Some small-scale features of the wind and damage distributions caused by the local downslope wind Hiroto-Kaze associated with typhoon TOKAGE

With the passage of the typhoon TOKAGE in 2004, a widespread wind damage has taken place within and around the Tsuyama basin which is located in Okayama prefecture, Japan. The large number of fallen trees is one of the characteristic features of the wind damage at that time. Some researchers indicate the possibility that the unusually strong local downslope wind "Hiroto-Kaze" was induced by the typhoon TOKAGE. "Hiroto-Kaze" is one of the strongest downslope winds in Japan, which blows on the southern foot of the 1,240m-high- Mt. Nagi situated to the northeast of the basin. Wind gust recorded at several observation stations in the basin have exceeded 50m/s. The rather large values of the gust to the mean (10 min average) wind speed ratio (gust factor) may be related to the extensive wind damage. The coauthor F. Kataoka derives the detailed distributions of the fallen trees from satellite data analysis with the horizontal resolution of several tens of meters. In the present study, we will investigate the relationships between the distributions of fallen trees and the wind field through simulations with a numerical fluid dynamics model and an elastic dry-atmosphere model. The former model is one-way nested in the latter model. Small-scale features of the relations among wind field, topography, and fallen tree distribution will be mainly concerned here.