98.8松嫩流域一次东北冷涡暴雨的数值模拟初步分析

利用MM5中尺度非静力模式对1998年8月8~9日松嫩流域的东北冷涡切变型暴雨过程进行了数值模拟和对比试验。结果表明, MM5能够对此次过程进行较成功的模拟; 同时发现, 此次过程首先由高层强的正涡度平流触发, 低涡区强降水的产生是由于高低空急流的耦合。其中, 高空急流的作用强烈而短暂, 低空急流的作用较为持久, 强度稍弱。阻高前部下沉气流形成的中层偏东干冷气流是切变形成的关键, 它与偏南气流在对流层中层形成的风向切变是产生上升运动的主要强迫机制。减弱阻高使其底部偏东气流减弱及后部偏南气流在高层减弱、低层增强, 导致降水减弱, 并使系统位置偏东、偏南。西北路冷空气增强使低涡和切变北侧高层偏南气流增强、低层偏南气流减弱, 导致降水强度增强、尺度增大。

Analysis of Numerical Simulation on A Northeastern Cold Vortex Heavy Rain in the Songneng River Basin

By using MM5 mesoscale non hydrostatic model, a numerical simulation and contrast test is conducted successfully of a northeastern cold vortex and its shear which caused heavy rain in the Songneng River Basin on August 8,1998. Results show that the rainstorm was triggered by the positive vorticity advection at the upper level atmosphere. Coupling of two jets at the upper and lower levels led to the strong precipitation within the area of low vortex coverage. The upper jet stream was strong and temporary, while the lower is weak and persistent. Resulted from the down draught in the front of a blocking high, the easterly dry and cold air current at the mesosphere was the crux of shear formation. The wind shear, which formed at the mid troposphere as a result of the easterly current meeting with a southerly, is the dominant forcing in producing ascending motion. As the blocking high weakened, the easterly current below and the southerly current at the rear weakened. As a result, the precipitation system abated and moved both easterly and southerly. With enhancement of the northwest cold air, the southerly currents at the upper and lower levels strengthened and weakened respectively. As a result, the precipitation intensity enhanced and the precipitation scale enlarged.