A NUMERICAL MODELING STUDY OF MESOSCALE CYCLOGENESIS TO THE EAST OF THE KOREAN PENINSULA

Numerical simulations and the analysis of observational data are emplo yed to understand the mesoscale cyclogenesis in a polar airstream that occurred over the sea to the east of the Korean peninsula on 28-29 Ja nuary 1995. The observational analysis shows that a mesoscale low deve lops over the southeastern East Sea (Japan Sea) on 29 January 1995. Sa tellite imagery also indicates that a meso-beta-scale vortex forms on the lee side of the northern Korean mountain complex (KMC), which is l ocated in the northern Korean peninsula, and that a meso-alpha-scale c yclone develops over the southeastern East Sea at a later time. The me soscale cyclone forms in the lower troposphere with strong baroclinici ty and cyclonic circulation under the influence of an upper-level syno ptic-scale cold vortex, Numerical simulation has captured major featur es of the observed cyclogenesis very well. The cyclogenesis occurs in a progressive manner: Basically, four distinctive stages of the cyclog enesis are identified. 1) First, a surface pressure trough forms on th e lee side of the KMC under a northwesterly synoptic-scale flow that i s deflected anticyclonically over the KMC. 2) Second, the lee trough d eepens further into a strong convergence zone and a meso-beta-scale vo rtex. 3) Next, the meso-beta-scale vortex develops into a meso-alpha-s cale vortex as the vortex and the trough begin to move southeastward f rom the lee of the KMC. 4) Finally, the surface trough deepens into a closed low and the meso-alpha-scale vortex becomes collocated with thi s deepening surface low to form a meso-alpha-scale cyclone over the so utheastern East Sea. Several sensitivity experiments are performed to isolate the effects of a topography, warmer sea surface, diurnal therm al forcing, and latent heat release. During stages 1 and 2, it is foun d that the KMC and low-level baroclinicity are responsible for generat ing the strong lee trough and vortex. During stage 3, the development of the meso-alpha-scale vortex is brought on by the tilting of horizon tal vorticity and vertical stretching in a synoptic-scale cyclonic cir culation. In the final stage, the condensational heating plays the key role for the development of the meso-alpha-scale cyclone under the in fluence of an upper-level synoptic-scale cold vortex. The presence of the warm sea surface is found to be a necessary condition for the deve lopment of a polar air convergence zone and the mesoscale cyclone. It is also found that the low-level baroclinicity is essential for the pr esent case of mesoscale cyclogenesis.