Classification of the Yamase (Cold Northeasterly Wind around Northeastern Japan) Based upon its Air-Mass Vertical Structures

Abstract

The Yamase wind -a cold northeasterly wind which produces cool weather 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 clusters. The mean features of the Yamase air mass in each cluster are investigated. The surface low temperature area is in proportion to the Yamase air mass height but the air mass height does not always correspond to the height of the easterly wind. In the case of the Yamase air mass and easterly wind being both high (over about 800hPa), the surface easterly wind blows over northeastern Japan and an extremely low temperature area spreads over the Pacific side (Clusters 2 and 6). In the case of the easterly wind being high (over about 800hPa) but the Yamase air mass being low (below about 800hPa), the surface easterly wind blows over northeastern Japan but the surface temperature is not extremely low, and the cold air is effectively blocked by the mountain range (Clusters 3 and 5). In the case of the easterly wind and Yamase air mass heights being both low, the surface easterly wind and low temperature area are restricted to the Pacific side (Clusters 1, 4 and 7). The Yamase wind appearances in each cluster show a seasonal dependence; the Yamase wind included in Clusters 6 and 7 blows only in June, that 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 the summer season. Also the surface pressure pattern, front distribution, and the mean 500hPa geopotential height field have the unique characteristics in each cluster. The Yamase-wind appearances in each year indicate that the severe cool summer years (1980, 1988, 1993) are induced by the Yamase winds classified in Clusters 2, 3, and 4; however, the Yamase wind in Cluster 2 having a thick and cold air mass plays an especially important role in causing a cool summer.