Urbanization Effects On Precipitation Over The Yangtze River Delta

With the rapid urbanization around the globe, urban climate have received considerable attemtion by scientists. In this paper, high resolution satellite data and gauge data were used to analyze the characteristics of precipitation over Yangtze River Delta. The paper foucused on rainfall about two aspects:one is spatial distribution and another is annual trend.Based on the 2003-2009 CMORPH (CPC Morphing Technique) satellite rainfall data and wind data from the NCEP/NCAR reanalysis,,the effects of urbanization on precipitation were analyzed in major cities over Yangtze River Delta. Results show that generally urbanization has an effect on the precipitation intensity significantly in summer and autumn. The rainfall intensity increases over and downwind of major urban cities under the 700hPa steering flow. The maximun value was generally found at a distance of 20-70 km downstream from the center of the city. There are little effects of urban on precipitation in spring and winter.Due to the geographical differences, the distribution of precipitation shows differences among urban cities. In summer and autumn, rainfall intensity was increased significantly downwind of Nanjing, Hangzhou and Suzhou and Changzhou. However, the rainfall center is located over the downstream of sea breeze circulation in Shanghai and Ningbo,. The precipitation intensity is maybe influenced by both sea breeze circulation and urban heat island effect together. It was also found that the rainfall intensity of Zhenjiang was deeply affected by upstream urbanization. It was found that the urban group, Ningzhenyang, Suzhou, Wuxi, Hangzhou and other major cities around the group average rainfall intensity in the 700hPa the direction of the steering flow increased significantly; exist between urban agglomeration is relatively low in the rainfall intensity.By using 69 stations dayly-precipitation data of the Yangtze River Delta from 1960 to 2007, the interannual variations trend of precipitation was analysised. Based on population data, cities were divided into three kinds:large-size cities, medium-size cities and small cities. From 1960 to 2007, whole-year, summer and winter precipitation fluctuated upward trend in the Yangtze River Delta region. The precipitation of large city stations rise obviously higher than small city stations. The whole-year precipitation of large city stations increase about 1.7 times faster than small city stations. On the view of day and night climate trend rate of precipitation, the precipitation growth rate of large-size city at night was 2.2-2.4 times higher than the medium and small cities, while the precipitation growth rate of all kinds of cities are almost the same. In addition, there is a precipitation decrease trends in spring and autumn.By analyzing precipitation features from 1980 to 2007, the trend of rainfall is different between recent 28 years and recent 48 years. In recent 28 years, the precipitation of whole-year, spring, summer and autumn are decrease, while only in winter it increases. The decrease trend of small citise is very obvious, its rate is 4 times faster than large-siza cities, whose rate is slowest. The rate of medium cities is in the middle. So we get the result that urbanization may slow down the precipitation decrease trend of recent 28 years. Thus, the strength of urbanization play an important role of the increase of precipitation.