VOLUME 126
MONTHLY WEATHER REVIEW
SEPTEMBER 1998
A Numerical Modeling Study of Mesoscale Cyclogenesis to the
East of the Korean Peninsula
TAE-YOUNG LEE
AND
YOUNG-YOUN PARK
Department of Atmospheric Sciences, Yonsei University, Seoul, Korea
YUH-LANG LIN
Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina
(Manuscript received 9 September 1996, in final form 26 August 1997)
ABSTRACT
Numerical simulations and the analysis of observational data are employed to understand the mesoscale
cyclogenesis in a polar airstream that occurred over the sea to the east of the Korean peninsula on 28–29 January
1995. The observational analysis shows that a mesoscale low develops over the southeastern East Sea (Japan
Sea) on 29 January 1995. Satellite imagery also indicates that a meso- b-scale vortex forms on the lee side of
the northern Korean mountain complex (KMC), which is located in the northern Korean peninsula, and that a
meso-a-scale cyclone develops over the southeastern East Sea at a later time. The mesoscale cyclone forms in
the lower troposphere with strong baroclinicity and cyclonic circulation under the influence of an upper-level
synoptic-scale cold vortex.
Numerical simulation has captured major features of the observed cyclogenesis very well. The cyclogenesis
occurs in a progressive manner. Basically, four distinctive stages of the cyclogenesis are identified. 1) First, a
surface pressure trough forms on the lee side of the KMC under a northwesterly synoptic-scale flow that is
deflected anticyclonically over the KMC. 2) Second, the lee trough deepens further into a strong convergence
zone and a meso-b-scale vortex. 3) Next, the meso-b-scale vortex develops into a meso-a-scale vortex as the
vortex and the trough begin to move southeastward from the lee of the KMC. 4) Finally, the surface trough
deepens into a closed low and the meso-a-scale vortex becomes collocated with this deepening surface low to
form a meso-a-scale cyclone over the southeastern East Sea.
Several sensitivity experiments are performed to isolate the effects of a topography, warmer sea surface,
diurnal thermal forcing, and latent heat release. During stages 1 and 2, it is found that the KMC and low-level
baroclinicity are responsible for generating the strong lee trough and vortex. During stage 3, the development
of the meso-a-scale vortex is brought on by the tilting of horizontal vorticity and vertical stretching in a synopticscale cyclonic circulation. In the final stage, the condensational heating plays the key role for the development
of the meso-a-scale cyclone under the influence of an upper-level synoptic-scale cold vortex. The presence of
the warm sea surface is found to be a necessary condition for the development of a polar air convergence zone
and the mesoscale cyclone. It is also found that the low-level baroclinicity is essential for the present case of
mesoscale cyclogenesis.
1. Introduction
Satellite imagery often shows the development of
meso-a-scale lows over the ocean to the east of the
Asian continent during winter outbreaks of polar air
(Ninomiya 1989; Tsuboki and Wakahama 1992). Some
of these lows are found over the East Sea (Japan Sea)
between the northeastern coast of the Korean peninsula
and the west coast of Japan. Once these lows form, some
of them keep developing when they are moving northeastward along the west coast of Japan. Although the
development of the mesoscale lows over the ocean
Corresponding author address: Dr. Tae-Young Lee, Department of
Atmospheric Sciences, Yonsei University, Seoul 120-749, Korea.
E-mail: [email protected]
q 1998 American Meteorological Society
around northern Japan have been studied extensively,
the genesis mechanism of these mesoscale lows to the
east of the Korean peninsula is still not well understood
and deserves further study.
Ninomiya (1989) found that polar lows were observed
to form about 500–1000 km north of major polar frontal
zones, where strong low-level baroclinicity is maintained by sea surface fluxes in the polar air mass between the continent and the relatively warm ocean. He
defined the polar low in this region as a meso-a-scale
low accompanied by spiral or comma cloud system with
a scale of 200–700 km. He also found that these mesoscale lows rarely appear over the Asian continent,
Yellow Sea, and East China Sea.
As the polar air streams out over the ocean from the
Asian continent, surface weather maps often show a
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VOLUME 126
FIG. 1. Geographic map and smoothed topography in the mesoscale model domain. Contour
interval is 200 m. The KMC represents the northern Korean mountain complex, located in the
northern part of the Korean peninsula.
sharp trough that starts from immediately to the lee side
of the mountains in the northern Korean peninsula, that
is, the northern Korean mountain complex (referred to
as KMC hereafter) (Fig. 1), and extends toward the
ocean. Satellite imagery sometimes shows mesoscale
vortices that form to the southeast of the KMC and
propagate southeastward. These characteristics suggest
that the topography of the Korean peninsula may play
some significant role in the mesoscale cyclogenesis to
the east of the peninsula.
Previous studies have revealed the importance of the
KMC on the mesoscale disturbances around the peninsula. Yagi et al. (1986) indicated that the low-level
polar air convergence zone to the east of Korea may
result from the dynamic effect of the KMC. Nagata
(1991) suggested that the blocking effect of the KMC,
the land–sea thermal contrast, and the characteristic SST
distribution equally contribute to the formation of the
convergence zone. Asai (1988) showed that zones of
frequent occurrence of mesoscale vortices were found
over the sea to the east of the Korean peninsula and to
the west of Hokkaido Island, Japan, and indicated that
these zones were collocated with the polar air convergence zone. Nagata (1993) proposed that barotropic
shear instability was the dominant development mechanism of meso-b-scale vortices along this zone. Direct
effects of the KMC on the formation of mesoscale cy-
clones over the East Sea, however, are rarely discussed
in the previous studies.
The KMC consists of various peaks with heights
greater than 2 km, and the length and width of the area
above 1-km height are about 320 and 100–160 km,
respectively. The shape of the KMC is asymmetric with
a steeper slope to the east coast. The Froude number
for a typical flow during night around the KMC is about
0.3–0.7, which is sufficiently low for the mountains to
affect the flow significantly according to the previous
studies [e.g., Smolarkiewicz and Rotunno (1989a,b);
Lin et al. (1992)]. Lee and Park (1996) indicated that
the KMC may be directly responsible for the formation
of some mesoscale disturbances around the Korean peninsula.
In simulating an inviscid flow over the Hawaiian
mountains, Smolarkiewicz et al. (1988) found that a pair
of vortices formed on the lee side of the island, which
then shed downstream at later times. Smolarkiewicz and
Rotunno (1989a,b) found that this type of lee vortex
may occur when a low Froude number, inviscid, nonrotating stratified flow passes over an isolated mountain.
According to Smolarkiewicz and Rotunno, the Froude
number, defined as U/Nh, where U is the basic wind
speed, N the Väisälä frequency, and h the mountain
height, needs to be less than 0.5 in order to produce the
lee vortices. Higher Froude number flows have also been
SEPTEMBER 1998
LEE ET AL.
found to be able to produce lee vortices (Lin et al. 1992).
The lee vortex is formed by either the baroclinically
generated vorticity (Smolarkiewicz and Rotunno
1989a,b) or the generation of potential vorticity (Smith
1989). These types of lee vortices have also been simulated in numerical experiments of the Denver cyclone
(Crook et al. 1990) and the Taiwan mesolow (Sun et al.
1991; Lin et al. 1992). In the Taiwan case, Lin et al.
(1992) also found that the cyclonic vortex collocates
with the mesolow in a rotating fluid flow system and
may therefore be classified as a mesocyclone.
