寒冬と暖冬における日本の１月の降水量・降雪量分布の比較

Abstract

It is often said that heavier snowfall is obtained in relatively colder Jaunuaries. However, lighter rainfall (not snowfall) may also be obtained then, since colder air contains less water vapour.This paper compares the distribution of precipitation and snowfall in Japan obtained in both colder and warmer Januaries with that in usual ones. Six Januaries-those of 1963, 1977, 1981, 1984, 1985 and 1986-are selected as the colder Januaries, while six-1964, 1972, 1973, 1979, 1988 and 1989-are selected as the warmer ones. In both cases, the distribution of precipitation and snowfall is discussed and compared with that of usual Januaries (Figs. 3 and 4). Also the “snowfall ratio”, which is defined as the value of the monthly total depth of snowfall per total monthly precipitation, is calculated (Fig. 5). Besides, in order to assume the relationship between the synoptic conditions, the atmospheric pressure pattern in both colder and warmer Januaries is discussed (Fig. 6).In five of the six colder Januaries, lighter precipitation was usually obtained in almost whole of Japan except for the Hokuriku District (the Japan Sea side of Central Japan) and the western part of Hokkaido, both of which correspond to the landing locations of JPCZ (Japan Seapolar Airmass Convergence Zone) or the generated locations of local anticyclones (or local low-pressure areas). The value of the snowfall ratio is larger, so it can be considered that in a colder January when a heavier snowfall occures, the precipitation usually obtained as rainfall is obtained as snowfall. The value of atmospheric pressure is not always smaller, but a steeper pressure gradient is found. However, in 1963 January, much heavier precipitation was obtained on the Japan Sea side and only lighter precipitation on the Pacific Ocean side. The value of the monthly mean atmospheric pressure was much smaller. Therefore, January 1963 must be considered as an abnormal case.In five of the six warmer Januaries, heavier precipitation was obtained on the Pacific Ocean side and lighter precipitation on the Japan Sea side, especially the Hokuriku District and the western part of Hokkaido; in 1988 lighter precipitation was obtained in the whole area of Japan. The value of the snowfall ratio was usually smaller, so it can be considered that the lighter snowfall in the warmer Januaries is the result of both lighter precipitation in itself and the rainfall usually obtained as snowfall. The value of monthly mean atmospheric pressure is usually larger, and gentler pressure gradient is found.This paper emphasizes the general features both in the colder Januaries and in the warmer ones. However some difference is found among both the colder and the warmer Januaries. And these general features do not always fit with Hokkaido or the Pacific Ocean side of Northeast Japan. These problems must be given further consideration.