摘要
利用中尺度数值模式MM5和多普勒雷达观测资料，对2002年7月22-23日发生在长江中游一次梅雨暴雨过程进行模拟和分析。模拟结果与观测资料相比基本吻合。主要研究内容包括暴雨过程α中尺度到γ中尺度的回波结构以及动力特征和云降水粒子的分布。研究结果表明：在长江中游地区存在一条东北西南走向低空切变线，切变线北侧偏东气流中回波较弱，而南侧西南气流中不断出现强对流云团发生发展、合并分裂现象，形成沿着切变线分布的α中尺度对流带。对流带中有多个东北西南向的β中尺度波列，这些波列由从西南向东北方向移动的γ中尺度回波所构成。新回波大多产生在老回波的后部。γ中尺度回波具有相应尺度的气流辐合辐散结构。各种云降水粒子与动力场相互配合，上升运动位置和强弱决定云水的位置和强弱。云发展初期降雨首先产生于低层，以暖雨过程为主，成熟期云中冰相粒子对降水非常重要。强回波区的降水会在近地面产生出流。在西南气流中，远离切变线的回波移速大于靠近切变线的回波，容易产生回波合并；强回波由于降水而产生下沉和辐散气流，易导致回波分裂。
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 LouXiaofeng （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,γscale 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.
出处
《大气科学》
CSCD
北大核心
2007年第1期1-18,共18页
Chinese Journal of Atmospheric Sciences
基金
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
关键词
梅雨
中尺度对流系统
雷达回波
数值模拟
Meiyu, mesoscale convective system, radar reflectivity, numerical simulation