Smith (1984, 1986) proposed a theory which views
lee cyclogenesis as the formation of the first trough of
a standing baroclinic wave. The theory requires that the
basic wind reverses its direction at a certain level. This
wind reversal height (i.e., critical level in a steady-state
flow) satisfies the general conditions observed to accompany lee cyclogenesis in the Alps, since lee cyclogenesis is often associated with the passage of a cold
front there. This theory has also been applied to explain
cyclogenesis in the lee of the Appalachians (Smith
1986). This type of lee cyclogenesis is found to be more
effective in a nonlinear flow (Lin and Perkey 1989). The
splitting of low-level flow is more pronounced for a low
Froude number flow. The ageostrophic advection of cold
air is able to strengthen the mountain-induced high and
the lee cyclone. In addition, lee cyclogenesis is strengthened by both the low-level sensible heating and the
turning of the wind associated with boundary layer processes. Therefore, the presence of low-level baroclinicity across the east coast of the Korean peninsula and
the boundary layer forcing associated with the warm
sea surface may play a similar role in the formation of
the lee cyclone over the KMC.
Mechanisms for the formation and development of
meso-a-scale lows have been a subject of considerable
interest, especially for the polar lows over the North
Atlantic Ocean, the Norwegian Sea, the Barent Sea, and
the Gulf of Alaska, as well as the lows around Japan.
Mansfield (1974) suggested that polar lows were shallow baroclinic disturbances, whereas Rasmussen (1979)
viewed the polar low as an extratropical disturbance
driven by conditional instability of the second kind
(CISK). Later, Forbes and Lottes (1985) suggested that
both the baroclinic and CISK mechanisms were important for polar low development. Based on observations,
Bond and Shapiro (1991) found that mesoscale cyclogenesis exists in the large-scale parent low over the Gulf
of Alaska and suggested frontogenesis at low levels as
a polar low genesis mechanism. Douglas et al. (1991)
suggested that the observed evolution of the polar low
over the Gulf of Alaska may be significantly influenced
by 1) flow modification by the high mountains ringing
the Gulf of Alaska, 2) the varying synoptic-scale flow
over the Gulf of Alaska, and 3) heat and moisture fluxes
from the underlying ocean surface.
Ninomiya (1991) suggested that a similar mechanism,
as proposed by Bond and Shapiro (1991), was respon-
2307
sible for the polar low genesis to the east of the Asian
continent. He suggested that the mesoscale low over the
east coast of Asia formed, under the influence of a cold
vortex aloft, in the west–east-oriented trough within the
northwestern quadrant of the synoptic-scale low that
developed over the northwestern Pacific. Tsuboki and
Wakahama (1992) suggested that the meso-a-scale cyclones off the west coast of Hokkaido Island, Japan,
were due to baroclinic instability associated with a particular baroclinic flow. It appears that the formation
mechanism of the meso-a-scale cylone or polar low in
this region is extremely complicated and deserves further study. In this study, we will focus on the earlier
stages of meso-a-scale cyclone formation to the east of
the Korean peninsula.
The meso-a-scale cyclone presented in this study occurred on 29 January 1995 over the southeastern East
Sea in the vicinity of the polar air convergence zone.
In this study, we will analyze the mesoscale cyclogenesis event and discuss its mechanism. In section 2, we
describe the formation and development of a mesoscale
vortex to the east of the peninsula based on synoptic
analyses and satellite imagery. Several numerical experiments are performed to investigate the observed mesoscale cyclogenesis. The description of the numerical
experiments is given in section 3. The results from the
control experiment and the comparison of its results
with observations are presented in section 4. In section
5, results from five idealized experiments and three sensitivity experiments with real data are discussed. Concluding remarks can be found in section 6.
2. Observational analysis
The mesoscale cyclogenesis of the present case is
observed on 28–29 January 1995 across the sea between
the KMC (northern Korean mountain complex) and the
west coast of Japan. Figure 2 shows the sea level pressure (SLP) patterns for the period of 1200 UTC 28–
1200 UTC 29 January 1995. At 1200 UTC 28 January,
two mesoscale troughs are found, one over the Sakhalin
Islands and the other over the northeastern coast of the
Korean peninsula. A mesoscale low is found off the
midwest coast of Japan. The pressure gradient is weak
throughout the eastern Asian continent and also over
the area to the west of the Korean peninsula. A significant trough has developed over Japan by 0000 UTC 29
January. At 1200 UTC 29 January, another mesoscale
low has developed off the midwest coast of Japan (near
398N, 1388E), at the location similar to that of the mesoscale low at 1200 UTC 28 January. The present study
is interested in the formation of this mesoscale low. The
pressure drops about 8 hPa during the period of 0000–
1200 UTC 29 January over the area of this low. With
the development of the mesoscale low, the zonal gradient of surface pressure has also significantly increased
over the sea between the Korean peninsula and the mesoscale low. Detailed surface pressure analysis over the
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MONTHLY WEATHER REVIEW
FIG. 2. Sea level pressure (hPa) for (a) 1200 UTC 28, (b) 0000
UTC 29, and (c) 1200 UTC 29 January 1995.
northern Korean peninsula shows that a low pressure
area is persistently found over the northeastern coast of
the peninsula, although the synoptic charts do not show
this feature well.
Satellite imagery shows that a mesoscale vortex develops during 1500–2100 UTC 28 January to the southeast of the KMC (Fig. 3a). The line of convective clouds
VOLUME 126
to the southeast of the vortex indicate the existence of
an elongated low-level convergence in a northwest–
southeast direction (Figs. 3a and 3b). The relationship
between the band of convective cloud and the lowerlevel convergence has been investigated by several studies [e.g., Yagi et al. (1986); Nagata et al. (1986)]. Hereafter, this cloud band will be called the convergent cloud
band following Nagata et al. (1986). During this early
stage, the lee vortex moves slowly southeastward (Fig.
3b).
The imagery shows the movement of the convergent
cloud band. At 0600 UTC 29 January, the convergent
cloud band is found between the northeastern coast of
the peninsula and the west coast of Japan. A vertex point
of the band is located near 38.28N, 130.88E at 0600
UTC. It keeps moving southeastward, and reaches the
point near 36.88N, 133.78E at 1200 UTC 29 January.
The speed of movement is faster during 0300–1200
UTC 29 January than before 0300 UTC. High-level
clouds are found at 0600 UTC around 408N, 1358E far
to the north of the cloud band. These high clouds have
been advected from the area to the southeast of Vladivostok (43.18N, 131.98E) and from the southwest. The
clouds are then advected northeastward.
A well-developed meso-a-scale cyclonic circulation
is found over the southeastern part of the East Sea at
1200 UTC 29 January (Fig. 3d). This cyclonic circulation is over the convergence zone, and is located to
the southwest of the mesoscale low shown in the corresponding surface map (Fig. 2c). The relatively large
cloud mass to the northeast of the cyclone consists of
high-level thin clouds advected from the west and the
clouds associated with the mesoscale low. The cloudtop temperature over the cyclone area (southern part of
the cloud mass) ranges from 2338 to 2378C. This range
of cloud-top temperatures indicates that the clouds associated with the meso-a-scale cyclone are below the
500-hPa level, where the air temperature over the cyclone area ranges from 2358 to 2408C.
Satellite imagery can be useful for understanding the
movement of the mesoscale pressure system over the
sea. The satellite observations described above indicate
that the mesoscale cyclone is associated with the polar
air convergence zone that extends with a V-shaped pattern from the lee of the KMC to the mesoscale cyclone.
