Long-Lived Medium-Scale Cumulonimbus Cluster in Asian Subtropical Humid Region

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Abstract
GMS (Geostationary Meteorological Satellite) observations reveal a long-lived mediumscale Cb cluster which migrates along -32N latitude circle over the Asian subtropical humid region in 12-16 July 1979. In the present synoptic and subsynoptic-scale study based on upper, surface, radar and GMS infrared data, we describe the features and structure of the Cb cluster and its developing process in relation to the large-scale situations. The Cb cluster is associated with a medium-scale upper westerly trough and migrates with speed of -1, 000km/day. Although the convective activity in the cluster indicates time variations with meso-scale period, the medium-scale cluster as a whole lasts for several days. TBB (equivalent black body temperature measured by GMS infrared observation, which is almost equal to the cloud top temperature) of -75--60°C is observed for several days. Intense rainfalls of 50-100mm/hour and radar echoes of 12-15km height are found under the high cloud shield where TBB is less than -70°C. Considering the features of cloud system and medium-scale circulation, we subdivide the life history of the Cb cluster into "Cb cluster stage (12 GMT 12-12 GMT 14)" and "medium- scale depression stage (12 GMT 14-00 GMT 16)". During passage over the China continent where baroclinicity in the lower troposphere is extremely weak, the Cb cluster remains as an isolated oval-shaped Cb cluster and the depression in the lower level is not formed (Cb cluster stage). When the Cb cluster moves into the baroclinic zone in the Pacific area, the isolated cloud system changes into elongated cloud system and the depression is formed in the lower layer (medium-scale depression stage).