Changes of wind speed and extremes in Beijing during 1960-2008 based on homogenized observations

【摘要】：Daily observations of wind speed at 12 stations in Greater Beijing Area during 1961-2008 were homogenized by using the Multiple Analysis of Series for Homogenization method. The linear trends in the regional mean annual and seasonal (winter,spring,summer and autumn) wind speed series are -0.26,-0.39,-0.30,-0.12 and -0.22 m s-1 10yr-1,respectively. Winter is of the greatest magnitude of declining wind speed,followed by spring,autumn and summer in order. The annual and seasonal frequencies of the wind speed extremes (days) also decrease,more prominent for winter than for other seasons. The declining trends in wind speed and extremes are mainly formed with some rapid declining changes during the 1970s-1980s. The maximum declining trend in wind speed occurs at Chaoyang (CY),a station within the Central Business District (CBD) of Beijing with the highest level of urbanization. The declining trends are in general smaller in magnitude away from the city center except for the winter case,in which the maximum declining trend shifts northeastward to rural Miyun (MY).The influence of urbanization on the annual wind speed is estimated about -0.05 m s-1 10yr-1 during 1960-2008,accounting for about one fifth of the regional mean declining trend. The annual and seasonal geostrophic wind speeds around Beijing,based on daily mean sea level pressure (MSLP) from the ERA-40 reanalysis dataset,exhibit decreasing trends as well,coincident with the results of the site observations. A comparative analysis of the MSLP fields between 1966-1975 and 1992-2001 suggests that the influences of both the winter and summer monsoons on Beijing were weaker in the recent decade than in the early. It is suggested that the bulk of wind in Beijing is considerably influenced by urbanization,while changes in strong winds or wind speed extremes are prone to large-scale climate change in the region.