DISTRIBUTION OF WINTER PRECIPITATION BROUGHT ABOUT BY THE PASSAGE OF DEPRESSIONS OVER THE JAPANESE ISLANDS

Abstract

The winter climate of Japan is dominated by the northwesterly monsoon, which brings: abundant precipitation in the form of snow to the regions facing the Japan Sea. Most climatological studies on winter precipitation have concentrated upon the relationship betweenn the heavy precipitation near the Japan Sea and the winter monsoon, but a considerable amount of precipitation is brought about also by the passage of depressions or fronts. The purpose of this paper is to reveal some characteristics of the precipitation brought about by the passage of depressions, especially its areal distributions and the time variation of precipitation intensity. The results obtained are summarized as follows: 1) The depressions which pass over the Japanese Islands in winter may be classified into two types: the Japan Sea Low type, in which depression forms over the southwest of the Japan Sea, moves northeastward, and passes through the Soya or the Tsugaru Strait, andd the Pacific Coast type, in which depression rises over the East China Sea and travels eastward along the Pacific Coast of Japan (Fig. 1). 2) The Pacific side has little precipitation during the passage of Japan Sea Lows. The distribution of precipitation associated with these depressions is similar to that of monsoonall precipitation. Furthermore, it is fairly irrespective to the location of the depression. On.n the other hand, Pacific Coast Lows bring more precipitation to the Pacific side than to the Japan Sea side, and the distribution of precipitation changes as the depression moves eastward. The ordinary monsoonal precipitation pattern disappears during the passage of a. Pacific Coast Low, and then recovers once more after the passage (Figs. 2, 3 and 4). 3) There are conspicuous regional differences in the characteristics of precipitation during the passage of depressions. On the basis of the depression type which brings precipitation and the period of the maximum intensity of precipitation, eight major regions can be identified.as follows (Figs. 5, 7 and 9). On the Pacific side: (a) West of Kanto-more precipitation occurs during the passage of a Pacific Coast Low than during the passage of a Japan Sea Low. The intensity of precipitation increases rapidly as the depression approaches, reaches a maximum just before the arrival of de pression, and decreases with the depression moving away to the east. (b) Inland Honshu-similar to region (a), except that period of maximum intensity of pre cipitation is longer. (c) Tohoku-similar to region (b), except that the intensity of precipitation decreases much more slowly after the depression has passed. (d) Hokkaido-there is little difference in the amounts of precipitation brought by the two types of depression, and either type produces a little precipitation, i, e. this region is not so affected by the passage of depressions. On the Japan Sea side: (e) San-in-more precipitation occurs during the passage of a Pacific Coast Low than during the passage of a Japan Sea Low, but unlike the Pacific side regions, the maximum intensity of precipitation occurs after the depression has passed. (f) Hokuriku-there is little difference in the amounts of precipitation brought by the two types of depression. More precipitation is brought to this region by the winter monsoon than by depressions. (g) Tohoku-more precipitation occurs during the passage of a Japan Sea Low than during the passage of a Pacific Coast Low. Precipitation reaches maximum intensity before a Japan Sea Low arrives; that is, while it is still located over the central Japan Sea. (h) Hokkaido-similar to region (g), except that the absolute amount of precipitation is less than in any other region except the Pacific side of Hokkaido, and there is no distinct period of maximum precipitation intensity.