The extratropical transitions of eastern Pacific Hurricane
Lester and Atlantic Hurricane Andrew (1992): A comparison
Michael J. Dickinson12, Kristen Corbosiero3, Lance F. Bosart4
2
3
WeatherPredict Consulting,
Wakefield, RI
Department of Atmospheric and Oceanic Sciences
University of California, Los Angeles,
Los Angeles, CA
4
Department of Earth and Atmospheric Sciences
University at Albany, State University of New York,
Albany, NY
1
Corresponding Author Address: Michael J. Dickinson, WeatherPredict Consulting, 50 South
County Commons Way, Suite E8, Wakefield, RI 02879. Email: mid@weatherpredict.com
Abstract
1. Introduction
The mid-latitude flow can interact with the tropics in many ways. One such way
involves the progression of a tropical cyclone (TC) into the mid-latitudes where it can
come under the influence of transient upper-level disturbances. This tropical-extratropical
interaction known as extratropical transition (ET) has been the focus of a number of
studies in the recent literature (see Jones et al. 2003 for a thorough review). The
interaction between the tropical cyclone and the increasingly baroclinic environment is
called (ET). Broadly speaking, ET is the conversion of a symmetric, vertically stacked,
warm-core tropical cyclone with a maximum intensity in the lower troposphere into an
asymmetric, cold-core and tilted extratropical cyclone with a maximum intensity in the
upper troposphere. A second way in which the extratropics can exert an influence of the
tropics is by providing a non-classical mechanism for tropical cyclogenesis to occur.
Typically, the incipient cyclogenesis often develops much further poleward (25˚- 30˚N)
than TCs that develop from African easterly waves or along monsoon troughs in the
Atlantic and Pacific Ocean basins.
Numerous studies in the literature have shown that ET is a common occurrence in
the Atlantic (Hart and Evans, 2003), western Pacific (Klein et al, 2000), and Australian
(Foley and Hanstrum 1992, Sinclair 2002) basins. Hart and Evans (2003) found that
relative to the number of TCs, ET is much more frequent in the Atlantic than in any other
basin. Hart and Evans estimate that nearly 50% of all Atlantic tropical cyclones undergo
ET. In the western Pacific, approximately 25% of all western Pacific tropical cyclones
undergo ET (Klein et al., 2000). The only basin to experience frequent tropical
cyclogenesis and not experience ET is the eastern North Pacific basin. The potential for
ET events in the eastern Pacific has come under increasing scrutiny because of the impact
these systems have on the precipitation totals in the southwestern United States
(Corbosiero et al, 2009).
The evolution of the ET is sensitive to the interaction of the decaying tropical
cyclone and the mid-latitude circulation. Over the last several years, several different
studies have attempted to classify the evolution of an ET event. Recently Hart (2002)
developed a cyclone phase diagram from two parameters calculated directly from the
three-dimensional height field. One parameter determines whether the cyclone exhibits a
warm or cold core. The second parameter provides a measure of the asymmetries in the
thermal structure. The Hart phase space diagrams can be used to examine the lifecycle of
all cyclones and can determine the approximate time that extratropical transition
occurred.
As a TC translates poleward, it is often accelerating poleward over cooler seasurface temperatures (or moving over land) and into increased vertical shear as it begins
to interact with a higher latitude upper-level trough/jet system during the ET process. In
satellite imagery, the inner core of the tropical cyclone loses its symmetric appearance
and gradually takes on the appearance of an extratropical cyclone (EC). The nearly
symmetric wind and precipitation distributions that are concentrated about the circulation
center of the TC evolve to produce strong and expansive asymmetric wind and
precipitation distributions. The increase in translation speed further contributes to the
asymmetric structure. Klein et al. (2000) showed that the inner-core of the tropical
cyclone becomes asymmetric as strong frontogenesis develops on the poleward side of
the tropical cyclone center. The system gradually takes on the appearance of a traditional
mid-latitude cyclone. Further indications include an increase in the radius of gale force
winds, asymmetries in the wind and precipitation fields, and a decrease in sea surface
temperature beneath the tropical cyclone.
