“99·6”梅雨锋暴雨低涡切变线的数值模拟和分析

Numerical Simulation and Analysis for “99·6” Meiyu Front Rainstorm and the Low Vortex with Shear Line

在天气分析的基础上 ,利用非静力中尺度模式MM5和四维资料同化逼进方法及双向三重嵌套网格技术 ,对 1 999年 6月 2 3～ 2 5日 (简称“99·6”)发生在长江中下游地区的梅雨锋暴雨过程进行了数值模拟。结果表明 :模拟结果与观测结果的比较指出 ,高分辨数值模式MM5可以成功地模拟梅雨锋中尺度低涡切变线的发生和发展 ;模拟结果显示 ,在α中尺度低涡切变线发展过程中 ,低层强的西南急流和东北气流增强了低层的辐合 ;而高空的西风急流和东风急流则增强了高空的辐散 ;正是由于这种从高空到低空环流的配置 ,才促进了α中尺度低涡不断发展 ;模拟低涡切变线不同部位的垂直环流和物理量场表明 ,“99·6”梅雨锋低涡切变线的结构非常复杂 :在梅雨锋的发展期 ,暖锋附近的垂直上升运动最强 ,低涡中心次之 ,冷锋附近最弱。模拟结果也表明 ,由于下垫面特征的不同 ,中国和日本的梅雨锋暖锋附近环流结构有较大的区别 ;模拟结果显示 ,在α中尺度低涡发展过程中 ,不断有扰动在低涡前部发展 ,激发并分裂出一系列的 β中尺度系统 ,β中尺度系统运动剧烈 ,但由于其低层辐合强于中空辐散 。

Based on the synoptic analysis for the meiyu front rainstorm event occurred in the Yangtze River area during 23～25 June 1999(“99·6”), the nonhydrostatic version of mesoscale numerical model MM5 was used to simulate this case with three nested domains run in two way interactions of full physics and four dimensional data assimilation (FDDA) techniques The main results are following:(1) The comparison between simulated and observed data shows that MM5 can successfully reproduce the genesis and development of low mesovortex with shear line on meiyu front;(2) The simulated results indicated that as the meso α low vortex with shear line developing, the strong southwest jet on low levels and the northeast flow increased the convergence on low levels, and the westly jet and the eastly jet on high levels increased the divergence on high levels This collocation from low levels to high levels urged the meso α low vortex to develop (3) The vertical circumfluence and physics fields in different parts of the low vortex with shear line indicated that the structure of “99·6” meiyu front low vortex shear line is very complicated In the developing period of meiyu front, the vertical ascent around the warm front is strongest; that around the low vortex center is the second and that around the cold front is weakest (4) The simulated results suggested that the circumfluence around warm fronts shows pronounced differences between Japan and China meiyu front owing to different underlaying surfaces (5)Simulated results also indicated that in the process of the meso α low vortex development, there are many disturbances in front of the low vortex, a train of meso β systems inspired and splitted have acute motions Owing to the convergence on low levels stronger than the divergence on middle levels, they will attenuate while they are apart from their parent