Meso scale Weather Systems During the Opening Ceremony of Beijing Olympic Games

The Opening Ceremony is one of the most influential events during 2008 Beijing Olympic Games. And the meteorological service of the Opening Ceremony is of great importance. The weather situation is very complicated, on one hand, analysis of upper air and surface layer circulation at 20：00 8 August suggests there is a great possibility of precipitation, on the other hand, the strong radar echo belts of the southeast area developed at around 21：20 begin to constantly move northward, which may combine with the strong meso scale radar echoes from northeast, and lead to heavy rainfall in the urban area. But in fact, medium and heavy rainfall occurs in the southwest and northeast suburb, but almost no precipitation occurs in the urban area. The fact is beyond the explanation of conventional meteorological data. So high resolution ground based meteorological data observed by auto weather stations (AWS), wind and temperature data from the surface layer to the tropopause observed by thee wind profilers located at Beijing Weather Observatory, Haidian Station and Shangdianzi Station are used. The possible mechanism of the meso scale weather system developing and weakening which affects the precipitation in Beijing urban areas are discussed considering the influences of the environmental wind field, topography and urban heat island effect. It is concluded that the impact of secondary circulation loop formed by the heat island and topography effect to the south urban area is different from that to the north. In the south part of the city, the secondary circulation weakens the southerly, and its subsiding air weakens the northward branch of meso scale system. But in the north part, the secondary circulation strengthens the southerly, and the air convergence will become stronger, promoting the development of meso scale system. When weak southerly environmental wind exists, the secondary circulation loop will lead to air convergence below 1500 meters high. But the air divergence above the height of 1500 meters is unfavorable for the meso scale system to develop northward and bring heavy rainfall to Beijing urban area.