中尺度地形对“98.7”鄂东特大暴雨的动力作用

用一个高分辨的 η坐标模式对 1998年 7月 2 1日鄂东沿江特大暴雨过程进行数值模拟 ,得到与实况相吻合的结果。通过敏感性试验和分析 ,研究了局地中尺度地形对这次暴雨过程的影响及其机理。结果表明 ,虽然地形对这次特大暴雨过程的基本面貌并未起到决定性的作用 ,但也有一些重要影响。大别山对边界层南风暖湿气流的阻挡使位于其迎风面上游的暴雨带有所加强。幕阜山对边界层水汽流入的拦截则使位于其背风面下游的暴雨带的初期发展减缓。到了暴雨盛期 ,由于暴雨带南侧有中尺度低空急流出现并正好位于幕阜山区 ,因地形而抬高的摩擦层内的Ekman风矢偏转直指暴雨区 ,加强了暴雨区水汽的辐合。由于强对流暴雨系统盛期近地层气流转为辐散 ,对暴雨维持起关键作用的辐合层升高 ,因此地形抬高的摩擦辐合恰好叠加其上使之增强 ,这可能是此次鄂东沿江特大暴雨过程持续时间长的一个重要原因。

DYNAMIC EFFECT OF MESOSCALE TERRAIN ON "98.7" EXTREMELY HEAVY RAIN IN THE EAST OF HUBEI PROVINCE

The process of the heavy rain in the reaches of the Yangtze River in the east of Hubei Province on 21st July ,1998 is numerically simulated by a high-definition -coordinate model. The result of the simulation conforms to the reality. By sensitivity test and analysis, the effect of local mesoscale terrain on the torrential rain and the mechanism are studied. The result shows that the terrain plays an important part in the formation of the heavy rain although it is not the decisive factor. The southern warm and moist stream at boundary layer is obstructed by Dabie Mountain and the obstruction enforces the rain band at the upper part of the windward side. The water vapor at boundary layer is intercepted by Mubu Mountain which slows the initial development of the rain band at the lower part of the lee side. When the rain greatly develops, because there is a mesoscale low-level jet in the south of the rain band which just lies at Mubu Mountain , Ekman wind arrows lifted by the terrain at the frication layer turn around and point directly at the rain band and enforce the convergence of air and vapor. The divergence of the air stream at the ground layer during the period when the severe convective rain system develops at its highest degree, the rise of the convergence layer that plays an important role in the maintenance of the heavy rain, the superposition of the frication convergence that is lifted by the terrain and enforces the intensity of the rainfall are the reasons why the extremely heavy rain lasts for a long time.