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During cold-air outbreaks in winter, a thick cloud band frequently appears over the northern Sea of Japan and produces localized heavy snowfall in the western coastal region of Hokkaido Island, northern part of Japan. The formation mechanism of this thick cloud band is investigated through a series of nonhydrostatic numerical simulations with a horizontal grid spacing of 5 km. The control simulation well reproduces the characteristics of an observed cloud band. The cloud band forms between relatively warm north-northwesterly winds on the northeast side and relatively cold northwesterly winds on the southwest side. Sensitivity experiments in which upstream topography is modified indicate that the formation and intensification of the cloud band depend on the following two effects; one is the effect of a specific mountain located near the coastline in the middle part of Russia's Sikhote-Alin mountain range (SAMR), and the other is the effect of large-scale topography along the SAMR on synoptic-scale low-level cold northwesterlies. The specific mountain deflects the cold airflow and immediately a convergence zone forms downstream of the specific mountain, where the cloud band is initiated. On the northeastern side of this mountain, the Froude number is estimated to be about 0.4 from relatively high topography (1.2 km), stable stratification (0.02 s^[-1]), and synoptic-scale wind speed of 10 m s^[-1]. Thus, the relatively high topography strongly blocks a low-level cold air, whereas an upper air with high potential temperature flows downward over the sea. In contrast, on the southwestern side of the mountain, a low-level cold air can pass over the topography, because the Froude number is estimated to be about 1.6 from relatively low topography (0.8 km) and weak stable stratification (0.008 s^[-1]). These two airs with different potential temperature create a mesoscale frontal zone over the sea, which causes the further development of the thick cloud band initiated by the coastal specific mountain in the SAMR

The Formation Mechanism of a Thick Cloud Band over the Northern Part of the Sea of Japan during Cold Air Outbreaks