Journal of Natural Disaster Science

ENHANCEMENT OF RAINFALL BY TOPOGRAPHIC EFFECTS

Toshio HARIMAYA
Professor, Department of Geophysics, Faculty of Science, Hokkaido University, Hokkaido
Kazutoshi TOBIZUKA
Graduate Student, Division of Environmental Structure, Graduate School of Environmental Science, Hokkaido University, Hokkaido
and
Katsuhiro KIKUCHI
Professor, Department of Geophysics, Faculty of Science, Hokkaido University, Hokkaido

(Received 16 January, 1988 and in revised form 26 July, 1988)

Abstract

The radar observation was carried out in the Iburi District, Hokkaido to study the enhancement of rainfall by topographic effects and the actual interactions between low topographic clouds and rain clouds associated with disturbance. The amounts of rainfall and radar analyses showed that rainfall was enhanced only on the upwind slope even though rain bands passed over both the upwind and downwind slopes.

Updraft or low topographic clouds are formed on the upwind slope when moist air masses from the sea are pushed up the slope by south-easterly wind from the sea. If, under such conditions, rain clouds associated with disturbances arrive over the upwind slope, they are modified or interact with low topographic clouds of different rain drop-size distribution, producing rain. By contrast, this mechanism is not active on the downwind slope or in other areas; therefore, it is the formation of updraft or low topographic clouds that enhances the amount of rainfall on upwind slopes only.

Key words

heavy rainfall, topographic effect, enhancement