抄録
In the islands of Japan, some characteristic patterns of precipitation distribution result from the winter monsoon. This paper aims to describe the regional characteristics of the distribution of precipitation in meso-scale during the winter mcnsoon. To show parallel fluctuation in daily precipitation between the places of observation, a cluster analysis was made to the coefficient of correlation between observatories in each 4 districts ; Hokkaido (HD), Tohoku (T), Hokuriku (HR), Seinan-Nippon (S) (Fig. 1). The data used are daily precipitation caused by the winter monsoon in each district during the 1980/1981 winter season, normalized by the square-roots. The precipitation data of the station with more than 10 precipitation days (about 20% of the days chosen) was also analysed. From these results, the districts were divided to the areas using the same standard throughout the districts (Fig. 3). The areas were numbered in order of their average daily precipitation.
In Hokkaiko and Hokuriku, the correlations between areas (Table 1) are rather lower and same pairs have a negative-correlation. Fig. 5 indicates that the higher the precipitation, the lower the coefficient of variation. The percentage frequency of days with precipitation for each 850 mb-level wind direction (Fig. 6) indicates that the percentage is very large for each direction within the even-precipitation areas (the coefficient of variation is less than 1). The average precipitation frequency in the uneven-precipitation areas (the coefficient of variation is more than 1) as a whole, but some particular wind directions is relatively low. This indicates that wind directions which bring heavy precipitation to the area are limited in the uneven-precipitation areas. The correlation between the amount of precipitation and the upper air temparature over the Japan Sea coast (Table 2) shows the contrast between many areas of Hokkaido and of other districts. In the latter, precipitation increases under lower temperature conditions over southwest Japan, while in the former it increases under higher temperature conditions.
The synoptic conditions of the heavy precipitation days in each area were then studied. As conditions vary a little from area to area, the areas as a whole are classified into 2 groups : one group consists of areas located under or in front of the 500 mb cold air mass ; the other consists of areas located behind it. The former shows the following common characteristic conditions. The gradient of the upper air temperature over the Japan Sea coast is weak ; the temperature below the 700 mb level is relatively high over the Tohoku Hokuriku district ; the difference between the temperatures at the 500 mb and 850 mb level over the district is large ; the wind direction at 700 mb, 850 mb level is westerly. The latter shows the following conditions. The gradient of the upper air temperature is strong and the difference between temperatures over the district is smaller; wind direction at the 700 mb and 850 mb is northwesterly. The 500 mb level circulation patterns also differ between the groups. The detailed areal differences could be explained by the 700mb and 850 mb level winds. Especially in Hokkaido and Hokuriku, the heavy precipitation area shifts markedly with the change of wind direction.
Thus, dual differences between areas are found. One is classified as the even and unevenprecipitation areas, with wind directions at the 850 mb level. The other is classified as areas under heavy precipitation conditions located within the 500 mb cold air mass. The patterns of precipitation distribution during the winter monsoon in Japan can be better understood when these characteristics are taken into account.
