4. Composite Analysis
The distribution of event type for ice storms impacting the ALY CWA is
generally similar to the distribution of event type for all 137 ice storms. Of the 35 ice
storms affecting the ALY CWA, 24 (68.6%) were Type G events, seven (20.0%) were
Type BC events, and three (8.6%) were Type EF events (see Section 2.3 for definitions of
each event type). Thus, we have created composite maps and composite cross sections for
Type G (N=24) and Type BC (N=7) events.
4.1 Type G Events
4.1.1 Synoptic Evolution
Figures 4.1a–c illustrate the evolution of the large-scale circulation prior to Type
G events. At t−48, a deep 500-hPa trough is located over southeastern Canada and a lowamplitude ridge begins to develop across the Mississippi Valley (Fig. 4.1a). The
juxtaposition of the trough and ridge establishes a large meridional geopotential height
gradient across the Great Lakes region and a broad area of confluent flow throughout
central and eastern North America. Looking west, a short-wave trough is present over the
Great Basin, well upstream of the building ridge. As time progresses, the western U.S.
trough approaches the Central Plains, while the downstream ridge amplifies and moves
toward the East Coast (Fig. 4.1b). Although the northern trough exits the Canadian
Maritimes by t–24, the 500-hPa flow remains confluent near the U.S.–Canadian border. At
t0, the upstream trough is positioned over the Midwest and the downstream ridge
continues to amplify as it departs the East Coast (Fig. 4.1c).
49
Figures 4.2a–c illustrate the evolution of the synoptic-scale thermodynamic
environment prior to Type G events. At t–48, a large swath of standardized precipitable
water (PW) anomalies exceeding +1σ covers the southern Plains (Fig. 4.2a). The highest
PW values (> 20 mm) are accompanied by south-southwesterly 850–700-hPa layeraveraged winds extending northward from the western Gulf of Mexico. By t–24, the main
PW axis has migrated into the Mississippi Valley, where maximum standardized PW
anomalies now exceed +1.5σ (Fig. 4.2b). Based on the orientation of the 850–700-hPa
layer-averaged 0°C isotherm, southwesterly flow also implies low- to midlevel warm
advection and thermal ridge amplification throughout the Ohio Valley and southern Great
Lakes region. At t0, the highest PW values are found along the East Coast, and
standardized PW anomalies exceed +2σ over portions of the mid-Atlantic region,
southeastern New York, and southern New England (Fig. 4.2c). These large standardized
PW anomalies result from northeastward moisture transport via strong low- to midlevel
southwesterly flow. Furthermore, pronounced warm advection between t–24 and t0 has
displaced the 0°C isotherm north of Albany, NY. The combination of abundant moisture
and warm air aloft establishes a thermodynamic environment favorable for a prolonged
freezing rain event.
Figures 4.3a–c illustrate the evolution of the 300-hPa wind speed, 1000–500-hPa
thickness, and mean sea level pressure fields prior to Type G events. At t−48, we see an
upper-level jet maximum and a low-level baroclinic zone across the Great Lakes and
Northeast (Fig. 4.3a). The associated thickness trough is centered upstream of a
meridionally elongated surface pressure tough (i.e., cold front), which suggests that lowlevel cold advection precedes the event and establishes an air mass sufficiently cold for
50
freezing precipitation in the interior Northeast. As time progresses, an inverted surface
pressure trough initially over the Four Corners region migrates toward the Mississippi
Valley and approaches the equatorward jet entrance region. A surface anticyclone tracks
eastward from the Upper Midwest and strengthens as it becomes co-located with the jet
maximum and the baroclinic zone (Fig. 4.3b). Given the abundance of cold air near the
U.S.–Canadian border, one may speculate that the anticyclone plays an important role in
reinforcing subfreezing surface air across the northeastern U.S. (there is evidence of weak
cold-air damming east of the Appalachians). By t0, a surface cyclone has developed from
the existing pressure trough and moved rapidly toward the eastern Great Lakes (Fig.
4.3c). The enhanced curvature of the isobars along the Appalachians and the mid-Atlantic
coast corresponds to surface cold and warm fronts, respectively (the pressure trough axes
are parallel to the thickness contours). Meanwhile, the position of the upper-level jet
maximum places upstate New York within the equatorward jet entrance region, where we
would expect QG forcing for ascent and a thermally direct vertical circulation.
