Effects of Orographically Forced Upstream Lifting on Mesoscale Heavy Precipitation: A Case Study

Abstract

An exceptionally heavy rainstorm hit the coastal area of the western part of Japan on 23 July 1983. The 6-h rainfall accumulation exceeded 300 mm locally. At its peak period, the hourly precipitation rate was as high as 90 mm. Yet the area of heavy precipitation was limited in its extent so that the Maddox criteria for midlatitude Mesoscale Convection Complex was not met in terms of size, when viewed in satellite data. The precipitation occurred in the warm, moist southwesterly sector of a weak, eastward propagating medium scale cyclone that developed along the Baiu front. Prior to the onset of heavy precipitation, the atmosphere was very moist up to the 600 mb level, and became convectively very unstable. The main topographic feature in the area where heavy precipitation occurred is a mountain ridge that runs approximately parallel to the coastline, with modest elevations of generally less than 1000 m except for mountain peaks. An analysis of raingage records over land clearly indicates that the rainfall accumulation was maximized in the coastal area rather than in the mountainous area. A detailed analysis of PPI radar data reveals that, during the heavy precipitation period, convective cells formed in succession over the sea about 50 km off the coast. As they moved eastward and approached the coastline, they developed rapidly and organized into a band structure. They then weakened on the downwind side of the mountains. A one-layer model developed by Danard is applied to investigate the topographic effect on the surface flow in the situations under study. The model result indicates that the surface flow over land is deflected mainly by the effect of the topographic barrier and partly by the increased surface friction over land. A convergence zone forms over the coastal strip and the adjacent sea between this deflected flow and the relatively undeflected flow over water. A local maximum of convergence is located just over the area of the maximum rainfall accumulation. This feature accounts for the enhancement of traveling convective cells over the coastal strip and suggests that even a mountain ridge of modest height could enhance precipitation significantly.