A Numerical Modeling Study of Mesoscale Cyclogenesis to the East of the Korean Peninsula

A Numerical Modeling Study of Mesoscale Cyclogenesis to the East of the Korean Peninsula
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  V OLUME  126 S EPTEMBER  1998M O N T H L Y W E A T H E R R E V I E W   1998 American Meteorological Society  2305 A Numerical Modeling Study of Mesoscale Cyclogenesis to theEast of the Korean Peninsula T AE -Y OUNG  L EE AND  Y OUNG -Y OUN  P ARK  Department of Atmospheric Sciences, Yonsei University, Seoul, Korea Y UH -L ANG  L IN  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)ABSTRACTNumerical simulations and the analysis of observational data are employed to understand the mesoscalecyclogenesis in a polar airstream that occurred over the sea to the east of the Korean peninsula on 28–29 January1995. The observational analysis shows that a mesoscale low develops over the southeastern East Sea (JapanSea) on 29 January 1995. Satellite imagery also indicates that a meso-   -scale vortex forms on the lee side of the northern Korean mountain complex (KMC), which is located in the northern Korean peninsula, and that ameso-  -scale cyclone develops over the southeastern East Sea at a later time. The mesoscale cyclone forms inthe lower troposphere with strong baroclinicity and cyclonic circulation under the influence of an upper-levelsynoptic-scale cold vortex.Numerical simulation has captured major features of the observed cyclogenesis very well. The cyclogenesisoccurs in a progressive manner. Basically, four distinctive stages of the cyclogenesis are identified. 1) First, asurface pressure trough forms on the lee side of the KMC under a northwesterly synoptic-scale flow that isdeflected anticyclonically over the KMC. 2) Second, the lee trough deepens further into a strong convergencezone and a meso-   -scale vortex. 3) Next, the meso-   -scale vortex develops into a meso-  -scale vortex as thevortex and the trough begin to move southeastward from the lee of the KMC. 4) Finally, the surface troughdeepens into a closed low and the meso-  -scale vortex becomes collocated with this deepening surface low toform a meso-  -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-levelbaroclinicity are responsible for generating the strong lee trough and vortex. During stage 3, the developmentof the meso-  -scale vortex is brought on by the tilting of horizontal vorticity and vertical stretching in a synoptic-scale cyclonic circulation. In the final stage, the condensational heating plays the key role for the developmentof the meso-  -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 zoneand 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-  -scale lows over the ocean to the east of theAsian continent during winter outbreaks of polar air(Ninomiya 1989; Tsuboki and Wakahama 1992). Someof these lows are found over the East Sea (Japan Sea)between the northeastern coast of the Korean peninsulaand the west coast of Japan. Once these lows form,someof them keep developing when they are moving north-eastward along the west coast of Japan. Although thedevelopment 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: around northern Japan have been studied extensively,the genesis mechanism of these mesoscale lows to theeast of the Korean peninsula is still not well understoodand deserves further study.Ninomiya (1989) found that polar lowswereobservedto form about 500–1000 km north of major polar frontalzones, where strong low-level baroclinicity is main-tained by sea surface fluxes in the polar air mass be-tween the continent and the relatively warm ocean. Hedefined the polar low in this region as a meso-  -scalelow accompanied by spiral or comma cloud system witha scale of 200–700 km. He also found that these me-soscale lows rarely appear over the Asian continent,Yellow Sea, and East China Sea.As the polar air streams out over the ocean from theAsian continent, surface weather maps often show a  2306  V OLUME  126M O N T H L Y W E A T H E R R E V I E WF IG . 