The vertex point of the V-shaped convergent cloud band
has moved southeastward from a point near the vortex
formation area in the lee of the KMC. These satellite
observations may indicate that the development of the
mesoscale trough to the west of central Japan at 1200
UTC 29 January (Fig. 2c) is associated with the southeastward-moving band of polar air convergence.
The 500-hPa chart shows the presence of a cold-core
cutoff low over eastern Manchuria at 1200 UTC 28
January (Fig. 4). This low is collocated vertically with
the low at 850 hPa. A synoptic-scale ridge is found to
the east of the low. The low at 500 hPa moves slowly
southeastward and becomes more symmetric by 1200
SEPTEMBER 1998
LEE ET AL.
2309
FIG. 3. GMS satellite IR imagery for (a) 2100 UTC 28, (b) 0000 UTC 29, (c) 0600 UTC 29, and (d) 1200 UTC 29 January 1995.
UTC 29 January, when the ridge to the east becomes
stronger. The low at 850 hPa has also moved southeastward and shows two mesoscale lows around Hokkaido Island, Japan, at 1200 UTC 29 January. During
this 24-h period, the 850-hPa airflow around the northern Korean peninsula has changed from weak northwesterlies to stronger northerlies.
The 850-hPa thermal trough develops toward the peninsula as the 500-hPa cold-core low moves southeastward during the 24-h period. At the 850-hPa level, temperature decreases are found over the northern peninsula
and most of the East Sea, except for the northeastern
part of the sea where a temperature increase is found.
On the other hand, the temperature decrease at the 500hPa level is relatively large (4–5 K) over the middle
part of the sea due to the southeastward movement of
the 500-hPa cold-core low.
These changes in temperature at the 850- and 500hPa levels result in the decrease of atmospheric static
stability over the middle and northeastern part of the
East Sea. Figure 5 shows the potential temperature difference (Du) between the 500- and 850-hPa levels obtained using the 2.58 3 2.58 analysis data from the Japan
Meteorological Agency (JMA). At 1200 UTC 28 January, the minimum difference is found to the north of
the surface mesoscale low off the midwest coast of Japan. The difference increases moderately over the eastern part of the sea during the next 12 h. Then a significant decrease occurs during 0000–1200 UTC 29 January over the middle and eastern parts of the sea. The
relatively small difference at 1200 UTC 29 January over
the northeastern part may be due to the combined effect
of the southeastward movement of the upper-level cold
vortex and lower tropospheric heating associated with
the mesoscale low. The contribution of the latter part
will be discussed further in section 5d.
Figure 6 shows the 500-hPa absolute vorticity and
geopotential height and the 850-hPa divergence of Q
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VOLUME 126
FIG. 4. Height (m, solid) and temperature (8C, dashed) fields at 500 hPa: (a) 1200 UTC 28 and (b) 1200 UTC 29 January 1995. At 850
hPa: (c) 1200 UTC 28 and (d) 1200 UTC 29 January 1995.
vector for 1200 UTC 29 January. A vorticity maximum
is found over the northern part of the Korean peninsula.
The figure indicates that some positive vorticity advection exists near the mesoscale low off the midwest coast
of Japan, but it is not significant (Fig. 6a). Upper-level
vorticity advection is generally weak throughout the
present case (not shown). The Q vector and its divergence at 850 hPa indicate an upward motion around the
mesoscale low (Fig. 6b).
The mesocyclone shown in this section forms over
the area of polar air convergence over the East Sea. The
development of the meso-a-scale cyclone occurs in an
environment with strong baroclinicity, synoptic-scale
cyclonic circulation, and a cold vortex aloft whose center is located to the north of the surface cyclone. Satellite
data suggest that the depth of the meso-a-scale cyclone
is limited to be below the 500-hPa level. Observation
also indicates that the KMC may play some roles in the
mesoscale cyclogenesis, since the surface pressure
trough is found to the lee side of the KMC and the
satellite imagery shows that a mesoscale vortex devel-
ops to the lee side of the KMC and then travels southeastward.
3. Numerical experiments
In the following sections, we will use a mesoscale
model to investigate the present case of mesoscale cyclogenesis. This study employs the Colorado State University RAMS mesoscale numerical model (Tripoli and
Cotton 1982; Tremback et al. 1985). The model assumes
a hydrostatic and anelastic atmosphere in terrain-following sigma-z (z*) coordinates. A Smagorinsky-type
eddy coefficient is used for eddy diffusion and a simplified Kuo-type parameterization is used for cumulus
convection (Tremback 1990). The ground surface temperature is predicted by the soil model of Tremback and
Kessler (1985).
Several numerical experiments have been performed
to investigate the mesoscale cyclogenesis, and major
features of the experiments are summarized in Table 1.
Experiment RE is designed to simulate the observed
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LEE ET AL.
2311
flow fields and to obtain deeper insights into the mesoscale cyclogenesis. Experiments I1 through I5 study
the effects of the mountain on the airflow in an environment characterized by a significant low-level baroclinicity and warm sea surface to the downstream side
of an idealized terrain. Experiments E1 through E3 study
the roles of physical factors in the observed mesoscale
cyclogenesis.
All experiments with real data (RE, E1–E3) are carried out in a domain that contains 107 3 90 3 17 grid
points with a constant horizontal grid interval of 20 km.
The model domain and smoothed topography are shown
in Fig. 1. These experiments are carried out for 36 h
from 0000 UTC 28 to 1200 UTC 29 January 1995.
Initial meteorological fields and synoptic-scale tendencies at lateral boundaries are prepared using the 2.58 3
2.58 analysis data provided by JMA. The initial ground
surface temperature is obtained by extrapolating the initial air temperature above the ground. Climatological
data is used for the sea surface temperature. The roughness length of the land surface is assumed to be 0.05
m. The experimental design for the idealized experiments (I1–I5) will be described in section 5a.
4. The control experiment and comparisons with
observations
FIG. 5. Difference of potential temperature between the 500- and
850-hPa levels (Du, K) at (a) 1200 UTC 28, (b) 0000 UTC 29, and
(c) 1200 UTC 29 January 1995.
The simulated SLP is shown in Fig. 7 at 6-h intervals.
A mesoscale trough forms on the lee (southeast) side
of the KMC during the daytime and persists through the
evening (not shown). At 1800 UTC 28 January, the
trough becomes sharper (Fig. 7a). It extends over the
sea to the west coast of Japan. Near southern Hokkaido,
Japan, a mesoscale low has developed off the midwest
coast of Japan and moved to its present location during
the past 12 h. The lee trough departs from the KMC
after 2100 UTC 28 January and travels southeastward.
The trough deepens faster after 0000 UTC 29 January
at the rate of about 2 hPa (3 h)21. It then develops into
a closed low on its east side at 0900 UTC and deepens
further with its central pressure below 1014 hPa at 1200
UTC 29 January near the midwest coast of Japan (Fig.
7d). The trough extends from the East Korea Bay
(around x 5 2400 km, y 5 250 km) to this low with
a V-shaped pattern.
