Abstract
In order to clarify the process of cumulus development on the windward side of a mountain range in convectively unstable air mass, an orographic heavy rainfall over Kyushu (southwestern Japan) on August 29-30, 1974 is studied using satellite, radar, raingauge and radiosonde data. The heavy rainfall occurred under the situation that convectively unstable subtropical air mass was transported to a mountain area of 1, 004-1, 500 m high by strong lower tropospheric winds. The influence of the vertical motion of large-scale disturbances (typhoon and frontal system) on the heavy rainfall is supposed to have been very small since they were far from the analyzed domain.
The cumulus development began at about 150km off the coast line and maximum precipitation was found over the windward slope of the mountain range. The distribution of rainfall reflected the small-scale topography of the range. The predominant peak of precipitation amount in 24 hours (-170mm) was located at the leeward foot of an isolated mountain (-1, 400m) near the coast. Though the radiosonde data suggested the existence of a shallow convective cloud layer over the windward slope of the mountain range, the raingauge and radar data indicated that the nature of the rainfall was highly convective and that the heaviest rainfall mainly resulted from deep convective clouds (-8km high) triggered by the orographic ascent.
