Structure and Evolution of Deeply Developed Convective Cells in a Long-Lived Cumulonimbus Cloud under a Weak Vertical Wind-Shear Condition

摘要

The structure and evolution of deeply developed convective cells within a long-lived multicellular cumulonimbus cloud that developed over the Huaihe River Basin in China on 13 July 1998 during the GAME/HUBEX were studied, mainly using the observational data of Doppler radars. The lifetime of the cumulonimbus cloud was longer than 3.5 hours, and its maximum echo-top height was 19 km AGL. The atmospheric condition was characterized by a large CAPE (2300 J kg−1), and weak vertical wind shear (1.6 m s−1 km−1 toward the north-northeast below 5 km AGL).In the mature stage of the cloud, a large radar-echo region stronger than 40 dBZ was observed. It was almost upright, and showed almost the same horizontal areas between 5 and 15 km AGL. Two strong updrafts existed around the upshear and downshear parts of the strong echo region, and they tilted toward the downshear and upshear sides, respectively. A downdraft developed in the eastern part of the cloud, where a cell-relative northeasterly wind was present at the low and middle levels; however, the wind was not observed in the synoptic-scale environment. The downdraft was located between the two updrafts. A descending northeasterly airflow continuously lifted the low-level air coming into the upshear-side updraft from the southwestern (upshear) side. The downdraft, and its surface outflow, did not cut off the supply of low-level air coming into the downshear-side updraft from the northern (downshear) side. The vigorous development of the cumulonimbus cloud in the mature stage was caused by the development of these adjacent convective cells. Significant processes in the formation of the mature structure are the maintenance of the upshear-side convective cell, and the change of the downshear-side updraft from downshear-tilting to upshear-tilting in association with the intensification of the downdraft in the cell-relative northeasterly wind.