Streamlines at z* 5 95 m and 850-hPa level are
shown in Figs. 8 and 9, respectively. At 1200 UTC 28
January, a low-level convergence zone is found between
the lee side of the KMC and the mesoscale low near
Japan (Fig. 8a). Over the KMC, the incoming northwesterly flow splits and the northern branch undergoes
an anticyclonic circulation. This anticyclonic circulation
is associated with the mountain-induced high pressure
over an isolated mesoscale mountain. A sharp cyclonic
turning is found immediately to the lee side. This flow
pattern is similar to the baroclinic flow over an isolated
mountain found in linear theory (Smith 1984, 1986) and
numerical simulations (Lin and Perkey 1989), even
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VOLUME 126
FIG. 6. (a) Geopotential height (m, solid line) and absolute vorticity (10 25 s21 , dashed line) at
the 500-hPa level, and (b) the Q vector and its divergence (= · Q, 10216 km22 s21 ) at the 850hPa level at 1200 UTC 29 January 1995. Contour intervals for absolute vorticity and Q-vector
divergence are 2 3 1025 s21 and 4 3 10216 km22 s21 , respectively. (b) Solid and dashed lines
indicate positive and negative values, respectively.
though no wind reversal level exists in the present case.
The convergence zone on the lee side of the KMC becomes stronger at 1800 UTC (Fig. 8b). Airflow from
the north near Vladivostok and the Sikhote-Alin mountain range also blows into this convergence zone. This
convergence zone departs from the lee side of the KMC
after 2100 UTC 28 January and then moves southeastward. The vortex center, however, moves eastward
(Figs. 8c and 8d).
At 850 hPa, a cyclonic mesovortex appears over the
sea immediately to the lee side of the KMC at 1800
UTC 28 January (Fig. 9b). This vortex has formed at
about 1500 UTC and grows in size without a noticeable
movement until 2100 UTC and then moves southeastward growing into a meso-a-scale vortex. However, its
size remains quasi-steady after 0300 UTC 29 January.
The 850-hPa vortex is located within the mixed layer
above the convergence zone and elongates in a west–
east direction at earlier times and a southwest–northeast
direction at later times.
Figure 10 shows the y–z cross sections of equivalent
potential temperature, cloud water mixing ratio, and v–
w wind vector fields at x 5 2100 km for 0000 UTC
29 January. The depth of the mixed layer over the ocean
is less than 2 km in general, except over the area to the
north of the convergence zone where clouds develop up
to a 4-km height. The wind vector field shows that the
convergence associated with the northerly incoming
flow is found up to the 1315-m level (Fig. 10b). This
indicates that the 850-hPa vortex is developing at the
top of the layer with the polar air convergence. Figure
11 shows the cloud water mixing ratio fields for 1200
UTC 29 January at two levels, z* 5 2402 and 4019 m.
The southern boundary of clouds at 2402 m coincides
with the location of the low-level convergence zone.
The clouds at 4019 m are found to the north of the
mesoscale low off the midwest coast of Japan.
Simulated heat fluxes from the sea surface show significant spatial variation with a local minimum over the
trough region where the wind speeds are low (not
shown). Sensible and latent heat fluxes are relatively
large off the east coast of the Korean peninsula with
TABLE 1. Summary of numerical experiments.
Experiment
RE
I1
I2
I3
I4
I5
E1
E2c
E3
a
b
c
Topography
Real
Idealized mountainb
Same as I1
Same as I1
Same as I1
Same as I1
No KMC
Real
Real
Ocean
Latent heat
release
Diurnal heating
Yes
No
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
No
No
No
No
No
Yes
Yes
No
Yes
No
No
No
Yes
Yes
Yes
No
Yes
Horizontal
temperature
gradienta
Applies only to the idealized expts I1–I5.
The idealized mountain possesses the characteristics of the northern Korean mountain complex in the Korean peninsula.
No sensible and latent heat fluxes at the surface.
No
No
Yes
No
Yes
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LEE ET AL.
2313
FIG. 7. Simulated sea level pressure (hPa) at (a) 1800 UTC 28, (b) 0000 UTC 29, (c) 0600 UTC 29, and (d) 1200 UTC 29 January 1995.
Terrain heights of 100, 500, 1000, and 1500 m are shaded with increasing darkness, respectively.
maximum value larger than 250 and 400 W m22 near
the east coast, respectively.
The simulated SLP patterns (Fig. 7) agree reasonably
well with those of observations. But the simulated SLP
near the center of the mesoscale low off the midwest
coast of Japan at 1200 UTC 29 January is somewhat
higher than the observed SLP. The model results show
more significant mesoscale features over the East Sea
than the synoptic weather charts, especially the movement of the trough. The synoptic SLP chart (Fig. 2)
does not show the mesoscale features over the sea due
to sparse observations and may not be adequate for the
verification of the model results. The simulated streamlines at 850 hPa (Fig. 9) agree very well with the analyzed streamlines (Fig. 12), except that the simulation
shows more mesoscale features in the lee of the KMC.
Both the simulated and observed streamlines show the
transition of flow in the upstream of the KMC from
northwesterlies at 1800 UTC 28 January to general
northerlies at 1200 UTC 29 January. The location of
the simulated mesoscale cyclonic vortex over the southeastern East Sea at 1200 UTC 29 January agrees very
well with that of the analyzed vortex (Figs. 9d and 12b).
The simulation is also in good agreement with the satellite observations. The development of the simulated
850-hPa vortex to the lee side of the KMC (Fig. 9b)
matches well with the cloud pattern in the satellite imagery during 1800–2100 UTC 28 January (Fig. 3a). The
simulated cyclonic circulation and its location also
match reasonably with those deduced from the cloud
imagery for 1200 UTC 29 January. A good agreement
can be found between the location of simulated clouds
over the low-level convergence zone and that of the
convergent cloud band found in the satellite imagery
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FIG. 8. Simulated streamlines at z* 5 95 m for (a) 1200 UTC 28, (b) 1800 UTC 28, (c) 0000 UTC 29, and (d) 1200 UTC 29 January 1995.
during the later stage (e.g., Figs. 3d and 11a). The relatively clear area near southern Hokkaido also matches
reasonably well with the simulated mesoscale ridge area
(Figs. 3d and 7d).
It is difficult to find the meso-a-scale vortex in the
satellite imagery for 0600 UTC 29 January (Fig. 3c).
According to the simulated results, the vortex is embedded in the mixed layer where clouds form primarily
by turbulent mixing or by ascent of air over the convergence zone. A relatively strong upward motion associated with the lower-level convergence is found in
the northern and western parts of the vortex where relatively deep clouds develop, while a weak downward
motion is found over the southern part where no cloud
is found above the mixed layer (Fig. 13). It appears that
the northern and western parts of the vortex are covered
by the relatively deep clouds that developed over the
convergence zone. The southern boundary of the simulated cloud at z* 5 2402 m (Fig. 13c) matches very
well with that of the convergent cloud band in satellite
imagery (Fig. 3c). These results together suggest that
the simulated clouds may not show the meso-a-scale
vortex pattern. This may also imply that there can be
more meso-a-scale vortices over the sea than what is
shown in the satellite imagery. In addition, Figs. 10 and
13 indicate that the cyclone is shallow and basically is
a boundary layer disturbance since the major upward
motion is below 3 km. This is in agreement with the
satellite observations that indicate that the clouds associated with the meso-a-scale cyclone are limited below 500 hPa.
The present results have shown that a mesoscale
trough forms over the sea on the lee side of the KMC
and then develops into a strong convergence zone and
SEPTEMBER 1998
LEE ET AL.
2315
FIG. 9. Same as Fig. 8, except at 850 hPa.
a vortex in a relatively complicated environment: a
mixed layer over the sea with strong low-level baroclinicity. These results are further investigated to determine the processes that are responsible for the development of the leeside disturbance.
