Synoptic-Scale Variations in the Polar Air-Mass Transformed over the Japan Sea

The present report studies synoptic-scale variations of the polar air-mass transformed over the Japan Sea, during the ‘‘East Japan Sea Observation’’ in the winters of 1976 and 1977. Rawinsonde observation data at a research vessel are used, in addition to data at operational upper observation stations and the objective analysis data. The multi-layer structure of the transformed air-mass, including the unstable lowest layer and the mixed layer capped by the stable layer, is formed in the transformed airmass over the Japan Sea. The mixed layer, which includes the sub-cloud layer and the cloud layer, is characterized by nearly vertically-uniform equivalent potential temperature. The height of the mixed layer top is 1000–1500 m, on the average, over the Japan Sea. The height increases toward the south, and reaches 2000–2500 m over the coastal area of Japan. The height of the mixed layer top, and the apparent total heat source in the transformed air-mass, vary widely in association with the synoptic-scale changes of polar-air outbreak. The height of the mixed layer top tends to increase in association with the increase of the apparent total heat source in the mixed layer, and the decrease of the vertical stability in the lower-middle troposphere. The snowfall depth over the coastal area of Japan tends to increase in association with the increase of the apparent total heat source over the Japan Sea. The important synoptic conditions for the occurrence of the heavy snowfalls are the intense polar-air outbreak, and/or the passage of the cold vortex or the deep cold trough over the