一次北京大暴雨过程低空东南风气流形成机制的数值研究

2002年6月24~25日，北京门头沟附近发生了一次大暴雨过程.观测资料和数值模拟均发现，在暴雨发生前和发生过程中，北京地区边界层内出现了一支强盛的东南风气流.东南风气流沿太行山东坡爬升，触发了对流.为探讨这支低空东南风气流的形成原因，本文通过数值模拟和敏感性试验，对这支东南风气流的形成机制进行了研究.结果表明，这支低层东南风气流是一支冷湿的、伴有较强风速辐合的气流，主要是在天气尺度系统作用下生成的.东南风气流形成过程中，地表感热加热作用对其强度有加强作用.大暴雨开始后的潜热加热作用对这支东南风气流有正反馈作用，使气流的强度大大增强，因此，在降水开始后气流强度也增强，降水最强时低空急流的强度达最强.暴雨开始后，由于夜间地表降温造成山风效应，导致在北京西部山脚下出现偏北风.

Numerical study for the formation mechanism of the southeasterly airflow for a Beijing torrential rain event

On 24~25 June 2002, a torrential rain event occurred in Beijing near Mentougou. The study of this case showed that there is a southeasterly airflow in the boundary before and during the torrential rain. This southeasterly airflow climbed along the east slope of the Taihang Mountain and triggered the convection. In order to study the formation mechanism of this southeasterly airflow, numerical simulation and sensitivity experiments are conducted in this study to analyze the dynamic and thermal mechanism of the southeasterly airflow. The results show that this southeasterly airflow is a cold and moist airflow coming from the Bohai Sea with strong wind speed convergence. It is mainly caused by the dynamic effect of the synoptic-scale system. The sensible heat flux of the surface can affect the strength of the southeasterly airflow significantly. After the outbreak of the torrential rain the latent heat can enhance the strength of the southeasterly airflow significantly, as a result, the southeasterly flow is strongest when the torrential rain is heaviest. The northerly flow at the foot of the mountain near Beijing after the outbreak of the rain is the mountain wind due to the decrease of the surface temperature.