The Synoptic-Climatological Characteristics of Heavy Rainfalls in the Region of Pacific Ocean of Tohoku District

Abstract

The appearance-distribution of heavy rainfalls (above 50 mm per day) in the region of Pacific Ocean side of Tohoku district is analysed synoptic-climatologically. The main results obtained are as follows;1) The heavy rainfalls in warm season tend to appear in the eastern (coastal) region of Tohoku district. This tendency is more obvious as the maximum amount in rainfall-area is larger.2) The heavy rainfalls in the western and central regions are mianly due to the front, thunderstorm and northwesterly monsoon. On the other hand, those in the eastern (coastal) region are mainly due to the typhoons and subtropical cyclones. Regarding the latter, almost every case (80-90%) having the maximum rainfall-amount more than 100 mm per day is due to the typhoon or subtropical cyclone.3) The larger the daily mean sea-level pressure difference (Δp) between Nemuro and Choshi, the more frequently thd heavy rainfalls (above 100 mm per day) appear in the eastern region.4) The fact mentioned in 2) is more obvious in the eastern (coastal) region when positive Δp is larger, and in the western and central regions when negative Δp is larger.5) The annual variation of frequency of heavy rainfalls in an area with the size of a prefecture corresponds to that of precipitable water. This relation, however, is not obvious in smaller regions covering a half or one third of a prefecture, because the heavy rainfall areas are dislocated according to synoptic condition specially in summer and in winter.