ET is clearly reflected in the cloud and precipitation patterns of the transitioning
TC. ET events can be tremendous rain makers as the large-area of synoptically-driven
ascent can act on abundant tropical moisture. Hurricanes Agnes (1972, Bosart and Dean
1991; Bosart and Carr 1978; DiMego and Bosart 1982a,b) and Floyd (1999, Atallah and
Bosart 2003; Colle 2003) are two such historic examples. Due to the expansion of the
area covered by clouds and precipitation when the tropical cyclone moves poleward,
heavy precipitation can occur over land without the tropical cyclone center making
landfall.
In addition to interacting with a pre-existing tropical cyclone, mid-latitude troughs
can also trigger disturbances that could evolve into tropical cyclones. Higher latitude
upper-level cold-core disturbances on occasion can initiate ordinary baroclinic
cyclogenesis in the subtropics. A subset of these baroclinic developments may in turn
become TCs via the tropical transition (TT) process where TT is used to describe the
transition of a cold-core baroclinic EC into a warm-core TC.
Vertical wind shear
associated with the higher latitude upper-level disturbance acts to organize the
developing convection and generate low-level vorticity (Bosart and Bracken 2002).
Forcing for large-scale ascent associated with the higher-latitude trough can be effective
in triggering deep convection because deep instability can be created equally well by
cooling aloft ahead of the upper-level disturbance as opposed to hating from below. In
the environment favorable for ascent a positive feedback process between deep
convection and cyclonic vorticity production by stretching in convectively driven
updrafts acts to weaken the vertical wind shear and allows the growing disturbance to
develop a warm core. Shear reduction and attendant upper-level potential vorticity (PV)
destruction through diabatically driven processes can weaken and eliminate the original
triggering baroclinic disturbance, resulting in the development of a warm-core
disturbance and anticyclonic outflow aloft via the TT process provided sea surface
temperatures (SSTs) are sufficiently warm (at last 23-24 C) to sustain the deep
convection . Notable examples of TT include TC Diana (1984; Bosart and Bartlo 1991),
TC Michael (2000; Davis and Bosart 2003, 2004), TC Humberto (2001; Davis and Bosart
2006), and TC Catarina (2004), the first documented western South Atlantic TC
(McTaggart-Cowan et al. 2006) .
The purpose of this paper is to examine in detail the remarkable events of late
August 1992. During this period, the ET of TC Lester over the southwestern United
States, the first-ever documented ET in this region occurred. At the same time, powerful
TC Andrew devastated South Florida and made a second landfall along the central
Louisiana coast. Andrew was then steered poleward ahead of the same baroclinic system
responsible for the ET of Hurricane Lester just days prior. Finally, the upscale impact of
Lester and Andrew on the synoptic-scale flow led to the downstream development and
the generation of a “vorticity seed” in the western Atlantic. Despite favorable sea-surface
temperatures and low vertical wind shear, no tropical system developed. All of these
events are directly related to each other.
The paper is organized as follows: Section 2 describes the data source and
methodology. Section 3 provides a brief description of Hurricanes Lester and Andrew.
The climatology and results are presented in Section 4.
2. Data and Methodology
The tropical cyclone track information for both the eastern Pacific and Atlantic
tropical cyclones was obtained from the National Hurricane Center Best Track dataset
(HURDAT). This dataset contains 6-hourly records of tropical cyclone locations and
intensities (maximum 1-min 10 m wind speeds and minimum central pressures). As noted
by Farfán (2004), the best track information in the eastern Pacific is largely derived from
satellite measures of position and intensities and not direct observations. In the Atlantic,
reconnaissance flights are commonplace for storms approaching land, thus location and
intensity estimates are determined, in large part, through direct observation.
To describe the evolution of each of the three systems in question, the European
Center for Medium Range Weather Forecasts (ECMWF) Re-Analysis (ERA) dataset
(Uppala et al. 2005) available from the National Center for Atmospheric Research
(NCAR) on an equivalent resolution of 1.125˚ x 1.125˚ (the native ERA data are stored at
T159 spectral resolution). The dataset consists of geopotential heights, temperatures, and
winds on 23 vertical levels extending from 1000 hPa to 1 hPa every 6 h. The National
Center for Environmental Prediction-National Center for Atmospheric Research
(NCEP/NCAR) reanalysis dataset (Kalnay et al 1996) is used to supplement to ERA
dataset. It is important to note that the NCEP-NCAR reanalysis data has a horizontal
resolution of 2.5˚, which implies that this data are only able to resolve large-scale
circulations. Specifically, the NCEP-NCAR reanalysis is used to investigate large-scale
climatological features of the eastern Pacific region.
