A Numerical Simulation on Torrential Rain During the Meiyu Period and Analysis of Mesoscale and Microscale Structure of Convective Systems
SUN Jing~(1,2,3),LOU Xiao-Feng~2,HU Zhi-Jin~2, and ZHAO Si-Xiong~11 Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 1000292 Key Laboratory for Cloud Physics and Weather Modification of China Meteorological Administration,Chinese Academy of Meteorological Sciences,Beijing 1000813 Graduate University of Chinese Academy of Sciences,Beijing 100049
Mesoscale convective systems(MCSs) usually develop in the middle and lower reaches of the Yangtze River during the Meiyu period.MCSs include multi-scale structure from meso-α scale convective system to meso-γ scale convective system.Many researches have been done about the characteristics of meso-α scale convective system,while in many cases meso-β scale convective system is the main cause of rainstorm occurrence.There also exist meso-γ scale convective cells in meso-β scale convective system.In this paper, a case of torrential rain during the Meiyu period of 22-23 July 2002 is simulated using mesocale model MM5(v3).The observed rainband with a northeast-southwest orientation exists in the middle and lower reaches of the Yangtze River.The accumulated 24 h rainfall is over 150 mm.Doppler radar data observed in Changde and Yichang are used together with the simulation results to analyze the characteristics of MCSs.A new explicit microphysical scheme implemented to MM5 by Lou Xiaofeng(2002) is used in the simulation.Three-level nesting structure of domains is designed.The grid intervals of three domains are 30 km,10 km and 3.33 km,respectively,and correspondingly the horizontal areas are 2400 km,900 km and 300 km.Such a design of domains is able to describe not only the large scale weather background but also the cloud with a few kilometers size.To compare easily with observation data,the radar reflectivity of simulation is calculated according to the relationship between radar reflectivity and water substances. The time interval to output model results is intensive to 5 minutes in order to have a good look at echo's continuously developing processes.Simulation results basically coincide with observation data.Structure and dynamic characteristics of meso-α,β,γ scale echo and distribution of cloud and precipitation particles are discussed.Results show: there exists a low level shear line of northeast-southwest orientation over the middle reaches of the Yangtze River.Echo in the east airflow on the north side of the shear line is weaker,while in the southwest airflow on the south side the convective echo develops more strongly, merging and separating continually,which form the meso-α scale convective zone along the shear line.In this zone there exist meso-β scale wave trains of northeast-southwest orientation which are composed of several meso-γ scale echo moving from southwest to northeast.New echo is mainly produced on the backside of old echo.Meso-γ scale echo has corresponding scale convergence and divergence structure.Cloud and precipitation particles cooperate with dynamical structure.The location and intensity of cloud water are decided by those of the ascending motion.Rain water is produced first at low level through warm rain process during the initial stage of precipitation,and ice phase particles are very important to rain water during the mature stage.Strong echo produces outflow near the surface.In the southwest airflow,the echo far from the shear line moves faster than that near the line,which causes echo merging;strong echo produces downdraft and divergence which cause the echo separating.A conceptual model of Meiyu cloud system structure is derived from the numerical simulations in this paper.
【Fund】： 国家重点基础研究发展规划项目2004CB418306;; 国家自然科学基金资助项目40305001;; 社会公益研究专项2004DIB3J116
【CateGory Index】： P458.121
【CateGory Index】： P458.121