4.1.2 Synoptic–Mesoscale Linkages
Figure 4.4 illustrates the 700-hPa QG forcing (from the Q-vector perspective)
associated with Type G events at t0. As previously inferred from Fig. 4.3c, the strongest
QG forcing for ascent (indicated by warm colors and Q-vector convergence) is located
within the equatorward jet entrance region, just northeast of the surface cyclone.
Equatorward jet entrance regions are typically characterized by thermally direct
ageostrophic transverse circulations in which warm (cold) air ascends (descends) in
response to ageostrophic divergence (convergence) at upper levels and ageostrophic
51
convergence (divergence) at lower levels. Meanwhile, southerly (northerly) ageostrophic
flow occurs at upper levels (lower levels) in the horizontal branches of the circulation.
Assuming a predominantly meridional temperature gradient, low-level cold advection by
ageostrophic northerlies provides a critical mechanism for maintaining a shallow layer of
cold air near the surface. Keyser et al. (1988) and Sanders and Hoskins (1990) recognized
that one could diagnose regions of frontogenesis and frontolysis based on the orientation
of the Q-vectors with respect to the temperature gradient. In Fig. 4.4, the Q-vectors are
perpendicular to the isotherms such that they point from colder air to warmer air (parallel
to the temperature gradient) below the jet entrance region, and thus we would expect lowto midlevel frontogenesis from upstate New York eastward across New England.
In order to further examine the ageostrophic transverse circulation and
frontal structure, we created a composite cross section centered on Albany, NY (42.75°N,
73.80°W), and extending 10°N and 10°S (Fig. 4.5). Looking north, we see a thermally
direct vertical circulation, with deep ascent on the warm side, and northerly (southerly)
ageostrophic flow in the lower (upper) troposphere. Looking south, we see a region of
frontogenesis [> 1 K (100 km)−1 (3 h)−1] sloping upward from the surface near Atlantic
City, NJ to roughly 900 hPa above Albany, NY. Previous studies (e.g., Roebber and
Gyakum 2003; Ressler et al. 2012) have suggested that low-level frontogenesis may play
a crucial role in prolonging freezing rain events and providing a focus for low-level
convergence and ascent. Sloped ascent above the warm front is indicative of frontal
overrunning, while ageostrophic cold advection below 925 hPa reinforces subfreezing air
near the surface. These two processes acting simultaneously help maintain a
thermodynamic profile conducive to freezing rain.
52
4.2 Type BC Events
4.2.1 Synoptic Evolution
Figures 4.6a–c illustrate the evolution of the large-scale circulation prior to Type
BC events. At t−48, the 500-hPa geopotential height pattern is characterized by a broad,
low-amplitude ridge over the eastern U.S., a positively tilted short-wave trough near the
southern Rockies, and a second short-wave trough over south-central Canada (Fig. 4.6a).
Between t−48 and t−24, the amplification of the northern trough and eastern U.S. ridge
results in the intensification of the meridional geopotential height gradient and enhances
the confluence throughout eastern North America. By t−24, the southern trough is located
over the southern Plains, but the rapid progression of the northern trough has caused the
two troughs to become out-of-phase with each other (Fig. 4.6b). This out-of-phase
relationship allows shallow cold air to travel southeastward in wake of the northern
trough before the southern trough approaches the eastern U.S. At t0, the trough–ridge
dipole is centered on 60°W and the greatest confluence occurs over southern Quebec
(Fig. 4.6c). Meanwhile, the southern trough weakens and becomes negatively tilted as it
migrates toward the Tennessee Valley.
Figures 4.7a–c illustrate the evolution of the synoptic-scale thermodynamic
environment prior to Type BC events. At t–48, an axis of high PW extends from the
western Gulf of Mexico through the Mississippi Valley and into the Ohio Valley (Fig.