Daily total precipitation data (from 12 UTC to 12 UTC) was obtained from the
National Centers for Environmental Prediction/Climate Prediction Center (NCEP/CPC)
Unified Precipitation Dataset (UPD, Higgins et al. 1996)). This national dataset
incorporates National Oceanic and Atmospheric Administration (NOAA) first-order
station precipitation measurements, daily cooperative observer measurements, and data
from NWS River Forecast Centers (RFCs), representing over 13,000 stations in the
contiguous United States. Precipitation amounts represent 24-h accumulation ending at
1200 UTC, interpolated to a 0.25 latitude–longitude grid.
All of the gridded datasets (ERA-40, UPD, and SST) were stored, analyzed, and
displayed using the Generalized Meteorological Analysis Package (GEMPAK, Koch et
al. 1983).
A potential vorticity (PV) approach is ideally suited to the study of ET events.
Several studies (e.g., Thorncroft and Jones 2000, McTaggert-Cowan et al. 2001, Attallah
et al. 2007) have utilized the PV approach to identify the important interactions between
the lower-level tropical cyclone PV maximum and upper-tropospheric PV maximum
associated with the approaching trough. Ertel PV is calculated on isobaric surfaces using
q  g
 v  u  
 
v u 
 f     g 
 ,

p 
x y 
 p x p y 
where q represents Ertel PV, θ represents the potential temperature, u and v represent the
zonal and meridional components of the wind, respectively, g is gravity, f is the Coriolis
parameter, and the other terms have their usual meteorological meaning. To track the
upper-level disturbances tied to each event, we employ a potential vorticity (PV) –
dynamic tropopause (DT) perspective (Hoskins et al. 1985; Hoskins 1990, Morgan and
Neilsen-Gammon 1998). The PV/DT perspective has been utilized in a number of studies
(Hoskins and Berrisford 1988; Dickinson et al. 1997; Hakim et al. 1996; Davis and
Bosart 2003). The DT is determined from the orientation of the 1.5 PV unit (PVU, 1 PVU
= 10-6 K m2 kg-1 s-1) surface and value of u, v,  and p are interpolated to that surface
from above (uT and pT will refer to the values of u and p on the DT, respectively).2
Finally, archived satellite imagery available for both Hurricanes Lester and
Andrew were acquired from the National Climate Data Center (NCDC) HurricaneSatellite Dataset (Knapp and Kossin, 2007). The data is available every 3 h with a
horizontal resolution of 8 km. Each satellite image is centered on temporally-interpolated
tropical cyclone best track. This dataset is the basis for the satellite-tropical cyclone
reanalysis project (Kossin and Knapp 2006). For each storm, the infrared window
brightness is plotted to show the distribution of the cloud and, presumably, precipitation
shields with respect to the salient large-scale features. This data was stored and displayed
2
Due to diabatic effects, PV values in the lower troposphere occasionally may exceed 1.5 PVU. Thus,
interpolation upward from the surface may intercept these low-level positive PV anomalies resulting in an
artificially low tropopause height.
using the MatLab graphics package. Additional diagnostics using the tropical cyclone
best track datasets were performed using MATLAB.
3. Storm Histories
a) Hurricane Lester
The track of Hurricane Lester is shown in Figure 1. Hurricane Lester developed
from a tropical wave that moved westward across the Atlantic Ocean and Caribbean Sea
and crossed Central America (Lawrence and Rappaport 1994). The disturbance was
classified a tropical depression on 20 August and moved to the northwest and
strengthening to tropical storm intensity in 24 h. By 18 UTC 22 August, Lester had
obtained hurricane strength, roughly 400 km west of La Paz and 290 km south of Punta
Abreojos, Baja California. At this point, Lester was moving nearly due north ahead of an
elogated positively-tilted trough that was moving into the eastern Pacific (Figure ). Lester
made landfall as a tropical storm just north of Punta Abreojos near 1000 UTC 23 August.
Lester continued north-northeastward into the southwestern United States. The Lester
circulation produced a significant amount of precipitation over the desert southwest,
including amounts in excess of 80 mm in central Arizona a 40 mm in a band extending
from northwest New Mexico, through Colorado and Nebraska.