The development of the leeside trough during the
daytime appears to be associated mainly with diurnal
heating and thermal advection. The solar heating of the
elevated plateau (i.e., the KMC) and differential thermal
advection create a thermal ridge over the mountaintop
and downslope area, consequently producing a leeside
trough. The impact of diurnal heating will be discussed
later using idealized experiments. Differential thermal
advection is important throughout the event and will be
described in detail below.
Figure 14 shows the changes in SLP and average
temperature in the lowest 444-m layer during the 3-h
period from 1800 to 2100 UTC 28 January (0300–0600
LST 29 January). SLP has increased over the continent
and over the sea to the west of the peninsula, while it
has decreased over most of the area to the east of the
continent and the peninsula. A decrease in pressure is
found in the lee side of the KMC, where a temperature
increase is found. The KMC contributes both directly
and indirectly to the leeside temperature change. As it
splits and deflects the approaching airstream, air above
it descends over the leeside resulting in a warmer region
over the downslope area through adiabatic warming. In
the mean time, the deflected airstream to the northeast
of the KMC advects colder air to the area surrounding
the lee trough, while thermal advection is weakened
over the downslope area and in the lee trough (Fig. 15).
In the trough over the sea, heating of the mixed layer
by the warm sea surface is generally stronger than the
2316
MONTHLY WEATHER REVIEW
FIG. 10. The y–z cross sections of (a) equivalent potential temperature (K, solid) and cloud water mixing ratio (rc , dashed), and (b)
y2w wind vector fields at x 5 2100 km for 0000 UTC 29 January
1995. The isoline of rc starts from 0.1 g kg21 with an interval of 0.1
g kg21 .
VOLUME 126
advective cooling, while the opposite is true in the area
surrounding the trough. These results suggest that the
differential thermal advection and the adiabatic descent
are the key processes that contribute to the development
of the strong leeside trough. Analysis also indicates that
the differential heating of air in between and surrounding the trough is important for the deepening of the
trough over the ocean throughout the period.
The mesoscale vortex, which appears on the lee side
of the KMC at 1800 UTC (Fig. 9b), develops over the
strong convergence zone. In order to explain the development of this vortex, the vorticity budget has been
investigated. Figure 16 shows the relative vorticity, the
stretching, and tilting terms that are found to be mainly
responsible for the vorticity generation. It can be found
that the locations of maximum vorticity at 585- and
1315-m levels coincide with that of the maximum vertical stretching at 585 m (Figs. 16a–c). In addition, the
values of the maximum vorticity at the two levels are
similar (about 2.4 3 1024 s21 ), even though the combined stretching and tilting effects at 1315 m are weaker
than the stretching at 585 m. This is due to the turbulent
mixing of vorticity in the mixed layer. This indicates
that the vertical stretching is the major source of vorticity near the area of vorticity maximum. The tilting is
important at higher levels (i.e., 1315 m) in the eastern
part of the vortex where its importance is comparable
to that of the stretching at lower levels. It also contributes to the anticyclonic vorticity to the north of the
vortex. Note that another vorticity source may be derived from upstream of the KMC either through potential vorticity generation (Smith 1989) or through the
tilting of horizontal vorticity as proposed by Smolarkiewicz and Rotunno (1989a,b).
FIG. 11. Simulated cloud water mixing ratio (1022 g kg21 ) where z* equals (a) 2402 m and (b) 4019 m for 1200 UTC 29 January 1995.
Contour interval is 0.2 g kg21 .
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LEE ET AL.
2317
FIG. 12. Streamlines at 850 hPa obtained using the wind analysis data from the Japan Meteorological Agency for (a) 1800 UTC 28 and
(b) 1200 UTC 29 January 1995.
In this vortex area where vorticity does not significantly vary with height in the layer below 1315 m (Figs.
16a and 16b), the flow maintains a convergence pattern
at lower levels (Fig. 8b), although a vortex forms near
the 850-hPa level. This variation of flow pattern with
the height can be explained by the anticyclonic vorticity
to the north of the large positive vorticity. As found in
Figs. 16a and 16b, the anticyclonic vorticity to the north
of the positive vorticity area is much stronger at 1315
m than that at 585 m. The vortex can be better defined
with the significant contrast of positive and negative
vorticity. As mentioned previously, the tilting is important for the anticyclonic vorticity to the north of the
vortex. Another way to look at this feature is that the
flow at 850 hPa behaves more like an inviscid, low
Froude number flow over a mountain [e.g., Smolarkiewicz and Rotunno (1989a,b)], while the convergence
associated with the flow near the surface is influenced
by friction in the mixed layer.
The departure of the leeside trough from the KMC
after 2100 UTC 28 (Fig. 7b) is mainly caused by the
cold air advection in the lee coastal area. It occurs gradually. The air deflected by the KMC to the left-hand
side (facing downstream) turns anticyclonically and
flows into the lee side trough, bringing colder air into
the area (Fig. 15b). This cold air advection pushes the
thermal ridge on the lee coastal area southwestward and
weakens the middle portion of the extended thermal
ridge. In the meantime, the lee trough over the sea
steadily intensifies and a significant increase of wind
speed is found in the northwestern part of the trough
(not shown). The enhanced airflow in the lee coastal
area pushes the trough farther southwestward, and causes the trough over the sea to become detached from the
KMC. After the detachment, the strong northerly in the
northwestern part of the trough forces the western part
of the trough to move southward fast.
This simulation has reproduced the observed features
of the present mesocyclogenesis fairly well and has revealed some valuable insights into the event. The cyclogenesis is occurring in a progressive manner. It appears that four distinctive stages of the cyclogenesis can
be identified. 1) First, a surface pressure trough forms
on the lee side of the KMC under a northwesterly synoptic-scale flow which is deflected anticyclonically over
the KMC. 2) Second, the lee trough deepens further into
a strong convergence zone and a meso-b-scale vortex.
3) Then the meso-b-scale vortex develops into a mesoa-scale vortex as the trough and the vortex begin to
move southeastward from the lee of the KMC. 4) Finally, the surface trough deepens into a closed low and
the meso-a-scale vortex becomes collocated with this
deepening surface low to form a meso-a-scale cyclone
over the southeastern East Sea where static stability of
the 500–850-hPa layer is relatively weak. The simulated
results suggest the importance of the KMC for the genesis of the trough and the convergence zone. The results
also indicate the importance of the warm sea surface
and low-level baroclinicity for the mesoscale cyclogenesis considered here.
5. Roles of physical factors in the meso-a-scale
cyclogenesis
Explanation of the mesoscale cyclogenesis shown in
the previous sections requires further understanding of
the roles of some physical factors, such as the KMC,
warm sea surface, condensational heating, etc., in the
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VOLUME 126
formation of the lee trough–vortex and the transition
from vortex into cyclone. This section describes the
roles of these physical factors in the mesoscale cyclogenesis based on the results of numerical sensitivity
experiments.
a. Roles of the KMC (northern Korean mountain
complex)
FIG. 13. (a) Simulated streamlines at z* 5 1315 m, (b) vertical
velocity (cm s21 ) at z* 5 1548 m, and (c) cloud water mixing ratio
(1022 g kg21 ) at z* 5 2402 m, respectively. Solid and dashed lines
in (b) indicate positive and negative values, respectively, with contour
interval of 5 cm s21 .