4.7a). This moisture plume corresponds to a swath of standardized PW anomalies
exceeding +1σ and low- to midlevel southwesterly flow. A thermal ridge exists over the
southern Great Lakes region, with west-southwesterly winds supporting weak warm
53
advection across the interior Northeast. As time progresses, persistent southwesterly flow
maintains the northeastward surge of Gulf moisture. By t−24, an extensive area of
standardized PW anomalies exceeding +1σ envelopes the Tennessee Valley, the Ohio
Valley, and the Appalachians, with the greatest standardized PW anomalies (> +1.5σ)
along the western slopes of the central Appalachians (Fig. 4.7b). A secondary PW
maximum develops near the Southeast coast (within the western quadrant of a synopticscale anticyclonic circulation) and suggests that the Atlantic Ocean also serves as a
primary moisture source during Type BC events. Although the thermal pattern in Fig.
4.7b is clearly more amplified than in Fig. 4.2b, the 850–700-hPa wind vectors are nearly
parallel to the 0°C isotherm, and the implied thermal advection in Fig. 4.7b is
considerably weaker than in Fig. 4.2b. Between t−24 and t0, the low- to midlevel flow
becomes southerly and supports increased warm advection and Atlantic moisture
transport over eastern New York and New England. At t0, the moisture plume resembles
the comma cloud structure often associated with mature extratropical cyclones, in which
the highest PW is found ahead of the cold front and along the warm front (Fig. 4.7c).
Maximum standardized PW anomalies now exceed +2σ throughout the mid-Atlantic
region, southeastern New York, and southern New England (as in Fig. 4.2c).
Figures 4.8a–c illustrate the evolution of the 300-hPa wind speed, 1000–500-hPa
thickness, and mean sea level pressure fields for Type BC events. At t−48, an upper-level
jet maximum and a low-level baroclinic zone extend across the Great Lakes region (Fig.
4.8a). Whereas the jet streak in Fig. 4.3a is cyclonically curved and coincident with a
thickness trough, the jet streak in Fig. 4.8a is anticyclonically curved and coincident with
a thickness ridge. Thus, unlike Type G events, Type BC events are not preceded by a
54
deep cold-air intrusion. By t−24, the upper-level jet maximum has migrated over southern
Quebec and intensified in excess of 60 m s−1, while surface cyclogenesis is occurring
south of the equatorward jet entrance region (Fig. 4.8b). The strengthening of the jet
maximum corresponds to a significant increase in the thickness gradient as a thickness
trough enters eastern Canada. Despite the general lack of cold air aloft, the eastward
progression of the thickness trough and trailing anticyclone allows shallow cold air to
gradually migrate across the interior Northeast. Between t−24 and t0, the surface cyclone
turns northeastward and intensifies considerably as it approaches mid-Atlantic coast (Fig.
4.8c). As in Fig. 4.3c, the enhanced curvature of the isobars south and east of the low
pressure center correspond to surface cold and warm fronts, respectively. The meridional
pressure dipole (higher pressure to the north and lower pressure to the south) implies
northeasterly flow throughout northern New York and New England, which may act to
reinforce shallow cold air and retard the northward progression of the surface baroclinic
zone. At t0, the upper-level jet maximum is located over the Canadian Maritimes and
upstate New York is juxtaposed with the equatorward jet entrance region. New York also
lies just north of the left exit region of a second jet streak that develops along the East
Coast. The resulting coupled jet structure provides a favorable environment for both
surface cyclone intensification and ascent across the interior Northeast.
4.2.2 Synoptic–Mesoscale Linkages
Figure 4.9 illustrates the 700-hPa QG forcing associated with Type BC events at
t0. As in Fig. 4.4, the greatest QG forcing for ascent occurs within the equatorward jet
entrance region over upstate New York, but the Q-vector forcing is markedly stronger.
55
During Type BC events, the presence of a coupled jet structure along the East Coast
likely enhances upward vertical motion near the surface cyclone. Across eastern New
York, the Q-vectors are parallel to the temperature gradient and thus imply low- to
midlevel frontogenesis. However, given the difference in Q-vector magnitude between
Type G and Type BC events, we can infer that the frontogenesis is more pronounced
during Type BC events. Considering that Q-vector magnitude is related to the strength of
the ageostrophic circulation, we may also expect enhanced low-level ageostrophic
northerlies and ageostrophic cold advection below the jet entrance region.
