1998年夏季长江流域大气季节内振荡的结构演变及其对降水的影响

利用中国逐日降水格点资料和NCAR/NCEP再分析资料,对1998年发生在我国东部长江中下游流域的夏季持续性强降水过程中显著的大气季节内振荡(ISO)的三维结构演变等活动特征进行了分析。1998年夏季长江及江南地区的异常强降水对应着该地区强的ISO活动。利用位相合成方法,对长江流域两个典型的季节内循环周期的ISO降水、850 hPa水平风场以及水汽和垂直速度等循环过程的时空分布特征进行了诊断分析。在低频环流场上,对流层低层的低频气旋和反气旋环流表现出交替在热带西北太平洋增强并向西偏北方向移动发展的特征,当异常气旋环流移动到长江流域上空时,长江流域正好位于气旋环流西南侧的东北风异常和西北太平洋上向西移动的反气旋环流西北侧的西南风异常环流汇合处的下方,引起该地区强降水的发生。在强降水阶段的ISO的垂直结构上,上升运动和水汽表现出从华南到长江流域自南向北移动的特征,强烈的垂直上升运动以及来自南方充足的水汽为增强长江流域地区的降水起到了重要作用。

Structure and Evolution Characteristics of Atmospheric Intraseasonal Oscillation and Its Impact on the Summer Rainfall over the Yangtze River Basin in 1998

The spatial and temporal structures of intraseasonal oscillation (ISO) associated with the summer rainfall of 1998 over the middle-lower reaches of the Yangtze River basin (YRB) are investigated using China gauge-based daily precipitation analysis data and NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data. It is found that summer rainfall over the YRB exhibits marked ISO activities in 1998. Based on phase-composite analysis, the alternating cyclonic-anticyclonic circulation pattern propagates northwestward from the western North Pacific. A maximum rainfall anomaly occurs over the YRB when an anomalous low-level cyclone appears in situ. An anticyclonic circulation anomaly, with pronounced southwesterlies, appears to the south of the YRB. The vertical motion and specific humidity anomalies associated with ISO exhibit notable northward propagation from the tropics all the way to the latitude of the Yangtze River during the summer of 1998. The enhanced upward motion, low-level vorticity and moisture favor strong precipitation over the YRB.