1998年一次梅雨锋暴雨中尺度对流系统的模拟与诊断分析

NUMERICAL SIMULATION AND DIAGNOSTIC ANALYSIS ON MESOSCALE CONVECTIVE SYSTEMS OF A TORRENTIAL RAIN CASE IN MEIYU PERIOD OF 1998

文中利用观测资料 (包括部分‘四大科学试验’资料 )和高分辨率数值模拟结果 ,对 1998年 6月 16～ 17日发生在赣闽浙沿武夷山北麓地区的梅雨锋暴雨中尺度对流系统特征进行了分析研究。分析表明 :(1)本次梅雨锋暴雨发生在对流层中低层中 β尺度低压南侧的中尺度辐合线上 ;在弱的风垂直切变环境下 ,梅雨锋中α对流云系中有数个中 β尺度云呈塔状强烈垂直发展 ,它们是造成暴雨的中 β尺度对流系统。(2 )基于加密探空观测的对流有效位能计算显示 ,赣闽浙沿武夷山北麓地区的强暴雨发生前 ,最大对流有效位能可达到 2 6 0 0J/kg ;通过时间加密的探空观测有可能捕捉对流有效位能的中尺度变化特征。(3)利用高分辨率模拟结果对赣闽浙沿武夷山北麓的暴雨中β尺度对流系统 (中 β降水云塔 )的结构分析显示 ,强烈发展的中 β降水云塔为有利的中尺度动力配置结构 ,即对应着一个狭窄的、从地面伸展到 2 5 0hPa的正涡度区 ,其 1.5m/s的垂直上升运动与低层强辐合和高层强辐散相伴随。(4)通过分析与诊断 ,提出了低层中尺度辐合线上强烈发展的梅雨锋暴雨中 β尺度对流系统的气流运动图像 ,即 :在对流层低层 ,空气从西南和西北两个方向流入中 β降水云塔区 ,在云塔中垂直 (略向东倾斜 )上升 ;靠近云塔南 (北 )

The characteristics of the mesoscale convective systems of a torrential rain case occurring over south Yangtz River on 16-17 June 1998 are investigated by using mesoscale numerical simulations and observations as well as 1998 field experiments' data. It has been revealed that the torrential rain occurs associated with the generation of a mesoscale convergence line in the region to the south of a mesoscale low in lower troposphere; under the weak vertical wind shear condition, several meso β clouds, the convective systems directly related to the torrential rain, develop strongly in vertical like towers within a meso α cloud; and before the occurrence (about 8 h) of the torrential rain, the maximum value of convection available potential energy could reach 2600 J/kg, which is gained through the intensive sounding data from 1998 field experiments . The diagnosis on the structures of the meso β convective systems, based on the mesoscale simulation results, shows that the strong development of meso β convective systems is supported by the favorable mesoscale dynamic configuration, namely, a very narrow positive vorticity zone extending from surface up to 250 hPa is coupled with strong lower level convergence and upper level divergence, which is associated with 1.5 m/s vertical velocity at 500 hPa. In addition, investigation implies that the temperature structure of meso β convective systems, i.e., warmer in middle troposphere and colder in upper and lower troposphere than the environment, may relate closely to the latent heat release of the meso β convective systems. On the basis of the study of this paper, the air motion image of the meso β convective systems (or say meso β cloud towers), which develop concomitantly with lower troposphere mesoscale convergence line in Meiyu period, is sketchily given as follows: the air flows into the meso β cloud tower in lower troposphere from both southwest and northwest directions, and then moves upward with slight tilt toward east; at the south and north edges of the meso β cloud tower, there are un closed vertical mesoscale circulations like spirals in 800-400 hPa layers; however, the upward motions in middle region of the meso β cloud tower are quite deep, extend to 200 hPa and then turn to horizontal movement toward southeast direction out of the cloud tower.