1985 Volume 63 Issue 5 Pages 859-871
The detailed analysis on the strucure of the polar maritime airmass is made for two typical cases of summertime cold spell (11-20 August 1976 and 25 July-4 August 1980) over Tohoku District. The summertime cold weather over Tohoku District is caused by the westward cold air outbreak from the polar maritime airmass formed over the Okhotsk Sea, the Bering Sea and the northern Pacific. The thin (the surface to about 1 km) layer of the cold easterly wind shows the character of the mixed layer and is capped by stable layer. The large thermal gradient (∂T/∂x--2K/100km), and the thermal advection (∨•∇T-0.2K/hour) are restricted within the mixed layer. The amount of total heat energy supplyed from the sea to the atmosphere over the sea adjacent to Tohoku District is estimated to be -1001y/day. The polar maritime air can reach Tohoku District without the full airmass transformation, because the sea water is cold in the Pacific adjacent Tohoku District.
The analysis shows the influence of topography and the ground temperature on the air temperature and cloud (sunshine duration) in the cold air. It is infered that the orographically induced downward motion in the west side (lee-side for the easterly wind) of the mountain ranges in Tohoku District disperses the cloud formed in the upper part of the mixed layer, and causes the temperature rise due to the insolation over the Japan Sea side of Tohoku District. The thin cold air is rapidly warmed over the ground and/or sea surface with the higher temperature. This explains that the summertime cold and cloudy weather is localized over the Pacific side of Tohoku District.
