Mesoscale and Convective Scale Features of Heavy Rainfall Events in Late Period of the Baiu Season in July 1988, Nagasaki Prefecture

Abstract

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, 1990). Spatially and temporally dense (-100 km, 3-6 h) sounding data and Doppler radar data were collected during this special observation. In this study, in order to investigate the mesoscale and convective scale features of the heavy rainfall events in the late period of the Baiu season, the low-level wind field and evolution processes of radar echoes were analyzed by using the Hokkaido University Doppler radar, which was located in Saikai town, Nagasaki Prefecture. Two successive mesoscale 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 cloud 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, respectively). The Doppler radar analyses revealed important mechanisms of the heavy rainfall events, which are described briefly as follows. In Case 1, enhancement of the Baiu frontal convergence by gust fronts from convective clouds, and the merging between the Baiu frontal rainband and isolated echoes generated south of the Baiu front played an important 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 arc-shaped echo caused the heavy rainfall, and gust fronts contributed to the organization of this echo. 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 convective scale features influencing the formation and maintenance of Baiu frontal rainbands are (1) enhancement of low level convergence by convective outflow (gust front), (2) merging effect of convective clouds, and (3) mesoscale contrast of temperature and rainfall produced by mesoscale rainfall.