A comprehensive analysis of the meso-γ scale local heavy rain event around Xiangshan in Beijing on 29 July 2009

Based on a variety of intensive observational data including intensified automatic surface weather observation system, wind profiler and microwave radiometer, and combined with the thermodynamic and dynamic fields retrieved by the Variational Doppler Radar Analysis System (VDRAS), we have conducted a comprehensive analysis of a local heavy rain event occurred around Xiangshan in Beijing on 29 July 2009 (hereinafter referred to as"7.29"event). The results show that the"7.29"event is caused by a newly-born meso-γ scale thunderstorm with formation and development being abrupt, short life history, highly localized and high precipitation rate. Looking from the early stage environment, unstable atmospheric condition with low lifting condensation level (LCL) and free convection level (LCF) makes the initiation of the newly-born thunderstorm not need much convergence lifting mechanism. The convergence between the southerly in environment field and the northerly from cold outflow that extended to 300 m above the surface, which happened in the northern mountain area of Beijing, is a key factor for the initiation of the Xiangshan thunderstorm. Favorable local water vapor environment supplies sufficient moisture for formation of the "7.29"event. After the initiation of the Xiangshan thunderstorm, due to the lack of the interaction between favorable low-level wind shear and the strong cold outflow produced by the convection itself, it dies out rapidly since it becomes difficult to get systematically further development after its maturation. In addition, the complicated terrain also shows an important role during the formation and development of convection. On the one hand it plays a triggering role for the initiation of the newly-born thunderstorm by topographic forcing effect. On the other hand, it plays an enhancement role for the vertical ascending movement of air for the convections.