As shown in Figure 1a, much of the precipitation associated with Hurricane
Lester fell to the west of the storm track. Recent work by Atallah and Bosart (2003) and
Atallah (2004) has found that the majority of the precipitation from tropical systems
undergoing ET occurs to the left of the storm track. This is a result of the strong upperlevel dynamical forcing of the approaching trough interacting with the tropical cyclone
vorticity and moisture fields. The precipitation distribution associated with Hurricane
Lester supports the assertion that this is the first documented case of ET of an eastern
Pacific storm.
b) Hurricane Andrew
It is certainly understandable why the landfall and subsequent extratropical
transition of Hurricane Lester in the eastern Pacific was largely ignored. At the same
time, Hurricane Andrew was bearing down on south Florida. Andrew made landfall as a
Category 5 (after reanalysis) storm near Homestead FL. Much has been written about the
history of Hurricane Andrew (e.g., Landsea et al., 2004) and a detailed synopsis of this
entire event will not be provided here. Hurricane Andrew crossed south Florida and
emerged in the eastern Gulf of Mexico as a Category 4 storm and made a second landfall
near Pt. Chevreuill, LA as a Category 3 storm. Andrew continued northward and turned
northeastward across eastern Tennessee.
Unlike Lester, the Andrew’s precipitation is primarily along the track initially
after landfall (Figure 1b), suggesting the Andrew circulation was not significantly
initially impacted by the approaching trough. However, as Andrew continued
northeastward, the maximum precipitation shifted to the west of the track, indicating the
influence of the mid-latitude trough and potentially signifying ET.
4. Results
a) Climatology
It has been argued that the synoptic-scale environment in the eastern Pacific is
typically not conducive to ET events. Figure 2a shows all landfalling eastern Pacific
tropical cyclones that propagated northward reaching at least 25˚ N for the period 19582004. The 25˚N threshold is designed to focus on events that could impact northwestern
Mexico and the southwestern United States. During this 46 year period, 40 tropical
cyclones (roughly 9 every 10 years) satisfied this criterion. The most active periods were
the 1960s (red) and 1970s (blue) featuring 12 and 10 events, respectively. The 1980s
(green), however, had only 4 events satisfy the criteria. The causes of the observed multidecadal variability are unknown and are beyond the scope of the present study. Figure 2b
shows how often an eastern Pacific tropical storm of any intensity, comes within 250 km
of any given point. This figure can be considered to show the preferred storm tracks for
eastern Pacific tropical cyclones. The construction of this figure is similar to Figure 1 of
Larson et al. (2004). The most frequent activity occurs at the southern portion of the
domain, with return rates of approximately 1, corresponding to one event every year
satisfies the criteria. The return rate decreases markedly poleward such that roughly one
tropical storm every ten years reaches the southwestern United States.
b) Large-scale evolution
In the days leading up to the landfall of Hurricane Lester, there was an anomalous
ridge in place over the west coast of the U. S. with an anomalous trough over the central
Pacific (Figure 3a, d). The orientation of the ridge-trough couplet was oriented to steer
eastern Pacific tropical systems northward. As Hurricane Lester made landfall and
proceeded inland, the 200 hPa pattern over the U. S. over the following three day period
has become highly amplified (Figure 3b, e). Strong positive height anomalies (amplified
ridge) developed over eastern Canada while strong negative anomalies developed over
the central portions of the U.S. It will be shown in subsequent figures that this pattern
amplification was due in large part to diabatic heating from both Tropical Storm Lester
and Hurricane Andrew. Note the development of a narrow tongue of negative height
anomalies over the western Atlantic (Figure 3 e). This configuration is the precursor to a
cyclone wave breaking event (Thorncroft et al. 1993). The result of the cyclonic wave
breaking is an upper-level closed circulation.
After Hurricane Andrew’s second landfall in Louisiana, the final three day period
(Figure 3 c, f) reveals a strong ridge over New England and eastern Canada. Height
anomalies over the central and western U. S. have weakened as the tropical remnants
have moved off the east coast. The aforementioned cutoff cyclone has developed over the
western Atlantic just off the southeast coast of the United States. From this cutoff
cyclone, a low-level cyclonic circulation developed. As will be illustrated later in the text,
this system attempted to undergo tropical transition (Davis and Bosart reference) but did
not complete the transition. The factors inhibiting the tropical transition process will be
discussed.
