华南沿海一次暴雨中尺度对流系统的形成和发展过程

THE FORMATION AND DEVELOPMENT OF A HEAVY RAINFALL MESOSCALE CONVECTIVE SYSTEM ALONG SOUTHERN CHINA COASTAL AREA

关键词:  地形辐合  中层扰动  中尺度对流系统(MCS)  数值模拟

Key words:  topographic convergence  mid-level disturbance  mesoscale convective system(MCS)  numerical simulation

摘要:

应用观测分析和数值模拟方法研究了2004年5月13～14日发生在华南西部沿海的一次暴雨中尺度对流系统(MCS)。结果发现:MCS开始发生在低层切变线南侧的偏南气流中,强盛时其水平范围达中α尺度,生命史近10个小时。在MCS的形成和发展过程中,低层的涡旋环流不明显,但其上空对流层中层500hPa上有流场的扰动与之对应。观测分析及数值模拟结果均表明沿海地形引起的辐合在对流的启动中有重要的影响,而对流发展起来后,由于凝结加热的作用,中层的扰动得到加强和发展,主要表现为涡度的增大,气旋性环流在500hPa高度附近表现最为明显,并在MCS东移发展的过程中起着重要的组织作用。MCS在有利地形触发作用下发生,并通过对流层中层扰动组织发展的过程,有别于华南其它一些暴雨过程常伴有低层涡旋系统向上发展的形式,有其独特的特征。还对中层扰动的增强过程和物理机制进行了分析,并提出了一个用于解释中层扰动对MCS组织发展影响的观点。

Abstract:

Observational analysis shows that a Mesoscale Convective System(MCS) occurred on May 13～14 2004 along the coastal area in Southern China.The MCS initiated among the southwesterly flows within a west-east orientation low-level shear line.Associated with the system,in its subsequent development stages,no distinct vortex circulation developed in low-level.Instead,a cyclonic flow disturbance was observed in the mid-troposphere.How the convection starts to develop and evolve into a MCS? With observational analysis and numerical simulation,the problems are studied.The high-resolution MM5 simulation shows that topographic convergence along the coastal line and the nearby mountains in western Southern China played an important role to initiate the MCS convection.Once the convection occurred,due to the condensation heating,a cooperative interaction between the preexisting mid-level disturbance and convection created,which may greatly affect the MCS development during periods when the system continued moving eastward.Compared to some typical MCS that happened in Southern China,which are usually accompanied with upward development of cyclonic vorticity,the development and evolution of the investigated MCS shows distinguishing features.In the article,the physical mechanisms responsible for the intensification of mid-level disturbance are discussed,and a viewpoint to interpret the effects of mid-level disturbance on the MCS organizational development is proposed.