江南丘陵和云贵高原地形对一次西南涡暴雨影响的数值试验

运用MM5数值模式对1998年7月22~23日一次由西南涡引发形成湖南大暴雨的个例进行了数值试验。模式中设计了全地形、去掉江南丘陵地形、去掉江南丘陵和云贵高原地形三种方案。对比试验表明:在去掉江南丘陵和云贵高原后,由于地形的狭管作用消失,南北热力差异减弱,低空急流随之减弱;由于低空急流对西南涡和切变线的正涡度平流输送减弱,以及500 hPa高空槽引导切变线向南移与西南涡结合,造成西南涡增强,切变线减弱、消失。同时,本文还对三种模拟试验中地形作阶梯化处理,进一步验证江南丘陵、云贵高原对偏南风存在狭管作用,对低空急流的形成有促进作用。

Simulation of Topographic Effect of Hilly Region to the South of Yangtze River and Yunnan-Guizhou Plateau on the Southweat Vortex during a Heavy Rain Process

The heavy rain happened in Hunan province on July 22~23,1998 triggered by the Southwest Vortexis numerically simulated using MM5 model.Three numerical experiments are designed-with real topography,without hilly region to the south of Yangtzeriver,without hilly region to the south of YangtzeRiver and Yunnan-Guizhou Plateau.The obtained results are as follows: With the last two experiments,the canalization effect of topography disappears,the thermal difference between south and north was reduced.Then,the lower level jet reduces.Reducing of positive vorticity transport by lower level jet to southwest vortex and shear line,combining with the shear lineled by 500 hPa upper trough,thesouthwest vortex strengthen.All these make the shear line weaker and disappeare finally.The stepped terrain in three experiments are designed also and the canalization effect of hilly region to the south of YangtzeRiver and Yunnan-Guizhou Plateau was validated further.It could accelerate the forming of lowerlevel jet.