1. Geographic map and smoothed topography in the mesoscale model domain. Contourinterval is 200 m. The KMC represents the northern Korean mountain complex, located in thenorthern part of the Korean peninsula. sharp trough that starts from immediately to the lee sideof the mountains in the northern Korean peninsula, thatis, the northern Korean mountain complex (referred toas KMC hereafter) (Fig. 1), and extends toward theocean. Satellite imagery sometimes shows mesoscalevortices that form to the southeast of the KMC andpropagate southeastward. These characteristics suggestthat the topography of the Korean peninsula may playsome significant role in the mesoscale cyclogenesis tothe east of the peninsula.Previous studies have revealed the importance of theKMC on the mesoscale disturbances around the pen-insula. Yagi et al. (1986) indicated that the low-levelpolar air convergence zone to the east of Korea mayresult from the dynamic effect of the KMC. Nagata(1991) suggested that the blocking effect of the KMC,the land–sea thermal contrast, and the characteristicSSTdistribution equally contribute to the formation of theconvergence zone. Asai (1988) showed that zones of frequent occurrence of mesoscale vortices were foundover the sea to the east of the Korean peninsula and tothe west of Hokkaido Island, Japan, and indicated thatthese zones were collocated with the polar air conver-gence zone. Nagata (1993) proposed that barotropicshear instability was the dominant development mech-anism of meso-   -scale vortices along this zone. Directeffects of the KMC on the formation of mesoscale cy-clones over the East Sea, however, are rarely discussedin the previous studies.The KMC consists of various peaks with heightsgreater than 2 km, and the length and width of the areaabove 1-km height are about 320 and 100–160 km,respectively. The shape of the KMC is asymmetric witha steeper slope to the east coast. The Froude numberfor a typical flow during night around the KMC is about0.3–0.7, which is sufficiently low for the mountains toaffect the flow significantly according to the previousstudies [e.g., Smolarkiewicz and Rotunno (1989a,b);Lin et al. (1992)]. Lee and Park (1996) indicated thatthe KMC may be directly responsible for the formationof some mesoscale disturbances around the Korean pen-insula.In simulating an inviscid flow over the Hawaiianmountains, Smolarkiewicz et al. (1988) found that a pairof vortices formed on the lee side of the island, whichthen shed downstream at later times. SmolarkiewiczandRotunno (1989a,b) found that this type of lee vortexmay occur when a low Froude number, inviscid, non-rotating stratified flow passes over an isolated mountain.According to Smolarkiewicz and Rotunno, the Froudenumber, defined as  U/Nh,  where  U   is the basic windspeed,  N   the Va¨isa¨la¨ frequency, and  h  the mountainheight, needs to be less than 0.5 in order to produce thelee vortices. Higher Froude number flows have alsobeen  S EPTEMBER  1998  2307 L E E E T A L . found to be able to produce lee vortices (Lin et al. 1992).The lee vortex is formed by either the baroclinicallygenerated vorticity (Smolarkiewicz and Rotunno1989a,b) or the generation of potential vorticity (Smith1989). These types of lee vortices have also been sim-ulated 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 collocateswith the mesolow in a rotating fluid flow system andmay therefore be classified as a mesocyclone.Smith (1984, 1986) proposed a theory which viewslee cyclogenesis as the formation of the first trough of a standing baroclinic wave. The theory requires that thebasic wind reverses its direction at a certain level. Thiswind reversal height (i.e., critical level in a steady-stateflow) satisfies the general conditions observed to ac-company lee cyclogenesis in the Alps, since lee cyclo-genesis is often associated with the passage of a coldfront there. This theory has also been applied to explaincyclogenesis in the lee of the Appalachians (Smith1986). This type of lee cyclogenesis is found to be moreeffective in a nonlinear flow (Lin and Perkey 1989). Thesplitting of low-level flow is more pronounced for a lowFroude number flow. The ageostrophicadvectionofcoldair is able to strengthen the mountain-induced high andthe lee cyclone. In addition, lee cyclogenesis is strength-ened by both the low-level sensible heating and theturning of the wind associated with boundary layer pro-cesses. Therefore, the presence of low-level baroclin-icity across the east coast of the Korean peninsula andthe boundary layer forcing associated with the warmsea surface may play a similar role in the formation of the lee cyclone over the KMC.Mechanisms for the formation and development of meso-  -scale lows have been a subject of considerableinterest, especially for the polar lows over the NorthAtlantic Ocean, the Norwegian Sea, the Barent Sea, andthe Gulf of Alaska, as well as the lows around Japan.Mansfield (1974) suggested that polar lows were shal-low baroclinic disturbances, whereas Rasmussen (1979)viewed the polar low as an extratropical disturbancedriven by conditional instability of the second