Journal of Natural Disaster Science

ENHANCEMENT OF RAINFALL BY THE OROGRAPHIC EFFECT: A CASE STUDY

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

(Received 29 September, 1988 and in revised form 12 December, 1988)

Abstract

A case study was made of the response of a cloud cluster to the orography when a cloud cluster around a cyclone passed over the Iburi District of Hokkaido. Rain system associated with this cloud cluster travelled over the Iburi District, an area both of mountains and seacoast, but heavy rain occurred only in the mountainous part. This indicates that rainfall was enhanced by the orography.

On the basis of our analysis of l-minute rainfall we concluded that although the peak values of rainfall in the mountainous area were nearly equal to those in the coastal area, strong rainfall continued for an extended period only in the mountainous part. The time variations in the rainfalls of both areas were supported by radar observations that showed that top of the radar echoes in the mountainous part was nearly equal to that in the coastal area and that the radar echoes developed in the forward direction forming other radar echoes between them only in the mountainous section. This enhancement of rainfall is considered to be a type of response of rain system associated with atmospheric disturbance to the orography.

Key words

heavy rainfall, orographic effect, enhancement