Five idealized experiments have been performed to
study the effects of the KMC on the airflow in an environment characterized by significant low-level baroclinicity and a warm sea surface to the lee side (Table
1). All experiments, except for experiment I1, include
the warm sea surface downstream of the idealized mountain. Experiments I1 through I3 do not consider the
diurnal heating of the land surface. Experiment I1 is
designed to show the mountain effect without the sea.
Experiments I3 and I2 are to show the effects of the
mountain with and without the east–west baroclinicity,
respectively. Experiments I5 and I4 are to show the
effects of diurnal heating with and without the horizontal temperature gradient, respectively. These experiments have used an elongated bell-shaped mountain
which possesses the characteristic size and shape of the
KMC. The model domain has 86 3 71 3 17 grid points
with a constant horizontal grid size of 20 km.
The initial fields for I3 and I5 are obtained by running
the model for 4 h during which time cooling of the
atmosphere is gradually imposed over the land as a function of height and distance from the shoreline so that
the initial air temperature near the surface is about 8 K
lower at 800 km inland from the shoreline than that at
the shoreline. The cooling rate decreases to 0 at z 5 5
km. Initial temperature and humidity profiles for this 4h run are taken from the JMA analysis data for 1200
UTC 28 January at a location upstream of the KMC. A
uniform westerly wind of 7.5 m s21 is assumed initially.
The sea surface temperature is 274 K at the coast and
increases toward the sea at the rate of 1 K (100 km)21 .
Other model input data are the same as those used in
experiment RE. Initial fields for all other experiments
are also obtained from the 4-h run, but with no prescribed artificial cooling. This is necessary for the experiments with the sea to have the same atmospheric
boundary layers over the sea, although they are not
exactly the same due to the difference in upstream condition. The wind fields obtained from the 4-h run with
horizontal temperature gradient are not yet in thermal
wind balance. The vertical wind shear, however, becomes more consistent with the thermal wind as the
integration proceeds. The Froude numbers (U/Nh) for
the initial upstream flows with and without the horizontal temperature gradient are about 0.30 and 0.35,
respectively, if we assume U 5 7.5 m s21 .
Figure 17 shows the streamlines at z* 5 1315 m from
experiments I1, I2, and I3. When there exists a flat land
surface instead of sea on the downstream side of the
SEPTEMBER 1998
LEE ET AL.
2319
FIG. 14. Changes in (a) sea level pressure and (b) average potential temperature in the lowest 444-m layer during the 3-h period from
1800 to 2100 UTC 28 January 1995. Solid and dashed lines indicate positive and negative values, respectively. Contour interval is 0.4 hPa
(3h)21 in panel (a) and 0.5 K (3h)21 in panel (b).
mountain and diurnal heating is not considered (Fig.
17a), a weak trough appears on the lee side. The leeside
disturbance is stronger with the warm sea surface than
without it (Fig. 17b). At lower levels, however, the disturbance is slightly stronger in I1 (not shown). The difference between I1 and I2 becomes more noticeable at
later times. This may be explained by the following two
impacts of a warm sea surface. The perturbation induced
by the mountain is vertically well mixed over the sea
and the horizontal temperature gradient produced by the
land–sea thermal contrast enhances the development of
disturbance. When the horizontal temperature gradient
is introduced upstream in I3, the leeside disturbance
becomes stronger and a pair of cyclonic and anticyclonic
vortices appear on the downstream side (Fig. 17c). The
temporal variation of flow pattern in I3 is not shown
here, but it is similar to that of I5, which will be shown
later.
Figure 18 shows the SLP perturbation and the potential temperature fields at z* 5 585 m. The pressure
perturbation in I1 shows a circular pattern on the downstream side, while the perturbation in I3 shows a trough
pattern. Due to the addition of the temperature gradient,
the strongest contrast in temperature is found in I3 be-
FIG. 15. Simulated fields of (a) potential temperature (K) and (b) horizontal wind vector at z* 5 585 m.
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2321
FIG. 17. Streamlines at 1315 m from expts (a) I1, (b) I2, and (c) I3.
tween the warmer leeside area and its surroundings.
Note that the temperature gradient near the coastline is
enhanced by the land–sea thermal contrast (Figs. 18d
and 18f). The stronger development of the leeside disturbance in I3 is mainly due to the differential thermal
advection, as discussed in the previous section. While
the subsidence produced by the mountain induces warming over the downslope area, the differential thermal
advection produces further thermal contrast between the
lee trough and its surrounding area, consequently producing a stronger trough or vortex. This differential advection also helps the development of anticyclonic vortex in I3. The anticyclonically deflected air flows into
the trough that extends from the lee slope to the sea.
The air flowing toward the lee slope forms an anticyclonic circulation to the north of the trough with the
contribution of anticyclonic vorticity generation through
the column shrinking. Results of I2 and I3 are similar
to the lee cyclogenesis simulated for an idealized baroclinic flow over a bell-shaped mountain by Lin and
Perkey (1989).
The diurnal heating of the land surface without the
initial temperature gradient brings perturbations to the
airflow whose pattern and magnitude are similar to those
in I2 (not shown). But the introduction of the temperature gradient and the diurnal heating together produces
significant perturbations to the airflow (Fig. 19). At 1600
LST, the left branch (facing downstream) of airflow
passing over the mountain turns anticyclonically toward
the leeside trough, forming a strong convergence zone
on the right-hand side of the downstream area (Fig. 19a).
By 1900 LST, a pair of mesoscale vortices develop in
the downstream area (Fig. 19b). The disturbance, however, becomes weaker later as the convergence at low
levels over the mountain weakens (Fig. 19c). This may
be mainly due to the dissipation of the warm core over
the mountain. A detailed comparison indicates that the
overall development of the lee vortices at various levels
is stronger in I5 than that in I3. The importance of
diurnal surface heating is also found by Sun and Chern
(1993) in a study of the lee vortices in Taiwan. In their
case, however, they found that an existing vortex over
the sea eventually vanishes during the daytime due to
the influence of the strong solar heating of ground surface. In the present wintertime case, the heating of elevated ground surface contributes to the development
of the lee vortices in the presence of a significant horizontal temperature gradient.
The results of these idealized experiments indicate
that the idealized mountain with the characteristics of
the KMC can produce a mesoscale trough and vortex
on its lee side. The low-level baroclinicity, enhanced by
the land–sea thermal contrast, provides an important
source for the leeside disturbance development. Diurnal
heating of an elevated plateau (i.e., the KMC) is also
found to be important for the development of the leeside
disturbance.
In order to investigate the effect of the KMC further
in a real setting, the KMC is removed in experiment
E1. A small low appears in an area of weak pressure
gradient to the east of the middle Korean peninsula at
1800 UTC 28 (Fig. 20a). At 0000 UTC 29 January, the
low (1021 hPa) is embedded in the trough over the
middle of the sea (Fig. 20b). This low keeps moving
eastward and reaches near the midwest coast of Japan
at 1200 UTC 29 January (Fig. 20d). The low near the
midwest coast of Japan is similar to that for RE (Fig.
7d) in its location and strength. However, the SLP pattern in Fig. 20d does not show the well-defined trough
found in RE to extend from the lee of the KMC to the
west coast of Japan in a V-shaped pattern (Fig. 7d).
According to the streamlines at 850 hPa (Fig. 21), the
←
FIG. 16. Relative vorticity, stretching term, and tilting term are shown in (a), (c), and (e), respectively, for z* 5 585 m, and in (b), (d),
and (f ), respectively, for z* 5 1315 m. Contour interval is 20 3 1026 s21 in (a) and (b), and 50 3 10210 s22 in (c) through (f ).
