The Contrastive Analysis of Formation of Dry and Moist Thunderstorms in Beijing
Using the Variational Doppler Radar Analysis System (VDRAS) combined with local unconventional observation data, a more indepth contrastive analysis is carried on the initiation mechanism of two storm cases in Beijing, one is 814 (August 14, 2008) case with strong rainfall that we call it as moiststorm and the other is 824 (August 24, 2008) case with little rainfall that we call it as drystorm. The results show: (1) The synopticscale systems of 814 storm were stable Northeast cold vortexlow trough at 500 hPa and shear line at 850 hPa, the specific humidity that more than 12 g·kg-1 below 850 hPa and the relative humidity that more than 90% in the surface indicated that the atmosphere was very moist. It had convective instability caused by humidity advection in lower levels. The 824 storm had a prevailing straight west wind in high levels, an anticyclone in lower levels, and a surface cold front moving fast.The specific humidity that less than 6 g·kg-1, and the relative humidity that less than 30% below 850 hPa indicated that the atmosphere was very dry. It had convective instability caused by temperature advection. (2) There was much strong vertical wind shear in the whole vertical layer for 814 storm case, the clockwise wind direction with height within 500-1500 m intensified the warm and humidity inflow of lower layer advantageous to storm initiation and development. While there was weak vertical wind shear and unobvious warm and humidity inflow of lower layer for 824 case, which was not conducive to storm initiation and development. In addition, composited wind of the whole troposphere and storm movement speed were very low for 814 case, but they were very high for 824 case. (3) The 814 storm was formed by the collision and mergence of multicell storms, a convergence line was formed by the cold pool outflow produced by the precipitation of the upstream of thunderstorm cell and the east wind in low levels which forced the lowlevel warm and moist air to uplift, additionally the strong convective instability and vertical wind shear supported the formation and development of new storm. The interactions (collisions) of gust fronts in the leading edge of cold pool of multicell thunderstorm group, further exacerbating the lowlevel instability, leading to the regeneration and mergence of new convective thunderstorms. The 824 storm was a line convective system accompanied with cold front that rapid moved eastward and lasted for short time, there was no east wind with warm and moisture air accompanying the cold pool outflow produced by the downdrafts of thunderstorm. The absence of mesoscale lifting mechanism and moisture inflow couldn’t support the formation and development of new storm.