Figure 4.10 shows the composite cross section for Type BC events at t0. As in Fig.
4.5, we see a thermally direct vertical circulation centered north of Albany, NY, and a
sloping region of frontogenesis below the equatorward jet entrance region. However, the
ageostrophic transverse circulation is stronger and more horizontally confined, while the
low- to midlevel frontogenesis is deeper and more robust in Fig. 4.10 than in Fig. 4.5.
During Type BC events, the ageostrophic transverse circulation tilts toward the cold air
with height and closely resembles the idealized secondary circulation described by
Holton (2004; p. 278–279). Moreover, the ageostrophic circulation follows the region of
sloped frontogenesis extending from the surface (off the New Jersey coast) to at least 750
hPa (above the St. Lawrence Valley). On the poleward side of the warm front, strong
low-level ageostrophic northerly winds help reinforce cold air near the surface.
56
N = 24
(a)
N = 24
(b)
57
N = 24
(c)
Figure 4.1: Composite 500-hPa geopotential height (contoured every 6 dam) and
anomalies (shaded every 30 m) for Type G events at: (a) t−48, (b) t−24, and (c) t0. Green
star denotes the approximate location of Albany, NY.
58
N = 24
10 m s−1
(a)
N = 24
10 m s−1
(b)
59
N = 24
10 m s−1
(c)
Figure 4.2: Composite 850–700-hPa layer-averaged wind (arrows, m s−1) and 0°C
isotherm (dashed), precipitable water (green, every 4 mm), and standardized precipitable
water anomalies (shaded, every 0.5σ) for Type G events at: (a) t−48, (b) t−24, and (c) t0.
Green star denotes the approximate location of Albany, NY.
60
N = 24
(a)
N = 24
(b)
61
N = 24
A
A′
(c)
Figure 4.3: Composite 300-hPa wind speed (shaded, every 5 m s−1), 1000–500-hPa
thickness (dashed, every 6 dam), and mean sea level pressure (black, every 4 hPa) for
Type G events at: (a) t−48, (b) t−24, and (c) t0. Green star denotes the approximate location
of Albany, NY. Red line in Fig. 4.3c marks the y–z cross section (extending from A to
A′) plotted in Fig. 4.5.
62
N = 24
10 x 10 K m−1 s−1
Figure 4.4: Composite 700-hPa temperature (dashed, every 3 K), Q-vectors (arrows,
10−11 K m−1 s−1), and RHS Q-vector form of QG omega equation (shaded, every 2.5 ×
10−16 K m−2 s−1) for Type G events at t0. Green star denotes the approximate location of
Albany, NY.
63
N = 24
A
A′
−1
5ms
−1
5 cm s
Figure 4.5: Composite cross section centered on Albany, NY, extending from 52.75°N,
73.80°W (A) to 32.75°N, 73.80°W (A′). Frontogenesis [shaded, every 1 K (100 km)−1 (3
h)−1], potential temperature (black, every 2 K), wind speed (green, every 5 m s−1), vertical
velocity (dashed red, every 5 μb s−1, only negative values plotted), and ageostrophic
circulation (arrows) for Type G events at t0. The top arrow is the reference vector for
horizontal ageostrophic motion, and the bottom arrow is the reference vector for vertical
motion. Red star denotes the approximate location of Albany, NY.
64
N=7
(a)
N=7
(b)
65
N=7
(c)
Figure 4.6: As in Fig. 4.1, except for Type BC events.
66
N=7
10 m s−1
(a)
N=7
10 m s−1
(b)
67
N=7
10 m s−1
(c)
Figure 4.7: As in Fig. 4.2, except for Type BC events.
68
N=7
(a)
N=7
(b)
69
N=7
A
A′
(c)
Figure 4.8: As in Fig. 4.3, except for Type BC events. Red line in Fig. 4.8c marks the y–
z cross section (extending from A to A′) plotted in Fig. 4.10.
70
N=7
10 x 10 K m−1 s−1
Figure 4.9: As in Fig. 4.4, except for Type BC events.
71
N=7
A
A′
−1
5ms
−1
5 cm s
Figure 4.10: As in Fig. 4.5, except for Type BC events.
72