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MONTHLY WEATHER REVIEW
VOLUME 126
FIG. 18. Sea level pressure perturbations (hPa, upper panels) and potential temperatures (K, lower panels) at 585 m from expts I1 (a,b);
I2 (c,d); and I3 (e,f ).
lee vortex is not found, but a cyclonic vortex develops
over the surface low at about 0600 UTC 29 January.
The cyclonic vortex at 1200 UTC 29 in E1 is weaker
and less organized than that found in RE.
The differences between RE and E1 in SLP and the
surface winds are shown in Fig. 22 for 1800 UTC 28
January. Positive values of pressure difference are found
over the KMC (Fig. 22a), and they may be interpreted
mainly as the result of pressure increase due to radiative
cooling of the elevated mountainous area in experiment
RE. Negative values in the lee side of the KMC can be
related to the effects of both the KMC and warm sea
surface. The wind vector difference clearly demonstrates
the importance of the KMC in the formation of the convergence zone (Fig. 22b). However, the formation of the
convergence zone will be discussed later, since it is also
related to the effect of the warm sea surface.
Results of experiment E1 indicate that mesoscale cyclogenesis can also occur in the absence of the KMC.
This cyclogenesis appears to be initiated by the influences of preexisting mesoscale cyclone off the midwest
coast of Japan and the land–sea thermal contrast along
the east coast of the Korean peninsula during the nighttime. This will be discussed in the next section.
FIG. 19. Same as Fig. 17 except for I5 at three different hours.
SEPTEMBER 1998
LEE ET AL.
2323
FIG. 20. Simulated sea level pressure (hPa) from expt E1, in which the mountains over the northern Korean peninsula are removed, for
(a) 1800 UTC 28, (b) 0000 UTC 29, (c) 0600 UTC 29, and (d) 1200 UTC 29 January 1995.
b. Roles of the preexisting meso-a-scale cyclone and
land–sea thermal contrast
Results of experiment E1 are analyzed further to investigate the formation of the mesoscale low off the
mideast coast of the Korean peninsula in E1. Figure 23
shows the changes in SLP and temperature in the lowest
444-m layer during the 3-h period from 1200 to 1500
UTC 28 January when a small low forms in the area of
weak pressure gradient near (398N, 1308E).
A pressure decrease is found off the east coast of the
Korean peninsula (Fig. 23a). The figure indicates that
the pressure decrease is related to the differential heating
of the air (Fig. 23b). A heat budget analysis suggests
that the temperature increase over the area of low formation is due to the heating by the underlying warm
sea surface in the presence of relatively weak cold air
advection. To the east of this area, warming of the mixed
layer by the sea surface is offset or dominated by the
cold air advection in the western part of the mesocyclone
off the west coast of Japan. The temperature decrease
over the peninsula is due to the radiative cooling of the
land surface and cold air advection. This differential
heating of air results in the pressure decrease and, consequently, the formation of a low off the east coast of
the peninsula.
The results shown in Fig. 23 and the tendency of
isobars to be parallel to the coastline during the nighttime (Figs. 20a,b) suggest the importance of coastal
shape for the low formation. These effects of the land–
sea thermal contrast and coastal shape are also important
in the results of RE, as will be shown later. But in RE,
the KMC plays a more pronounced role in the meso-
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cyclogenesis by producing a trough on its lee side. The
preexisting mesoscale cyclone should play the same role
in the mesocyclogenesis in experiment RE as that found
in the results of experiment E1.
c. Roles of warm sea surface
FIG. 21. Simulated streamlines at 850 hPa from E1 for (a) 1800
UTC 28, (b) 0000 UTC 29, and (c) 1200 UTC 29 January 1995.
The effect of the ocean is investigated by deactivating
the sensible and latent heat fluxes (experiment E2). The
SLP pattern from E2 shows that a surface trough appears
to the south of the KMC at 1200 UTC 28 January (Fig.
24a). The pattern at later times, however, shows no significant disturbance to the lee side of the KMC (not
shown). It can be seen that the mesoscale cyclone and
the trough associated with it do not develop without the
effects of a warm sea surface (Fig. 24b). According to
the streamline analysis (not shown), the anticyclonic
deflection appears over the KMC and a low-level convergence zone develops on the lee side, but the convergence zone quickly moves southwestward to the
south of the KMC. A weak shortwave appears at higher
levels over the KMC, but it does not develop into a
vortex.
Figure 25 shows the differences between experiments
RE and E2 in SLP and wind vectors at z* 5 95 m for
0000 UTC 29 January. The pattern of pressure difference over the East Sea somewhat resembles that of SLP
in RE. The SLP is about 2–7 hPa lower in RE than that
in E2 over most of the area to the east of the continent
and the peninsula (Fig. 25a). Relatively large differences
in SLP and wind vector are located over the trough area
of RE. The wind vector difference shows a strong cyclonic convergence pattern along the trough (Fig. 25b).
The wind vector difference and the relatively strong
gradient of pressure difference along the east coast of
the Korean peninsula indicate the importance of land–
sea thermal contrast and the coastline shape for the development of a cyclonic circulation and convergence
zone to the east of the peninsula. The wind vector difference over the coastal area may be interpreted as a
mechanism analogous to the land breeze (cf. Figs. 22b
and 25b). Due to the coastal shape, this land-breezetype wind may contribute to the development of the
cyclonic circulation over the sea around the KMC. In
the ocean area near Vladivostok, where the synoptic
scale pressure gradient is relatively weak in the lower
troposphere (see Fig. 4c), a significant portion of the
prevailing northerly airflow can be explained by the
ocean effect (i.e., Fig. 25b). This northerly flow is important for the development of the convergence zone.
These also indicate that the synoptic-scale pressure pattern in the lower troposphere is important for the formation of the convergence zone. Atlas et al. (1983) also
found the importance of the coastal shape for the formation of a convergence line off the coast of Long
Island, New York, during a cold air outbreak. They suggested that the major difference in the air on either side
of the convergence line was due to the difference in the
SEPTEMBER 1998
LEE ET AL.
2325
FIG. 22. Differences between RE and E1 (RE 2 E1) in (a) sea level pressure (hPa) and (b) wind vectors at 95 m over the ocean at 1800
UTC 28 January 1995. Contour interval in (a) is 0.5 hPa.
temperature of underlying sea surface and the path
length of overwater travel, which depends on the coastal
shape.
Comparison of experiments RE and E2 indicates that
the East Sea produces a favorable atmospheric environment for the mesoscale cyclogenesis through the heating
of the lower troposphere, which results in a region of
lower pressure and low-level cyclonic circulations to
the east of the Korean peninsula. The KMC produces a
lee trough, but the warm sea surface plays the crucial
role for the development of the convergence zone. The
impact of the ocean may vary with the case depending
on the temperature of the airflow moving out off the
continent. As implied by the result of E2, the impact
would be weaker for an airflow with higher air temperature due to a smaller heat supply from the sea.
d. Role of condensational heating
This section discusses the results of E3, in which the
condensational heating is neglected. The patterns of SLP
and streamlines from E3 are not significantly different
FIG. 23. Changes in (a) sea level pressure [hPa (3 h)21 ] and (b) temperature [K (3 h)21 ] during the 3-h period from 1200 to 1500 UTC
28 January 1995. Contour intervals in (a) and (b) are 0.5 hPa (3 h) 21 and 0.5 K (3 h)21 , respectively.
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FIG. 24. Simulated sea level pressure fields from expt E2, in which the latent and sensible heat fluxes at the surface are assumed to be 0,
at (a) 1200 UTC 28 and (b) 1200 UTC 29 January 1995.
from those of RE until the trough reaches the area of
cyclone development, although the trough and convergence is weaker without the condensational heating. The
effect of condensational heating is noticeable at 1200
UTC 29 January (Fig. 26). The cyclone does not develop
and the SLP pattern remains as a trough, suggesting that
a major effect of condensational heating is to enhance
the deepening of the trough and produce a well-defined
mesoscale cyclone. The pressure decrease associated
with the condensational heating is about 2.5 hPa over
the mesoscale trough and cyclone. The difference between RE and E3 in Du [i.e., Du (RE) 2 Du (E3)] over
the trough and mesoscale cyclone is up to 3–4 K (Fig.
