暖冬年と寒冬年における日本の降水量分布の比較

Distribution of winter precipitation in Japan was compared for the cases of warm and cold win-ters. In winter, the Japan Sea coast area has a great deal of precipitation brought by the NW mon-soon, whereas the Pacific coast area is dominated by dry weather because the mountains have a “rain shadow” effect against the NW monsoon. Precipitation along the Pacific coast is ordinarily brought by extratropical cyclones passing through off the coast of the Pacific Ocean. Sometimes cyclones emerge in the Japan Sea and bring some precipitation in the area of the Japan Sea coast.<br> In cold winters the contrast in distribution of precipitation between the Japan Sea coast and the Pacific coast becomes more marked because of the more persistent and severe NW monsoon. In warm winters, however, such contrast becomes obscure and cyclone precipitation dominates the whole country (Fig. 5).<br> Six winters were selected as warm and cold winters from 27 winters (1953/54-1979/80) by the following method. A winter surface temperature (three months: Dec. -Feb.) was represented by nine 10-day mean values. When the 10-day value was higher (lower) than the normal by one standard deviation, it was counted as a warm (cold) 10-day period. The numbers of warm and cold 10-day periods were counted for ten meteorological stations (see Fig. 1-c). The winters that had a large number of warm (cold) periods and a very small number of cold (warm) periods were selected as warm (cold) winters (Fig. 2).<br> Figure 4 shows the average precipitation rate and its ratio to normal values for the periods. In a warm winter relatively heavy precipitation (above normal) was observed over a broad area except the Japan Sea coast area, whereas in a cold winter the distribution showed the reverse pattern.<br> Synoptic factors bringing precipitation were classified by using daily synoptic charts (Table 1). Figure 6 shows these factors: 1) M-type, in which the pressure patterns are “West-high East-low” type and NW monsoon is strong, 2) L-type, in which extratropical cyclones are situated in the vicinity of Japan and the NW monsoon is very weak or absent, 3) ML-type, which means a transi-tional pattern from L-type to M-type, and 4) 0-type, which means “others” The occurrence fre-quency for the six winters (Table 2) shows that about 70% of total days were classified as days with M- and L-type precipitation.<br> Figures 7 and 8 show the precipitation amounts and percentage of total amount for each factor, respectively. M-type precipitation dominated the Japan Sea coast area, especially in the central part of Honshu Island; the percentage value exceeded 50% in cold winters, whereas it was about 20% in warm winters. L-type is the only main factor in the Pacific coast area, so this type showed about 80% in either warm or cold winters. In addition, L-type precipitation is fairly abundant in warm winters even in the Japan Sea coast area, and this became the dominant factor in warm winters almost all over Japan except in the central part of Honshu Island.<br> In sum, during warm winters most precipitation in the whole country was caused by extratro-pical cyclones, whereas in cold winters the NW monsoon caused a great deal of much precipitation and dominated in the Japan Sea coast area. The only area with above (below) normal precipitation in cold (warm) winters was the central part of the Japan Sea coast of Honshu, Island, because of the greater increase (decrease) in M-type precipitation than the decrease (increase) in L-type precipitation.