Abstract
In the previous paper, the synoptic climatology of Hokkaido in the winter season was described, with regard to the detailed local weather features. In this paper, the author intended to sketch out the detailed weather distribution of the whole area of Japan according to actual weather patterns of winter monsoon, and to interpret the local and small scale weather features, applying the same method as the previous one. Air flow patterns for each region, shown in Fig. 1, were determined as follows. Firstly, a series of the sea level weather map was classified into 5 types; Northwesterly monsoon type, northeasterly type (NE), anticyclonic type (H), cyclonic type (L) and frontal type (F). Cyclonic and frontal types were excluded from this paper. Secondly, northwesterly monsoon type was classified into 16 types according to the direction of gradient flow at 850mb surface (N, NNW, NW, WNW, etc.). The detailed weather situations for each air flow type were illustrated in Figs. 4 and 11. Fig. 4 indicates the distribution of the percentage frequency of days with daily precipitation more than 5mm, for the northwesterly monsoon type, and Fig. 11 represents the same distribution for the northeastely air flow type. In the former type, the distribution of precipitation clearly corresponds to the topographical features and wind directions. Particularly, the condition of surface relief does not mean merely large scale mountain ranges, but relatively small scale ridges and valleys. Almost all areas of the Japan Sea side of Japan and some parts of the Pacific Ocean slope of Central and West Japan are characterized by snowy or rainy weather. The location of heavy snowfall area shifts with the change of wind direction. Moreover, special local climatic features are recognized in some parts of Japan. For example, as is shown in Fig. 5, snowy weather area occasionally crosses over the mountain divide (Ibuki and Suzuka Mts.), and spreads out into the Nobi Plain, situated in the Pacific side of Central Japan. This extension of snowfall area is affected by the climatic factors such as wind direction, wind speed, and upper air temperature (Fig. 7). Above all, the outbreak of severe cold air from continental Asia, associated with low temperature and high wind velocity, causes heavy snowfall over Japan Sea side and wider extension of snowy weather in some parts of the Pacific Ocean side. In northeasterly wind patterns arise an asimilar distribution of weather in Central and West Japan. Namely, gloomy rainy weather condition is persistent in the coastal region of the Pacific Ocean, and snow or rain shower continues on the Japan Sea side, though a small amount of precipitation is observed. When the Siberian High expands to and covers over Japan, weather becomes better, fine and calm. In the winter season, these weather patterns transits from one to another as shown in Fig. 3. Percentage frequency and persistency of each air flow type are indicated in Tabs. 1 and 2. These results show that the north- westerly monsoon pattern is dominant in winter, especially in the northern part of Japan, and besides, northeasterly air flow and anticyclonic types mainly affect Central and Southern parts of Japan.
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