1998年8月嫩江流域一次大暴雨的成因分析

The Reason of a Heavy Rain Event in Nenjiang Valley in August,1998

利用中尺度滤波、高分辨率数值模拟和诊断分析对 1 998年 8月 9～ 1 1日嫩江流域的甘南和白城大暴雨的成因进行了初步探讨。结果表明 :中 β尺度天气系统是大暴雨的主要影响系统。地形抬升、纬向次级环流和小股弱冷空气共同作用是大暴雨产生的重要因素。敏感试验结果表明 :凝结潜热释放对中 β尺度天气系统的形成起决定作用。中尺度地形对中 β尺度天气系统维持和发展也起作用 ,影响暴雨落区、强度 ,且有增幅作用。改变初始湿度场对雨区和雨量的模拟影响随积分时间增长逐渐减小 ,东北地区水汽不是完全由初始时刻大气中的水汽含量所决定的 ,而是不断地来自低纬向东北地区的水汽输送。

A heavy rain event near Gannan and Baicheng of Nenjiang valley during 9-11 August in 1998 is investigated. Diagnostic analysis, mesoscale filter analysis and numerical simulation have been conducted to inquire into the physical mechanism responsible for the above heavy rainfall. It shows that the existence of meso- β scale system is the direct reason of the event. A comparison of physical mechanism is made between Gannan and Baicheng. The results show that terrain elevation and latitudinal secondary circulation contribute to Ganan heavy rain, weak cold air and latitudinal secondary circulation and thermal effect in mid-low troposphere play important role in Baicheng heavy rainfall process. The MM5 model reproduces successfully the development of the two weather processes. Sensitive tests show that latent heat release acts a decisive role in the formation of the meso- β scale systems. Mesoscale terrain affects obviously the location and intensity of precipitation cell, and it makes Gannan precipitation increase. At the same time, it favors the maintenance and development of meso- β scale system. The effect caused by the variation of initial humidity field on simulation of rainfall area and intensity gradually decreases along with the increase of integration period. For Northeast China, the amount of water vapor which is necessary for the formation of precipitation is not only determined by the initial field, but also affected by the water vapor transportation from the tropical area to Northeast China.