The aim of the present paper is to investigate the formation mechanism of a convergent cloud band which appears east of the Korea Peninsula over the western Japan Sea under northwesterly winter monsoon.Numerical experiments with different surface conditions reveal that the land-sea contrast of thermal property between the Peninsula and the Japan Sea plays the leading role in the formation of the convergent cloud band. Less transformation over the cold land causes a mesoscale high pressure in the lower troposphere with its center at the southeast end of the Peninsula and a convergence zone forms over sea at the eastern margin of it. The convergence zone makes cumulus convections active and organized into a band. In addition, blocking effect of the mountains north of Korea acts to intensify the convergence zone.

A prediction experiment is performed with a very-fine-mesh (42km at 60*N) primitive equation model for a heavy snowfall event in the Hokuriku District associated with a convergent cloud band over the Japan Sea. Using the result of the experiment, together with observed data including special soundings, we elucidate the structure of the convergent cloud band with a line of active convection on its southwestern edge.The atmosphere around the cloud band has the following characteristic features: -a low-level convergence , middle-level divergence zone accompanied by intense positive vorticity along the line of active convection, -a warm and weak-wind zone along the line of active convection embedded in cold airmass , -a weak stable layer on the northeastern side of the line of active convection as a boundary between two layers: a lower layer with northerly cold air flow toward the line of active convection and an upper layer with southwesterly warm air flow which has been heated in the line of active convection, -a west-northwesterly air flow on the southwestern side of the line of active convection with weak vertical wind shear and nearly neutral stratification up to the top of the cold airmass, -a strong-wind zone at the divergent level (700-6OOmb) 300-500km northeast of the line of active convection. On the basis of cross-section and trajectory analyses, we can make a clear picture of the atmosphere around the cloud band.Heat and water vapor budget analysis indicates that over the southern Japan Sea the mesoscale thermal structure around the cloud band is largely maintained by localized release of latent heat in large-scale cold air advection field.

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Further numerical experiments are performed to supplement the numerical study of Nagata et al. (1986) on the formation of the convergent cloud band (CCB) over the Japan Sea in winter. The purposes are to evaluate the relative contributions of the three lower boundary forcings (the landsea thermal contrast, the blocking effect of the mesoscale mountains, and the characteristic SST distribution) to the formation of the CCB, and to investigate how the mountains north of the Korea Peninsula produce the low-level convergence zone.Simulated mesoscale fields show that the contributions of each lower boundary forcing to the formation of the CCB are comparable. Convective heating is less important than sensible heating over the northwestern part of the Japan Sea in contrast with over the southern part. We make cross-section analyses and examine horizontal inertial stability of the CCB over the northwestern part of the Japan Sea. The dynamical effect of the mountains is shown to produce a local baroclinic environment, including a zone of near neutrality both to inertial and gravitational stabilities. Surface sensible heating appears to maintain the CCB in this zone. These findings suggest that the CCB is a mesoscale longitudinal roll parallel to the vertical shear vector associated with the local baroclinity.