Orographic rainfall events in the orofure mountain range in Hokkaido, Japan

Abstract

Generally speaking, it is well known that the annual mean rainfall and the times of heavy rainfall in Hokkaido Island, Japan are smaller than those of Honshu Island, Japan. In the Orofure mountain range in Iburi sub-prefecture in Hokkaido, however, they have a relatively large annual mean rainl’all and several periods of heavy rainfall per year. Further, based on the previous researches by Konno and Kikuchi (1981), the distribution pattern of the rainfall of the southeastern slope of Orofure mountain range was classified into five patterns; that is to say, Orographic rainfalls, Seaside rainfalls, Plain-centered rainfalls, Northwest slope rainfalls and other type rainfalls, according to the locations where the maximum peak of rainfall was located. To investigate these patterns further we set up our special mesoscale raingauge stations mainly in the southeastern slope of the mountain range and carried out the observations from June through to October, 1980 and 1981, respectively. In the case of orographic rainfall, the maximum peaks of rainfall were very frequently located on the Orofure Pass (930m a.s.l.) and the Shiraoi Waterfall (380m a.s.l.). The fact that the maximum rainfall amount was on the Shiraoi Waterfall in spite of the relatively low altitude was studied from the viewpoint of topography of the southeastern slope of Orofure mountainous regions. As a result, it was considered that when the warm and wet air from the southeast over the Pacific Ocean flowed toward the mountain range, comparatively weak rainfall occurred on the seaside and plain regions owing to the gentle slope, and the heavy rainfall occurred on the mountainous regions owing to the steep slope. Thus, it was expected that the horizontal convergence would increase the rainfall amount on the mountainous regions because the valley became narrower from the seaside to the mountain side. Numerical experiments were carried out to simulate the above prediction. The results of the calculated rainfall taking into account only the forced updraft owing to the mountain slope were smaller than the observed rain in the regions where horizontal convergence appeared to be present.