kind(CISK). Later, Forbes and Lottes (1985) suggested thatboth the baroclinic and CISK mechanisms were impor-tant for polar low development. Based on observations,Bond and Shapiro (1991) found that mesoscale cyclo-genesis exists in the large-scale parent low over the Gulf of Alaska and suggested frontogenesis at low levels asa polar low genesis mechanism. Douglas et al. (1991)suggested that the observed evolution of the polar lowover the Gulf of Alaska may be significantly influencedby 1) flow modification by the high mountains ringingthe Gulf of Alaska, 2) the varying synoptic-scale flowover the Gulf of Alaska, and 3) heat and moisture fluxesfrom the underlying ocean surface.Ninomiya (1991) suggested that a similarmechanism,as proposed by Bond and Shapiro (1991), was respon-sible for the polar low genesis to the east of the Asiancontinent. He suggested that the mesoscale low over theeast coast of Asia formed, under the influence of a coldvortex aloft, in the west–east-oriented trough within thenorthwestern quadrant of the synoptic-scale low thatdeveloped over the northwestern Pacific. Tsuboki andWakahama (1992) suggested that the meso-  -scale cy-clones off the west coast of Hokkaido Island, Japan,were due to baroclinic instability associated with a par-ticular baroclinic flow. It appears that the formationmechanism of the meso-  -scale cylone or polar low inthis region is extremely complicated and deserves fur-ther study. In this study, we will focus on the earlierstages of meso-  -scale cyclone formation to the east of the Korean peninsula.The meso-  -scale cyclone presented in this study oc-curred on 29 January 1995 over the southeastern EastSea in the vicinity of the polar air convergence zone.In this study, we will analyze the mesoscale cyclogen-esis event and discuss its mechanism. In section 2, wedescribe the formation and development of a mesoscalevortex to the east of the peninsula based on synopticanalyses and satellite imagery. Several numerical ex-periments are performed to investigate the observedme-soscale cyclogenesis. The description of the numericalexperiments is given in section 3. The results from thecontrol experiment and the comparison of its resultswith observations are presented in section 4. In section5, results from five idealized experiments and three sen-sitivity experiments with real data are discussed. Con-cluding remarks can be found in section 6. 2. Observational analysis The mesoscale cyclogenesis of the present case isobserved on 28–29 January 1995 across the sea betweenthe KMC (northern Korean mountain complex) and thewest coast of Japan. Figure 2 shows the sea level pres-sure (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 SakhalinIslands and the other over the northeastern coast of theKorean peninsula. A mesoscale low is found off themidwest coast of Japan. The pressure gradient is weak throughout the eastern Asian continent and also overthe area to the west of the Korean peninsula. A signif-icant trough has developed over Japan by 0000 UTC 29January. At 1200 UTC 29 January, another mesoscalelow has developed off the midwest coast of Japan (near39  N, 138  E), at the location similar to that of the me-soscale low at 1200 UTC 28 January. The present studyis interested in the formation of this mesoscale low. Thepressure drops about 8 hPa during the period of 0000–1200 UTC 29 January over the area of this low. Withthe development of the mesoscale low, the zonal gra-dient of surface pressure has also significantly increasedover the sea between the Korean peninsula and the me-soscale low. Detailed surface pressure analysis over the  2308  V OLUME  126M O N T H L Y W E A T H E R R E V I E WF IG . 2. Sea level pressure (hPa) for (a) 1200 UTC 28, (b) 0000UTC 29, and (c) 1200 UTC 29 January 1995. northern Korean peninsula shows that a low pressurearea is persistently found over the northeastern coast of the peninsula, although the synoptic charts do not showthis feature well.Satellite imagery shows that a mesoscale vortex de-velops during 1500–2100 UTC 28 January to the south-east of the KMC (Fig. 3a). The line of convective cloudsto the southeast of the vortex indicate the existence of an elongated low-level convergence in a northwest–southeast direction (Figs. 3a and 3b). The relationshipbetween the band of convective cloud and the lower-level convergence has been investigated by severalstud-ies [e.g., Yagi et al. (1986); Nagata et al. (1986)]. Here-after, this cloud band will be called the convergent cloudband following Nagata et al. (1986). During this earlystage, the lee vortex