DISTRIBUTION OF THE EXTREME VALUES OF PRECIPITATION IN JAPAN

Abstract

In the present article, the writer studies some characteristics of annual and seasonal maximum daily precipitation. First, the frequency distributions of the maximum daily precipitation are assumed to be expressed by Gumbel's exteme distribution function (Fisher-Tippett distribution function), Q(x)=exp(-e-a(x-b)) Applying this theory, the annual and seasonal maximum daily precipitations for 5, 10, 25, 50, and 100 year's return periods are estimated respectively at about fifty stations in Japan. The geographical. distributions of the maximum daily precipitation are shown for each period (Fig. 1). There are some differences in the pattern of the distributions for each period. In the June-July period, the Bai-u period, the largest amount is found in the southern part of Japan decreasing gradually toward the north. In the August-November period, the typhoon period, large values are characteristic of the Pacific coast area, especially the regions which are exposed to the humid southeasterly winds originating in the frontal side of the typhoon area, such as the Kanto Plain, the southeast part of the Kii peninsula and the plains of east Kyusyu. Distribution of the extreme values of annual maxima is also similar to the August-Novernber period. This similarity means that the precipitations with large amount are brought frequently by the tropical cyclones. Second, the secular changes of the annual and seasonal maximum daily precipitation are examind for the last fifty years. By the ten-years moving average, the trend curves are drawn for each period and station (Fig. 2). The Gumbel's theoryemay be appiicable, because the secular changes are not generally so remarkable. However, the following tendencies are found: many stations show an in creasing tendency in the June-July period, while some stations, especially on the Pacific coast, show an decreasing tendency in the December-May period.