A Study of the Heavy Rainfall over the Ohsumi Peninsula (Japan) Caused by Typhoon 9307

On the night of 9 August 1993, Typhoon 9307 passed near the southern part of Japan and caused heavy rainfall and many land-slide disasters across the Ohsumi Peninsula. A notable feature of this event is that the rainfall amount was very large on the lee side of the mountains. The 9-hour rainfall exceeded 150mm at the greater part of the stations on the lee side, while it was only 80mm at the station on the windward side. All the land-slide disasters occurred on the lee side. The heavy rainfall was mainly produced between the eyewall and the rainband of the typhoon. Radar echoes showed strong reflectivity over the mountains during the rainfall event. The rainfall increasing process was studied by using a two-dimensional numerical model. The results suggested that the seeder-feeder mechanism contributed to the rainfall enhancement on the lee side. Namely, the precipitating particles from the upper cloud grew over the mountains by accreting cloud water produced by the orographic lifting, and the grown particles were carried to the lee side by the strong wind of the typhoon. In addition, it was also suggested that the downdraft of the mountain wave made an additional contribution to the rainfall enhancement by transporting many ice particles to the lower layer.