Structure and Evolution of a Cumulonimbus Cloud Developed over a Mountain Slope with the Arrival of Sea Breeze in Summer

Abstract

Cumulonimbus clouds frequently develop over mountains, a plain, and the sea in the summer in association with thermally induced local circulations. On July 5, 2000, when the sea breeze from the Pacific Ocean blew over the Noubi Plain and arrived at the slope of the Ibuki Mountains where a valley wind circulation developed, a cumulonimbus cloud occurred over the slope of the Ibuki Mountains. In this paper, the structure and evolution of the cumulonimbus cloud are investigated using the data of Doppler radars.The direction of the environmental vertical wind shear was southeast, which is parallel to the slope of the Ibuki Mountains, when the cumulonimbus cloud occurred. The cumulonimbus cloud maintained forabout 2 hours. The cumulonimbus cloud consisted of groups of precipitating cells; “Primary Cell” and “Secondary Cells.” The former developed with tilting toward the downshear side and moved down the slope. The latter developed almost uprightly on the upshear side of the Primary cell. There were 6 groups of cells in the cumulonimbus cloud.The developing process and structure of group C, which was the most intense group, were investigated in detail. After Primary Cell C1, with tilting toward the downshear side, developed, Secondary Cells C2, C3 and C4 of group C developed on the upshear side (the Ibuki Mountains side) of cell C1. An outflow from cell C1 toward the upshear side of cell C1 lifted the low-level air. Cells C2, C3 and C4 developed almost uprightly on the upshear side of cell C1, where the convection of cell C1 weakened the venical wind shear. Cells C3 and C4 had maximum reflectivety of over 50 dBZ and the echo top of 15 km above sea level (ASL). Cells C3 and C4 developed explosively in the group C due to the horizontal convergence at the middle layer, which was strengthened by the outflow from cell C2, the northeasterly inflow toward cells C3 and C4, and the lifted low-level air on the Ibuki Mountains side.