THe Analysis on an Infrequent Southerly Gust of Beijing in Summer

The infrequent southerly gust in Beijing in 2014 summer was analyzed by using observational data, NCEP/NCAR reanalysis data，Doppler radar data and VDRAS data. Under the unfavorable weather condition, there were 6-8 grade southerly gusts in most area of Beijing in the midnight of Jul. 14 2014. The causes of the gust can be concluded in two aspects. Firstly, the local heavy precipitation between southeast of Hebei province and north of Shandong province,which is about 300 kilometers away from the southern part of Beijing, caused local temperature decrease and pressure surge, therefore, the outflow of cold air was formed. Because of weak northerly wind, warm air in the north of cold pool and topography, the outflow turned to the southerly wind. Secondly, the positive effect between southerly wind and local precipitation in Pinggu District, which is in the northeast of Beijing, was beneficial to convergence for precipitation. With the development of southerly wind, the ascending motion also strengthened, therefore, the pumping action of precipitation promoted the acceleration of southerly wind. Because the temporal and spatial distribution, scale and development of convective clouds meshed perfectly, these several beneficial factors caused the southerly gust in Beijing.