2011年长江中下游旱涝急转及汛期暴雨的对流条件研究

RESEARCH ON THE CONVECTION CONDITION OF SHARP TURNING FROM DROUGHT TO MEIYU FLOOD AND MEIYU RAIN STORM IN THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER IN 2011

利用ERA-Interim及雨量和土壤水分观测资料,对比诊断了2011年5—6月长江中下游梅汛前极旱期急转为梅汛期洪涝的极端天气事件的对流条件(水汽、不稳定、抬升作用)差异及特征,并研究条件性湿位涡垂直通量(CMF)指数与暴雨之间的定量关系。结果表明:在极旱期,干冷的东北气流控制,西太平洋副热带高压偏东,低层水汽通量弱且以偏北风输送为主,中低层为下沉气流,无低空急流,等θse线稀疏,边界层抬升机制缺乏,是干旱加剧的主要因子;在梅汛期,西南气流增强,西太平洋副热带高压西伸,低层气流在长江地区辐合,低层水汽通量增加且转为西南和东南风输送为主,伴随高低空急流耦合和深厚的上升运动,等θse线密集形成梅雨锋,增强不稳定暖湿空气强迫抬升和垂直输送,造成暴雨频发,引起区域性洪涝。暴雨中心600 hPa以下为负湿位涡的不稳定层,对流不稳定与条件性对称不稳定共同作用是强降水发生的不稳定机制。CMF指数与旱涝变化、暴雨过程演变非常一致,在极旱(梅汛)期,CMF指数低(高),变化平缓(剧烈),CMF指数在暴雨开始时逐步剧增,结束时迅速减小。

The extreme event of the extreme drought in May sharply turning into flood in June 2011 was triggered by heavy rainfall during Meiyu period in the middle and lower reaches of the Yangtze River.With the Era-Interim reanalysis data and rainfall data from regional stations in China,the characteristics and differences of three conditions for convection such as water vapor,instability and lifting mechanism in drought period and in Meiyu period were diagnosed,and the quantitative relationship and characteristics between the conditional moist potential vortex vertical flux(CMF)index and rainstorm were also studied.Results showed that in the extreme drought period,dry and cold northeast air current prevailed,the west Pacific subtropical high(WPSH)located easterly,the water vapor flux in the lower layer was weak and water vapor was mainly transported by northerly wind.Downdraft dominated the lower and middle layers and there were no low level jet.The sparse isometric pseudo-equivalent potential temperature(PEPT)lines led to the lack of boundary uplift mechanism,and they were the main factors that intensified drought.However,in the Meiyu period,the southwest airflow was intensified,the WPSH extended westward,the convergence of the lower level airflow and the water vapor flux in the lower layer were enhanced.Water vapor was mainly transported by southwest and southeast wind,accompanied by the coupling of high and low level jets and deep ascending motion,and the isometric PEPT lines became dense and Meiyu front was formed,which enhanced the forced uplift and vertical transport of unstable warm and humid air,causing frequent rainstorms and regional floods.In the storm center,the middle and lower layers below 600hPa were unstable with being less than 0.The combined action of convective instability and CSI was the unstable mechanisms that induce heavy rainfall.The CMF index changed simoutaneously when drought sharply turned into flood and rainstorms generated.In the extremely drought period,the CMF index was low and changed steadily,whereas in Meiyu period,the opposite was true.CMF index also inceased when rainstorm occured and decreased as it vanished.