CLASSIFICATION OF THE YAMASE (COLD NORTHEASTERLY WIND AROUND NORTHEASTERN JAPAN) BASED UPON ITS AIR-MASS VERTICAL STRUCTURES

The Yamase wind - a cold northeasterly wind which produces cool weathe r in summer over northeastern Japan- was classified by using upper-air meteorological data. A cluster analysis was performed of the vertical structures of the Yamase air masses, which were divided into seven cl usters. The mean features of the Yamase air mass in each cluster are i nvestigated. The surface low temperature area is in proportion to the Yamase air mass height but the air mass height does not always corresp ond to the height of the easterly wind. In the case of the Yamase air mass and easterly wind being both high (over about 800 hPa), the surfa ce easterly wind blows over northeastern Japan and an extremely low te mperature area spreads over the Pacific side (Clusters 2 and 6). In th e case of the easterly wind being high (over about 800 hPa) but the Ya mase air mass being low (below about 800 hPa), the surface easterly wi nd blows over northeastern Japan but the surface temperature is not ex tremely low, and the cold air is effectively blocked by the mountain r ange (Clusters 3 and 5). In the case of the easterly wind and Yamase a ir mass heights being both low, the surface easterly wind and low temp erature area are restricted to the Pacific side (Clusters 1, 4 and 7). The Yamase wind appearances in each cluster show a seasonal dependenc e; the Yamase wind included in Clusters 6 and 7 blows only in June, th at in Clusters 2 and 4 appears mainly in the Baiu season, in Clusters 1 and 3 it blows from late Baiu to summer, and in Cluster 5 blows in t he summer season. Also the surface pressure pattern, front distributio n, and the mean 500 hPa geopotential height field have the unique char acteristics in each cluster. The Yamase-wind appearances in each year indicate that the severe cool summer years (1980, 1988, 1993) are indu ced by the Yamase winds classified in Clusters 2, 3, and 4; however, t he Yamase wind in Cluster 2 having a thick and cold air mass plays an especially important role in causing a cool summer.