江淮流域6-7月极端强降水事件时空变化及环流异常

Spatial-Temporal Variation of Extreme Precipitation Events from June to July over Yangtze-Huaihe River Basin and the Circulation Anomalies

基于江淮流域近53年(1961—2013年)逐日降水观测资料和NCEP/NCAR再分析资料,利用百分位法定义了江淮流域6—7月极端强降水阈值,通过经验正交分解(EOF)方法将极端强降水事件分为沿淮型、沿江型和江南型三种类型。选取各类型极端强降水事件的典型个例,对环流形势进行诊断分析,结果表明:三种典型极端强降水事件发生时,850 h Pa上均存在由华南西部向长江中下游的低空西南急流,沿淮型急流强度最强、急流轴位置最北,沿江型次之,江南型最弱、急流轴位置最南;500 h Pa西太平洋副热带高压脊线和西伸脊点异常主要通过改变急流轴位置,来影响极端强降水的落区;200 h Pa上高空西风急流均表现出强度偏强,急流带偏窄,高空急流入口的右侧与对流层低层西南急流北侧相重合。在高低空急流的共同作用下,极端强降水发生区域上方垂直速度异常偏强,比湿异常偏大,梅雨锋位置的差异决定了极端强降水的发生区域;无论是沿淮型、沿江型还是江南型极端强降水事件发生时水汽主要来自孟加拉湾到中国南海附近海域。

Based on the recent 53 years（1961—2013） daily observational precipitation and NCEP/NCAR reanalysis data,extreme precipitation thresholds from June to July over Yangtze-Huaihe River Basin are defined by using percentile method. Extreme precipitation events are classified into three patterns according to EOF analysis,which are patterns along Huaihe River,along Yangtze River and south of Yangtze River. The typical cases of each pattern are selected and the atmospheric circulation situations are diagnosed subsequently. The results show that： When three patterns of extreme precipitation events happen,southwest low-level jets exist from western south China to middle and lower reaches of Yangtze River at 850 hPa without exception. Jet intensity of pattern along Huaihe River expresses the strongest with the jet axis locating northernmost,pattern along Yangtze River is the second,while pattern south of Yangtze River is the weakest and southernmost. Ridge line and western ridge point of West Pacific Subtropical High at 500 hPa mainly change the location of jet axis to affect the extreme precipitation region. At 200 hPa,upper westerly jet is stronger with a narrow jet band,the right side of upper jet entrance coincides with north side of lower jet reasonably. It is pointed that under the corporate effect of lower and upper jets,vertical velocity above the extreme precipitation region represents abnormally stronger with anomalous larger specific humidity,the location of Meiyu front is attributed to the region of extreme precipitation. Water vapors for all three patterns mainly come from the Bay of Bengal to South China Sea.