Another method of evaluating the evolution of the large scale is through the use of
Hovmoller diagrams. The trough of interest (labeled T1 in Figure 4) enters the domain
around 18 August and proceeds steadily eastward. The trough strengthens, as indicated
by the lowering of the pressure on the DT (to below 480 hPa) and the tightening of the
horizontal gradient with the approach of Tropical Storm Lester (labeled L). Notice the
interaction of Lester with the trough has stalled the eastward progression of the system to
about 26 August. After this point, the trough steadily moves eastward. Again, there the
lowering of the pressure on the DT (to below 550 hPa) and a tightening of the gradient as
the remnants of Andrew (labeled A) interact with the trough.
The second feature of interest, T2, travels eastward across the eastern portion of
the country. The trough is steadily weakening as reflected by the lowering of the
pressures on the DT (consistent with a lifting of the DT). Near 23 August, there appears
to be a split in the trough with a region of lower pressures on the DT splits from the main
trough and slowly drifts westward and gradually weakens. As will be shown, this feature
is responsible for the generation of the disturbance that failed to complete tropical
transition.
The evolution of the large-scale is presented in Figure 4. Beginning with 1200
UTC 22 August (Figure 4a), the positively-tilted trough is cross the California coast at
this time. Hurricane Lester is heading northward and is just west of the Baja Peninsula at
this time. Over the next 24 h (Figs 4b and c), Lester continues northward and interacts
with the approaching trough. The 700 hPa vorticity associated with Lester appears to
intensify over the 24 h period. It is unclear, however, whether this intensification is a
result of the interaction with the trough or if it is due to the system being better resolved
by the observations (and thus the reanalysis). By 1200 UTC 25 August (Figure 4d), the
remnants of Hurricane Lester appear over southwestern Nebraska within an elongated
band of vorticity oriented along the leading edge of the trough.
During the period from 1200 UTC 23 August (Figure 4b) to 1200 UTC 25 August
(Figure 4d), there is evidence of PV filamentation, consistent with anticyclonic wave
breaking (Thorncroft reference), and the development of a cold pool on the DT centered
near 60W and 30N (Figure 4d). This wave breaking event is associated with the
developing low-level cyclonic vorticity center. While the low-level vorticity center did
strengthen, tropical transition did not ensure.
By 1200 UTC 26 August (Figure 6e), Hurricane Andrew has made a second
landfall in central Louisiana. The trough has continued to progress eastward, with the
leading edge of the trough extending from the upper Midwest to New Mexico. Though
difficult to track at this time, the remnants of Lester are likely passing through Iowa.
Twenty-four hours later (Figure 4f), Andrew is steered to the north-northeast as the
trough approaches. By 1200 UTC 28 August (Figure 4g), Andrew is centered over
western portions of South Carolina. At this time, the potential temperature gradient on the
DT has tightened, reflecting a strengthening of the trough. In addition, low-level vorticity
to the north of the Andrew circulation has strengthened considerably over the last 24 h.
The rapid development of the vorticity center north of Andrew is a clear indication that
extratropical cyclogenesis has occurred. By 1200 UTC 29 August (Figure 4h), a major
extratropical cyclone is centered over southern Ontario. The remnants of Andrew,
however, remain displaced to the east and are centered over eastern New England at this
time.
Figure 4 illustrates the impact of weakening tropical cyclones on the large-scale
flow. For both Lester and Andrew, a strengthening of the approaching trough and
amplification of the downstream ridge occurred. The likely cause for these developments
is diabatic heating. Figure 5 is designed to isolate the impacts of the diabatic
contributions from Lester and Andrew by simply showing the difference between the
local change of potential temperature on the DT from the advection of potential
temperature on the DT. If the flow was adiabatic, this value would be 0.
c) Storm-scale evolution
The proceeding section evaluated the evolution of the synoptic-scale flow in
response to the interactions (largely through diabatic processes) of Hurricanes Lester and
Andrew. The following section focuses on the evolution of the storm-scale features. Here
we use archived satellite imagery (Knapp and Kossin reference) in conjunction with the
ERA gridded fields. Thus, there is a directly observed quantity that can be used to
validate the ERA products on the storm scale.
