1998年汛期长江流域洪灾中期天气过程分析

The Medium range Process Analysis of the FLood Disasters in the Yangtze River Basin in 1998 Flood Season

对影响长江流域强降水的夏季风、中高纬阻塞形势、西太平洋副热带高压活动（以下称副高）及南半球越赤道气流进行统计分析。结果表明：１９９８ 年长江流域夏季风环流建立是东亚夏季风环流在弱 Ｉ Ｔ Ｃ Ｚ位相时向北扩张的结果，它与中高纬偶极子双阻形势，构成长江流域暴雨洪灾发生、持续的大尺度环流形势。它们分别向长江流域暴雨区提供南方暖湿气流和北方冷空气，并在长江流域交绥；暴雨的落区是由副高脊线、西脊点的位置确定的；南半球越赤道气流对长江流域洪灾有重大影响，它一方面加强低空西南急流，另一方面它具有４～７ 天低频振荡特点，可为中期暴雨预报提供有用信息。

The statistical analysis of influence of summer monsoon,the blocking patterns at high and middle latitudes,the west Pacific subtropical high and cross equatorial flow on the heavy precipitation in the Yangtze river area was done.The results show that summer monsoon circulation is the result of northward progression of the summer monsoon and the Easterm Asia summer monsoon circulation in weak phase of ITCZ.Summer monsoon circulation and double blocking high with low cyclone in high middle latitude made the large circulation of the continuous heavy rain gush.They provide south warm moist current and north cold current separately.The current meets in the Yangtze Area.Rainfall areas were decided by the line of subtropical ridge and western points of the subtropical ridge.The cross equatorical flow influences the Yangtze basin Area flood disaster mainly.On the one hand,it reinforces the low level southwest jet,on the other hand,it has a low frequency vibration feature with 4—7 days.Undoubtedly,it provides a useful information for the medium range forecast.