moves slowly southeastward (Fig.3b).The imagery shows the movement of the convergentcloud band. At 0600 UTC 29 January, the convergentcloud band is found between the northeastern coast of the peninsula and the west coast of Japan. A vertexpointof the band is located near 38.2  N, 130.8  E at 0600UTC. It keeps moving southeastward, and reaches thepoint near 36.8  N, 133.7  E at 1200 UTC 29 January.The speed of movement is faster during 0300–1200UTC 29 January than before 0300 UTC. High-levelclouds are found at 0600 UTC around 40  N, 135  E farto the north of the cloud band. These high clouds havebeen advected from the area to the southeast of Vlad-ivostok (43.1  N, 131.9  E) and from the southwest. Theclouds are then advected northeastward.A well-developed meso-  -scale cyclonic circulationis found over the southeastern part of the East Sea at1200 UTC 29 January (Fig. 3d). This cyclonic circu-lation is over the convergence zone, and is located tothe southwest of the mesoscale low shown in the cor-responding surface map (Fig. 2c). The relatively largecloud mass to the northeast of the cyclone consists of high-level thin clouds advected from the west and theclouds associated with the mesoscale low. The cloud-top temperature over the cyclone area (southern part of the cloud mass) ranges from  33  to  37  C. This rangeof cloud-top temperatures indicates that the clouds as-sociated with the meso-  -scale cyclone are below the500-hPa level, where the air temperature over the cy-clone area ranges from   35   to   40  C.Satellite imagery can be useful for understanding themovement of the mesoscale pressure system over thesea. The satellite observations described above indicatethat the mesoscale cyclone is associated with the polarair convergence zone that extends with a V-shaped pat-tern from the lee of the KMC to the mesoscale cyclone.The vertex point of the V-shaped convergent cloud bandhas moved southeastward from a point near the vortexformation area in the lee of the KMC. These satelliteobservations may indicate that the development of themesoscale trough to the west of central Japan at 1200UTC 29 January (Fig. 2c) is associated with the south-eastward-moving band of polar air convergence.The 500-hPa chart shows the presence of a cold-corecutoff low over eastern Manchuria at 1200 UTC 28January (Fig. 4). This low is collocated vertically withthe low at 850 hPa. A synoptic-scale ridge is found tothe east of the low. The low at 500 hPa moves slowlysoutheastward and becomes more symmetric by 1200  S EPTEMBER  1998  2309 L E E E T A L .F IG . 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 becomesstronger. The low at 850 hPa has also moved south-eastward and shows two mesoscale lows around Hok-kaido Island, Japan, at 1200 UTC 29 January. Duringthis 24-h period, the 850-hPa airflow around the north-ern Korean peninsula has changed from weak northwes-terlies to stronger northerlies.The 850-hPa thermal trough develops toward the pen-insula as the 500-hPa cold-core low moves southeast-ward during the 24-h period. At the 850-hPa level, tem-perature decreases are found over the northern peninsulaand most of the East Sea, except for the northeasternpart of the sea where a temperature increase is found.On the other hand, the temperature decrease at the 500-hPa level is relatively large (4–5 K) over the middlepart of the sea due to the southeastward movement of the 500-hPa cold-core low.These changes in temperature at the 850- and 500-hPa levels result in the decrease of atmospheric staticstability over the middle and northeastern part of theEast Sea. Figure 5 shows the potential temperature dif-ference (    ) between the 500- and 850-hPa levels ob-tained using the 2.5  2.5  analysis data from the JapanMeteorological Agency (JMA). At 1200 UTC 28 Jan-uary, the minimum difference is found to the north of the surface mesoscale low off the midwest coast of Ja-pan. The difference increases moderately over the east-ern part of the sea during the next 12 h. Then a sig-nificant decrease occurs during 0000–1200 UTC 29 Jan-uary over the middle and eastern parts of the sea. Therelatively small difference at 1200 UTC 29 January overthe northeastern part may be due to the combined effectof the southeastward movement of the upper-level coldvortex and lower tropospheric heating associated withthe mesoscale low. The contribution of the latter partwill be discussed further in section 5d.Figure 6 shows the 500-hPa absolute vorticity andgeopotential height and the 850-hPa divergence of   Q
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