Climatic Changes Related To Thunderstorm Days And The Impact Of Land Cover Characteristic In Shanghai

The first section of this paper, the climate characteristics of thunderstorms in the Shanghai area from 1960 to 2004 are analyzed using wavelet analysis methods. The analysis shows the seasonal variation of thunderstorm is obvious. Most thunderstorms occur from May through September. The influence of the "urban heat island" may increase the thunderstorm occurrence in the vicinity of the urban areas. It is also shown that the dominant period of the annual thunderstorm variations is about 6 years in Shanghai. In thunderstorm anomalous years, the relationship between the average atmospheric circulation and the geopotential height at 500hPa from June to August suggests that an obvious negative anomaly exists in eastern Asia. The subtropical high is relatively weak and to the east in active thunderstorm years, and vice verse in less active thunderstorm years. Durative El Nino events are likely to induce the abnormal increment of the thunderstorm frequency while fewer thunderstorms occur in the period of ENSO (El Nino/Southern Oscillation) transformation years.The second section, numerical simulations have been carried out with MM5V3 model to examine the features of pure sea breeze, pure urban heat island (UHI) and the interaction between a sea breeze and a UHI whose initial conditions are ideal without considering the influences from other local and large-scale circulations. The result indicates that the probability of causing strong convective activity both in pure sea breeze and pure UHI is small. When the sea-breeze circulation (SBC) and the heat-island circulation (HIC) develop adequately and interact each other over the urban, such interaction intensifies the sea-breeze velocity and generates three horizontal convergent zones in Shanghai. The vertical flow over the urban generally consists of a close circulation characterized by strong daytime updrafts at the center city. In addition, the HIC takes effect of delaying the inward penetration of the sea-breeze front until the late afternoon. So the urban is in an unstably stratified environment with convergent vapor at low layer. It may play a significant role in the anomalies of convective precipitation and the cumulus formation. However, theShanghai UHI has not enough time to reach a mature stage before the arrival of the sea breeze because the urban of Shanghai is adjacent to the coastline. Therefore, the sea breeze is stronger, and the effect of urban is weaker. Anyway the interaction of the HIC and SBC is propitious to trigger the occurrence of local convective activity (e.g., thunderstorm) in the late afternoon.