Precipitation mechanism and organization of mesoscale convective systems.

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A heavy rainfall associated with the Baiu front was observed during an intensive observation period in the Kyushu District in July 1988. We studied a structure and mechanism of a rainband caused the heavy rainfall. A retrival method was developed to raveal a thermodynamic field in the rainband. The retrived field showed that the evaporation of raindrop and a vertical shear were important to maintain the inteese rainband.A numerical prediction experiment of the heavy rainfall was performed. The observed rainfall was simulated well and the environmental condition of the rainfall was stduied. A numerical model with high horizontal resolution was used to find a detailed structure of the heavy rainfall. The rainband was simulated and its vertical structure was clarified.A narrow cloud band developed over Kanto Plain on 22 April 1992 in association with the passage of an extratropical cyclone. Its unique features include the rapid formation and appcarance of an arc-shaped solid cloud wall, rather than a line of a number of isokated convective cells. It evolved along the southern edge of a much broader weak rain area in a manner similar to the embedded areal type as identified by Bluestein and Jain (1985).A mesoscale analysis of conventional observation data reveals that a local cold air mass propagated southeastward from the northwestern part of Kanto Plain. It was driven by the horizontal pressure gradient force associated with the eastward-propagiting cyclone. The cloud band was found to be forced to form when and where the cold surge collided with the synopic scale southwesterly flow which was prevailing over the southern part of Kanto Plain.It is further revealed that the cold surge was a leading part of a low-level mesoscale air mass which was termed as "forerunner". It was so named since it existed in front of the sysnoptic scale northwesterly flow and beneath the southwesterly flow ahead of the upper-level trough.