26b). This indicates that the condensational heating contributes most of the decrease of Du over the trough in
RE during the 6-h period from 0600 to 1200 UTC 29
January. The results of E3 also indicate that the mesoa-scale cyclone does not develop by the influence of
upper-level forcing alone.
6. Concluding remarks
Numerical simulations and the analysis of observational data are employed to understand the meso-a-scale
FIG. 25. Difference between expts RE and E2 (RE 2 E2) in (a) sea level pressure (hPa) and (b) wind vector at 95 m at 0000 UTC 29
January 1995.
SEPTEMBER 1998
LEE ET AL.
2327
FIG. 26. (a) Simulated sea level pressure from E3, in which the condensational heating is not considered, and (b) the difference between
the experiments RE and E3 (RE 2 E3) in the potential temperature difference (Du, K) for 1200 UTC 29 January 1995.
cyclogenesis in a polar airstream which occurred over
the sea to the east of the Korean peninsula on 28–29
January 1995. The observational analysis shows that a
mesoscale low develops over the southeastern East Sea
(Japan Sea) on 29 January 1995. Satellite imagery also
indicates that a meso-b-scale vortex forms on the lee
side of the northern Korean mountain complex (KMC),
which is located in the northern Korean peninsula, and
that a meso-a-scale cyclone develops over the southeastern East Sea at later times. The mesoscale cyclone
forms in the lower troposphere characterized by strong
baroclinicity and cyclonic circulation under the influence of an upper-level cold vortex.
Numerical simulations have captured major features
of the observed cyclogenesis very well and revealed
some valuable insights into the cyclogenesis process
discussed here. The cyclogenesis occurs in a progressive
manner. Basically, four distinctive stages of the cyclogenesis are identified. 1) First, a surface pressure trough
forms on the lee side of the KMC under a northwesterly
synoptic-scale flow which is deflected anticyclonically
over the KMC. 2) Second, the lee trough deepens further
into a meso-b-scale vortex and a strong convergence
zone, to which the cold air originating from the region
between Vladivostok and the Sikhote-Alin range blows.
3) Then the meso-b-scale vortex develops into a mesoa-scale vortex as the vortex and the trough begin to
move southeastward from the lee of the KMC. 4) Finally, the surface trough deepens into a closed low and
the meso-a-scale vortex becomes collocated with this
deepening surface low to form a meso-a-scale cyclone
over the southeastern East Sea.
Several numerical sensitivity experiments are performed to isolate the effects of topography, a warmer
sea surface, diurnal thermal forcing, and latent heat re-
lease. During stages 1 and 2, it is found that the KMC
and low-level baroclinicity are responsible for generating the lee trough. The presence of a warm sea surface
is found to be necessary for the development of the
polar air convergence zone. The present study has
shown that both dynamic and thermal effects associated
with the KMC are important. The solar heating and
differential cold air advection may help to build a warm
ridge over the lee slope of the KMC during the daytime.
As a result, a trough and convergence zone form over
the lee side of the KMC in the afternoon. It is found
that the KMC contributes both directly and indirectly
to the strengthening of the leeside trough during the
nighttime. While the subsidence produced by the mountain induces warming over the downslope area, the differential thermal advection caused by the deflected flow
produces further thermal contrast between the lee trough
and its surrounding area, consequently strengthening the
trough. A mesoscale vortex forms over the strong convergence zone and it is best defined at approximately
the 850-hPa level, which is near the top of the layer of
polar air convergence. The major source of vorticity in
the area of vorticity maximum is the vertical stretching
caused by the lower-level convergence, while the tilting
of the horizontal vorticity is also found to be an important source of vorticity in the eastern part of the
vortex. Both the tilting and vertical stretching in a synoptic-scale cyclonic circulation are found to be important for the development of the meso-a-scale vortex
during stage 3. In the final stage, condensational heating
plays the key role for the development of the meso-ascale cyclone.
This study suggests that the effects of KMC are important for the initial development of the mesoscale cyclone, and that the genesis and development of the cy-
2328
MONTHLY WEATHER REVIEW
clone require the influences of both a warm sea surface
and condensational heating. Differential cold air advection is found to be an important process throughout all
the stages of evolution, which allows for stronger development of the lee trough and vortex and also the
deepening of the trough over the ocean. In other words,
the low-level baroclinicity is essential for the present
case of mesoscale cyclogenesis. The southeastward
movement of the upper-level cold vortex and the condensational heating of the lower troposphere contribute
to the development of the meso-a-scale cyclone by decreasing the atmospheric stability. Sensitivity tests also
indicate that the present meso-a-scale cyclone does not
develop by the influence of upper-level forcing alone.
The possible contribution from the CISK is still not well
understood and needs to be investigated in the future.
This study also indicates that, under the atmospheric
conditions given in this case, a mesoscale cyclone is
still able to form without the influence of the KMC.
However, this mesoscale cyclone is initiated over the
sea, instead of over the lee side of the KMC. In that
case, the influences of the preexisting mesoscale cyclone
near the midwest coast of Japan and the land–sea thermal contrast contribute to the formation of the low off
the mideast coast of the Korean peninsula. The basic
wind direction around the KMC and the KMC orientation are found to be important for the development of
trough to the east of the KMC. The KMC may not be
able to produce a significant impact on the airflow over
the sea, when a strong synoptic-scale northerly flow
prevails around the KMC and over the sea.
According to this numerical study, more meso-ascale vortices may form over the convergence zone than
what is shown in the satellite imagery. The formation
and organization of these meso-a-scale vortices deserve
a further study, since they may organize into a mesoa-scale cyclone when they reach a location of weak
static stability, such as the area under the upper-level
cold vortex. The importance of the effects of the KMC
on the initial development of the mesoscale cyclone is
often missing in forecasting and from previous studies
due to the lack of data over the sea. This study implies
that a mesoscale numerical model with a fine grid resolution can be used to improve the prediction of mesoscale disturbances that develop to the east of the Korean peninsula.
Acknowledgments. This work is supported by the Korea Science and Engineering Foundation under Grant
94-0703-02-01-3. The authors wish to thank the anonymous reviewers for their valuable comments and Dr.
R. P. Weglarz for his proofreading and comments. They
would like to thank Drs. Pielke and Cotton at Colorado
State University for allowing them to use the CSURAMS model. They also thank the Korea Meteorological Administration (KMA) for providing the Japan Meteorological Agency analysis data. Thanks are also extended to Mr. Y.-H. Kim of KMA for processing the
VOLUME 126
satellite data. Computations were performed on the
CRAYC90 supercomputer of Systems and Engineering
Research Institute, Taejon, Korea.
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