Abstract
A case study is made of an exceptionally heavy rainstorm that hit the northwestern coastal area of Kyushu, Japan, on 23 July 1982. The 5hr rainfall accumulation was as high as 412mm at the city of Nagasaki. The precipitation occurred along the warm front as-sociated with a medium scale disturbance that developed along the Baiu front. The atmos- phere in the pre-storm period was very moist through virtually the entire troposphere and its static stability was conditionally unstable with a lifted index of -1.5°C, thus exhibiting characteristics more similar to the tropical atmosphere than the typical pre-storm environment over the midwestern United States in spring seasons.
Prior to the development of heavy precipitation over the Nagasaki area, an intensive rainband was propagating with a speed of 40km hr-1 eastsoutheastward over northwestern Kyushu. Its structure is found to be different from the structure of typical tropical squall clusters in that the heaviest rainfalll occurred in the middle of the rainband without a well defined leading edge. A dramatic change occurred in the rainband when its southern tip reached the Nagasaki area; it stopped propagating and stayed there for the next 5hr. Its line structure changed to a blob structure at the meso-β scale (-100km). The timing of the precipitation development at Nagasaki coincided with the peak period of the incursion of very moist air associated with the southwesterly low-level jet. During the period of heavy precipitation, a new cloud cluster formed over the sea 300km west of Nagasaki. It kept developing and propagating eastward with a speed of 60km hr-1, and eventually merged with the rainstorm over Nagasaki. These observations strongly suggest that the Nagasaki rainstorm was trapped orographically. Several other heavy precipitation events are cited which also occurred in association with medium scale disturbances along the Baiu front and in which the precipitation developed over the coast areas upwind of the low-level jets.
