The Effect Of Urban Heat Island On Summer Severe Convection Event In The Afternoon In Shanghai Region

In recent years, the urban waterlogging occurs frequently, which has caused to loss property of people and casualty. Therefore, in this paper, based on the observational daily precipitation data over the years in Shanghai area, the relationship between the process of urbanization and the short-time severe convection was explored. After that, a convection event which occurred in Shanghai at 13 September 2013 was selected to further verify the impact of the urbanization on the heavy precipitation. Finally, the comparative test was designed to analyze the mechanism of the interaction between city effect and heavy precipitation using numerical simulations.The results are as follows:The characteristics of spatial and temporal distributions of the short-time heavy rainfall in Shanghai region in the afternoon from 2003 to 2015 was studied by using the hourly rainfall data from 11 weather stations. The results showed that:Short-time heavy rainfall mainly occurred during June to September, especially during 14Z to 18Z. Results indicated that the rainfall mainly depended on the weather situations and the sun’s radiation. Besides that, there is a good corresponding relation between the occurrence frequency of short-time heavy rainfall and urbanization. The impacts of urbanization increases frequency of occurrence of short-time rainfall in downtown area.The influence of the city on the strong convection has the dynamic and the thermodynamic factor. We can learn that the gust front has a significant reduction in the urban area through the case. At the same time, the city’s thermal effect bring about the local stratification tend to be unstable and the severe convective weather can be easily formed under the favorable conditions.By comparing the control test and the comparison test replacing the city with farmland, the results indicated that the precipitation mainly depended on the environment. The existence of urban heat island changed the thermal status in the urban areas and enhanced the sea breeze convergence and convergence ascending motion of urban area, which lead to the occurrence of precipitation earlier. Compared to the surface condition covered by farmland, the existence of the city made the ascending area more concentrated and precipitation center more obvious.