The Characteristics of Precipitation Distributions in “Bai-u” Season in the Upper Reaches of the Agano River, Northeastern Japan
1979 Volume 31 Issue 1 Pages 58-67
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1979 Volume 31 Issue 1 Pages 58-67
Japan has two main rainy seasons, the one represented by “Bai-u” season or typhoon and the other caused by winter monsoon on the Japan Sea side. Generally speaking, in Japan “Bai-u” season ranges from the early June to middle July. In present paper, the writer tried to find the characteristics of precipitation distributions in “Bai-u” season (in June and July) in the upper reaches of the Agano River, Fukushima Prefecture, Northeastern Japan, using the statistics in the period of five years (from 1966 to 1970). The results obtained are as follows:
1) The regional difference between distribution of the total precipitation and that of occurrence frequency of rainy day (above 1.0mm/day) is recognized as one of the climatological characteristics in this region (Figs. 1, 3 and 4).
2) The distribution maps in each succeeding rainfall can be classified from E-type to X-type as shown in Table 2, those in the daily precipitation, however, can be classified in detail moreover as Table 3.
3) The occurrence of heavy rainf ails in this region is especially related to the activities of “Bai-u” fronts over Tohoku District. Furthermore, the extra-tropical cyclones moving from southwest to northeast over the Sea of Japan have the distinctive influences on precipitation distributions of this region.
4) Weathers of the Japan Sea side invade into the western part of this region, those of the Pacific side, on the contrary, extend to Mt. Adatara region, in the eastern part of this region. Therefore, the central part of this region (that is, the drainage area of the Inakawa River as well as that of the Okawa River) is not blessed with abundant rainfall.
5) In order to study the aerological situations in which heavy rainfalls occurred in this region, the vertical-time cross sections of wind velocity, wind direction and relative humidity on the one hand (Figs. 7 and 8), and the vertical-meridional cross sections of those on the other hand (Fig. 9) were drawn for the period from June to July in research years respectively. As the results, it is found that the heavy rainfalls will be expected in this region, when westerly moist (90%< in relative humidity) and strong winds (10m/s< in wind velocity) prevail in the upper layer between 800mb to 600mb.
