MESOSCALE AND CONVECTIVE-SCALE FEATURES OF HEAVY RAINFALL EVENTS IN LATE PERIOD OF THE BAIU SEASON IN JULY-1988, NAGASAKI PREFECTURE

A comprehensive observation during the late period of the Baiu season was performed in July, 1988, in the project ''A Study of Mechanism and Prediction on Heavy Rainfalls during Rainy Season in Japan'' (Asai, 1 990). Spatially and temporally dense (-100 km, 3-6 h) sounding data an d Doppler radar data were collected during this special observation. I n this study, in order to investigate the mesoscale and convective sca le features of the heavy rainfall events in the late period of the Bai u season, the low-level wind field and evolution processes of radar ec hoes were analyzed by using the Hokkaido University Doppler radar, whi ch was located in Saikai town, Nagasaki Prefecture. Two successive mes oscale rainfall events associated with the Baiu front (Case 1 and Case 2) which were observed on July 17 and 18, 1988, formed the main units of analysis. Both rainfall events occurred in association with a clou d cluster. Heavy rainfall amounts exceeding 100 mm were experienced in limited areas during Cases 1 and 2 (for example, rainfall amounts at Isahaya during Case 1 and Case 2 were 165 mm/3h and 104 mm/3h, respect ively). The Doppler radar analyses revealed important mechanisms of th e heavy rainfall events, which are described briefly as follows. In Ca se 1, enhancement of the Baiu frontal convergence by gust fronts from convective clouds, and the merging between the Baiu frontal rainband a nd isolated echoes generated south of the Baiu front played an importa nt role in the organization of a well developed rainband. The rainfall event of Case 2 occurred about 4 hours after Case 1 and in almost the same region. In Case 2, an organized are-shaped echo caused the heavy rainfall, and gust fronts contributed to the organization of this ech o. The advection of warm and moist air from the undisturbed region of the Case 1 rainfall also played an important role in its development. From these case studies, it can be concluded that mesoscale and convec tive scale features influencing the formation and maintenance of Baiu frontal rainbands are (1) enhancement of low level convergence by conv ective outflow (gust front), (2) merging effect of convective clouds, and (3) mesoscale contrast of temperature and rainfall produced by mes oscale rainfall.