Abstract
Two rainy periods might be identified in this country, occurring before and after fairly dry summer season. The former one is the Bai-u season which persists from the beginning of June to the middle of July and prevails mostly in southwestern parts of the country. And the latter one is the autumnal rainy period which starts from the end of August and lasts to the middle of October. In northern Japan, this rainy period is more remarkable than the Bai-u season and records heavier rainfall. It is generally believed that autumn is the fine and refreshing season in this country, but statistically as is shown on Fig. 1, early autumn is an obvious rainy period.
[1] Climatological Aspects.
The geographical distribution of total amounts of rainfall in September and October (1931-60) is demonstrated on Fig. 2. On this map, the following distribution characteristics might be pointed out:
1) Heavy rainfalls of more than 400mm have been reported along the coastal regions of the Pacific and also Japan Sea sides. At certain stations along the Pacific, they have measured heavier rainfall than 600mm for the total of these two months.
2) Along the central axis of the Japanese Islands, there has been a prolongation of fairly dry belt having less rainfall than 300mm.
3) Hokkaido, the northern island, is a rainless area in the country and reports only about 200-300mm of rainfall.
Fig. 3 illustrates geographical distribution of the ratios of September and October rains against annual amounts. This ratio might serve as an index of the degree of concentration of autumnal rains. Almost all stations, except the southern island, Kyushu, have registered higher ratios than 61/365 (supposed even occurring probability). Then, they suggest that the rainfall concentrations in September and October at various places are high and these two months may be regarded as the autumnal rainy period.
The geographical distribution of the ratio of the autumnal (September & October) rains against the Bai-u (June & July) rains is indicated on Fig. 4. The areas which the autumnal rains predominate more than the Bai-u rains are figured out along the coastal regions and in the northern parts of the country. And the Bai-u areas are distributed mainly in Kyushu and along the central axis of the Japanese Islands.
September and October in this country is also the period of typhoons. Therefore, the amounts of rainfall of these two months definitely include typhoon rains, besides of non-typhoonal rains. On this paper, it has been devoted to analyse principal characteristics of such non-typhoonal rains. Thus, typhoon rains and non-typhoon rains have been separated on these two months, based on synoptic analysis. Their amounts and ratios against total rainfall have been tabulated on Tab. 1, at 16 selected weather stations representing various climatic regions in the country. Typhoon rains reach 30-50% of total amounts for this season at almost all stations. The distribution of typhoon rains in September and October during the periods of 1956-60 have been demonstrated on Fig. 5.
Along the Pacific coasts and in the southern Japan, it has been observed heavy typhoon rains of more than 150mm. And this distribution pattern suggests complex distribution characteristics of the orographic band-structure and the coastal-rain type. These two are typical distribution patterns of daily typhoon rains.
Non-typhoonal rains which occupy 50-70% of total amounts of September and October rains are mainly caused by cyclones and frontsmeteorologically. Their occurring frequencies are tabulated on Tab. 2 and the geographical distribution of total amounts of the non-typhoonal rains is illustrated on Fig. 7.
The most remarkable characteristic of the distribution is few regional diversities. Almost every station has reported about the same amounts of rainfall around 200-300mm.
