Based on the daily precipitation data from 56 stations in Yangtze River Basin in summer,NCEP/NCAR reanalysis data,sea surface temperature data from Hadley center,the spatio-temporal variation of precipitation frequency for different categories in Yangtze River Basin and associated atmospheric and oceanic pattern were analyzed.More details are as follows:(1) There is a trend that total rain days happened more frequently in Yangtze River Basin in summer in recent 53 years,while the heavy rainfall frequency ratio of total rainfall frequency was increasing,and light rainfall frequency ratio of total rainfall frequency was decreasing.(2) Spatiotemporal variation of precipitation frequency for different categories in Yangtze River Basin and associated atmospheric and oceanic pattern are different from each other.In the heavy rainfall active period,early sea surface temperature in spring distributes as east-pattern El Nino,circulation is much stronger,Pacific-Japan teleconnection pattern shows “+-+” form at 500 hPa geopotential height,and the western Pacific subtropical high is stronger,moreover,the position of western Pacific subtropical high is more westward and southward.Dual-blocking pattern presents in Ural mountain and Okhotsk sea.Australian high in the Southern Hemisphere,cross-equatorial flow,westerly jet round 30°N at 200 hPa geopotential height are much more stronger,and presents cyclonic-anticyclone-cyclonic form at 850 hPa wind field.%利用长江流域56个站点1957-2009年夏季逐日降水资料、NCEP/NCAR再分析资料以及Hadley海表温度资料,分析了长江流域夏季不同强度降水日数的时空变化及其相关的海气异常型.结果表明:(1)近53 a来,长江流域夏季大到暴雨日数占总降水日数的比率呈明显增加趋势,而中小雨日数占总降水日数的比率呈明显减少趋势.(2)长江流域夏季不同强度降水日数的变化及其相应的海气异常型表现为明显不同的特征.当前期春季海温距平场表现为典型的东部型E1 Nino分布形态,500 hPa位势高度场呈现出“+-+”的经向PJ波列,西北太平洋副热带高压偏强,位置偏南偏西,中高纬地区乌拉尔山和鄂霍次克海地区出现双阻塞形势,南半球澳大利亚高压异常偏强,越赤道气流偏强,在30°N附近200 hPa纬向西风急流异常偏强,850 hPa风场在东亚上空经向方向上呈现出明显的反气旋—气旋—反气旋系统相间分布的特征时,有利于长江流域夏季大到暴雨降水日数偏多.
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