Hurricane Lester
Hurricane Andrew
The Non-developing event
Finally, the evolution of the non-developing case over the western Atlantic will be
discussed. As mentioned previously, the origin of the Null event is related to an upperlevel wave breaking event which deposited a cold-core cyclone over the central Atlantic.
This evolution of this system is summarized in the CPS diagram (Figure 12). According
to the CP diagram, the event develops as a cold-core extratropical system. Over the
period from 25 August to 28 August, the system shifted toward the tropical portion of the
phase space diagram suggesting that beginning of tropical transition. Ultimately tropical
transition never occurred as the system never became a warm core system. By 28 August,
the storm had moved poleward over cooler sea-surface temperatures (inset, Figure 10).
The subsequent discussion examines reasons why the tropical transition was not
successful in the time prior to 28 August.
The evolution of the system is illustrated by the evolution of the 850 – 200 hPa
thickness field (dashed) and 850 hPa relative vorticity (solid) in Figure 12. Beginning
1200 UTC 24 Aug, the thickness feature, oriented southwest – northeast, is still
connected to the flow at higher latitudes. Over the subsequent 24 hours, the thickness
tongue separates from the flow at higher latitudes and consolidates into a coherent closed
circulation center (Fig 11 a – d). In response to the development of the upper-level cold
core circulation, there is a corresponding organization and strengthening of low-level
cyclonic vorticity (solid contours, Figure 11) signifying the development of a nearsurface circulation center beneath the upper-level system. The low-level vorticity center
steadily strengthens reaching peak intensity between 0000 and 1200 UTC 27 August
(Figure 11 f – g, 12b). This related to a marked decrease in the vertical windshear (Figure
12, top). Beginning around 1200 UTC 26 August, the sea-level pressure steadily dropped
as the vertical wind shear fell below 10 m s-1. Despite the development of a robust
surface circulation, the deep convection (shaded. Figure 10) never consolidates around
the center of circulation. Further, the surface circulation remains under the upper-level
cold core. A warm 850-200 hPa thickness thermal ridge never develops near the surface
center.
While conditions appeared favorable for the tropical transition process to occur,
no development occurred. The key question to address is why the convection never
consolidated around the circulation center between 0000 UTC 26 August and 1200 UTC
27 August. During this time, the disturbance was located over sufficiently warm water (>
26°C). The answer lies in the vertical moisture profile around the storm center (Figure
13). While the low-levels where sufficiently moist, a band of dry air located between 500
and 350 hPa was present throughout the life cycle of this system. The presence of this dry
air prevented meaningful development of deep convection around the storm center.
Further, beginning near 1200 UTC 27 August, as the low-level circulation center begins
to move northward away from the 850 – 200 hPa cold core (Figure 10), the system moves
into a region where the deep-layer vertical wind shear begins increasing sharply and the
underlying sea-surface temperatures are cooling rapidly. The dry air together with the
increasing vertical wind shear and cooling sea-surface temperatures combined to inhibit
the tropical transition process.
Summary
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List of Figures
Figure 1: a) Tracks of all recurving E. Pac storms, b) Return rate
Figure 2: Track and accumulated precipitation for a) Lester and b) Andrew
Figure 3: Mean height and hght anomaly at 200Pa (18-21 Aug, 22-25 Aug, 26-29 Aug).
Add 500 hPa?, normalized height? Thickness?
Figure 4: DT θ and winds and 700 hPa ζ - every 12 h starting from 920822/1200 – large
view
Figure 5: Hovmoller diagram of DT P or (θ) – large view
Figure 6: PMSL, 850-200 hPa Thickness, and 700-400 hPa ζ – every 12 h – large view
Figure 7: 850 hPa hght, winds, and θe – large view
Figure 8: PMSL, 850-200 hPa thick ness, and 700-400 hPa ζ – every 6 h focused on
Lester ET
Figure 9: DT θ and winds and Kossin Imagery – same times as Figure 8
Figure 10: Les phase-space diagrams
Figure 11: as in Fig. 8 but focused on Andrew transition
Figure 12: As in Figure 9 but for Andrew
Figure 13: As Figure 10 but for Andrew
Figure 14: Available satellite imagery showing ND event – refer back to 3, 4, 5
Figure 15: Hart Phase space diagram (includes SST)
Figure 16: Time series of DT θ (or P) and vertical wind shear (850-200 hPa) and 700 hPa
ζ
Figure 17: Time-cross-section relative humidity for ND