两湖流域盛夏持续性旱涝过程诊断分析

基于区域的平均雨量,用Z指数方法确定发生于洞庭湖与鄱阳湖(简称两湖)流域持续10天及以上的少雨和多雨过程。利用NCEP再分析资料对这些旱涝过程进行诊断分析。结果表明:(1)区域出现连续性旱涝过程时,500 hPa高度场经向分布差异十分明显,在持续性干旱(洪涝)过程中,东亚-西太平洋地区从高纬到低纬表现为典型的"-+-"("+-+")波列分布型,旱涝过程的发生、维持和消退与西太平洋副热带高压的位置和强度变化紧密相关,同时中高纬阻塞形势的变化也十分明显。(2)区域出现连续性旱涝过程与东亚夏季风的强弱显著相关。持续性干旱过程,东亚夏季风环流处于偏强时期,伴随着弱的热带辐合带系统和强的副热带辐合系统,洪涝过程则完全相反。(3)区域持续性旱(涝)过程与区域强水汽辐散(辐合)相对应,且在水汽垂直分布上表现为低层水汽辐散(辐合),高层为水汽辐合(辐散),在850 hPa处的水汽辐散(辐合)最强。

DIAGNOSIS OF PERSISTENT DROUGHT/FLOOD EVENTS IN TWO INNER LAKES' REGION OF CHINA

Based on daily regional mean rainfall,persistent drought and flood events that lasted at least 10 days in two inner lakes(Dongting and Boyang)were selected.Using 4 times-daily National Center for Environmental Prediction(NCEP) reanalysis data,these drought and flood events have been preliminarily diagnosed.The results show as follows.(1) The composite standard geopotential height at 500 hPa exhibits an obvious longitudinal wave train from high to low latitude in the eastern Asian-Pacific region,i.e.,the "-+-"and "+-+" patterns are dominant over the eastern Asian-Pacific region in the persistent drought and flood events respectively.The developing,persisting and decaying phases in drought and flood events are closely linked with the intensity and location of the subtropical high as well as the mid-high latitude blocking high.(2) Furthermore,East Asian summer monsoon stays in the strong(weak) phase when persistent drought(flood) events happen.The persistent drought events are accompanied with a weak tropical convergent system and a strong subtropical convergent system.In turn,the persistent flood events are accompanied with a strong tropical convergent system and a weak subtropical convergent system.(3) At the same time,the persistent drought and flood events go respectively with the local integral water vapor flux divergence and convergence.As for the altitudinal profile of the water vapor flux,divergence(convergence) in the mid-lower layer and convergence(divergence) in the high layer are very obvious in the drought(flood) events.Both the strongest divergence in droughts and convergence in floods appear at the level of 850 hPa.