis 4). Figures 9–11 depict the evolution of three strong ECs leading to noteworthy cool-season EWEs that are currently considered case-study candidates. These three case-study candidates include 1) the 9–11
November 1998 central U.S. cyclone (Iacopelli and Knox 2001), which rapidly deepened in an environment characterized by tropospheric-deep baroclinicity (Fig. 9), 2) SS93
(Table 1), which rapidly deepened in the presence of widespread deep convection in an environment characterized by tropospheric-deep baroclinicity (Fig. 10), and 3) the 26–27
October 2010 central U.S. cyclone, which rapidly deepened in the poleward exit region of a >80 m s
−1
250-hPa jet in an environment where baroclinicity was confined to the uppertroposphere (Fig. 11). These three case-study candidates illustrate variations in the relative contributions of baroclinic, diabatic, and barotropic processes associated with rapid deepening ECs. The rapid deepening of the 9–11 November 1998 central U.S. cyclone (Fig. 9) is likely dominated by baroclinic processes; the rapid deepening of SS93
(Fig. 10) is likely dominated by both baroclinic and diabatic processes; and the rapid deepening of the 26–27 October 2010 central U.S. cyclone (Fig. 11) is likely dominated by baroclinic and barotropic processes.
Multiscale case studies may also be utilized to illustrate the structure, motion, and evolution of the upper- and lower-tropospheric features associated with the formation and motion of strong ECs that develop in different locations over central and eastern North
America. It is likely that the upper- and lower-tropospheric features associated with the
10

formation and motion of strong ECs lee of the Rockies will be different from those features associated with the formation and motion of strong ECs over the east coast of
North America. Multiscale case studies could be utilized to elucidate some of these differences, which would likely be reflected in the relative contribution of baroclinic, diabatic, and barotropic processes associated with each cyclone during its life cycle.
Additional multiscale case studies of strong ECs leading to particularly historic coolseason EWEs will be considered as appropriate. d.
Assessment of predictability of strong ECs dominated by various combinations of baroclinic, diabatic, and barotropic processes
11

Dissertation outline
1.
Introduction
1.1.
Motivation and purpose
1.2.
Literature review
1.3.
Research questions and hypotheses
2.
Data and methodology
2.1.
Datasets
2.2.
Selection of strong ECs leading to noteworthy cool-season EWEs over central and eastern North America
2.3.
Metrics for evaluating baroclinic, diabatic, and barotropic processes during the evolution of strong ECs
3.
Climatology of strong ECs leading to noteworthy cool-season EWEs over central and eastern North America (1979–present)
3.1.
Interannual and intraseasonal variability
3.2.
Baroclinic, diabatic, and barotropic processes
3.2.1.
Geographical location
3.2.2.
Intraseasonal variability
3.2.3.
Teleconnection pattern
4.
Cyclone-relative composite analysis
4.1.
Cluster selection
4.2.
Composite discussion
5.
Multiscale case studies
5.1.
Case studies representative of cyclone-relative composites
5.2.
Case studies representative of strong ECs forming in different locations
6.
Assessing the predictability of strong ECs
6.1.
Track forecast error
6.2.
Intensity forecast error
6.3.
Case studies
7.
Discussion and future work
8.
References
12

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