青藏高原云团东传过程及其中中尺度对流系统的统计特征

Statistical Characteristics of Eastward Propagation of Cloud Clusters from the Tibetan Plateau and Mesoscale Convective Systems Embedded in These Cloud Clusters

利用逐小时风云卫星TBB资料、逐小时中国自动站与CMORPH降水产品融合数据以及国家级地面观测站24小时累积降水量,统计分析2010~2016年夏季,伴随下游地区(104°E以东)降水的青藏高原云团东传过程以及东传过程中镶嵌于云团中的中尺度对流系统(Mesoscale Convective System,简称MCS)特征。结果表明,共出现120次伴随下游降水的高原云团东传过程,6月出现最频繁,但持续时间较长的过程多出现在7月。云团向东传播的主要三条路径是平直东传、沿长江折向东传和复合东传。其中路径二——沿长江折向东传中的过程是高影响过程,因为过程次数较多(46次),过程平均持续时间较长(62小时),在下游地区引发的降水日数和暴雨日数最多。属于东传过程的MCS在7月形成最多,集中分布在青藏高原东坡、云贵高原东部、长江沿岸及其以南地区。高原MCS影响长江中下游地区降水主要是通过向东传播的形式实现,因为即使生命史更长的中α尺度对流系统(Meso-α Convective System,简称MαCS)也鲜少直接移动至110°E以东地区。不同区域的中α尺度持续性拉长形对流系统(Permanent Elongated Convective System,简称PECS)的日变化特征显示,东传过程MCS更容易在夜间从高原东坡向东传播至下游地区。在三条路径中,路径二中的东传过程MCS数量最多、在下游地区发展最旺盛并与降水日数和覆盖范围存在更好的对应关系。

Based on TBB data from FY2E(2010-2014)and FY2G(2015-2016),the gauge-adjusted CMORPH hourly precipitation and daily gauge observations,statistical characteristics of eastward propagation of cloud clusters from the Tibetan Plateau(TP)and Mesoscale Convective Systems(MCS)embedded in these cloud clusters in the summers of 2010-2016 are analyzed.The results show that there are 120 eastward propagation processes accompanied with precipitation over the downstream region(east of 104°E).Most of these processes occurred in June,but those with longer durations more frequently occurred in July.Cloud clusters follow three prominent tracks to propagate from the TP to the middle-lower reaches of Yangtze River basin:1.propagating eastward directly,2.propagating along the Yangtze River,during which the cloud clusters first move southeastward and then turn eastward,and 3.propagating along complicated paths.The cloud clusters propagating along the second track has the highest impact due to their high occurrence frequency(46 processes),long duration and the most rainy days(heavy rain days)over the downstream region.The MCSs embedded in these eastward-propagating cloud clusters occur most frequently in July and more frequently over the eastern slope of TP,eastern part of Yunnan-Guizhou Plateau and the Yangtze River basin.The MCSs over the TP can only affect rainfall over the downstream region after they propagate eastward.This is because even the long-lived Meso-αConvective Systems(MαCS)with longer moving tracks over the TP or the eastern slope of TP cannot reach the region east of 110°E.The diurnal cycles of the Permanent Elongated Convective System(PECS)over different regions show that they propagate downstream more easily during the night.The MCSs embedded in the cloud clusters that follow the second track to propagate eastward are the most and also develop most robustly over the downstream region.They are highly associated with heavy rainfall events and areas affected by heavy rainfall.