Distribution of Heavy Rainfall Frequency and its Seasonal Patterns in Tohoku District

Abstract

In Japan, frequent heavy rainfalls appear bringing floods somewhere every year. In this paper, the author considered the geographical distribution of the frequency of heavy rainfalls in Tohoku District. Drawing the distribution map of the frequency of precipitation above 40mm/day, 60mm/day, 80mm/day and 100mm/day in each month (1922-1941), the author obtained the following results.1) The distribution of high frequency areas of heavy rainfalls is fairly specified in each month, and the more the daily amount of precipitation, the clearer is that tendency. 2) High frequency areas do not always coincide with, the so called rainy areas, for exmaple, heavy snow falls appear often on the eastern slope of the Kitakami Mountains where there is little snow usually. 3) The distribution patterns of the frequency of heavy rainfalls change seasonally. Grouping the patterns, a year is divided into four periods: (a) Feb. -Jun., (b) Jul. -Aug., (c) Sept. - Nov. and (d) Dec. - Jan. 4) High frequency areas are on Pacific side at (a) period (eg., Fig. 1, 2), on the Japan Sea side at (b) period (eg., Fig. 3), on Pacific side at (c) period (eg., Fig. 4), on the backbone range at (d) period (eg., Fig. 5). 5) The four periods do not always correspond to the four seasons of the year, but the seasonal change of the patterns is to be understood in a sense of dynamic climatology. In other words, the pattern of the frequency distribution changes from (a) period to (b) period when the Baiu front passes over Tohoku District, the pattern of (c) period prevails in the season of Typhoon, the pattern of (d) period prevails at the northwestern monsoon time, and that pattern disappears when the monsoon becomes weak at the end of January.