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.