南宁市一次非汛期暖区极端短时强降雨形成原因分析

Analysis on causes of the extreme warm-sector short-time heavy rainfall during non-flood season in Nanning

利用探空、卫星云图、多普勒雷达及加密自动观测资料,分析南宁市2020年3月25日极端短时强降雨的形成原因。结果表明,此次过程没有切变线及锋面参与,是一次弱强迫环境下的暖区暴雨过程,两支高空槽的存在提供天气背景场:高原槽东移使南宁上空处于上干冷下暖湿的对流不稳定状态,高原槽与副高的对峙促进低层双低空南风急流的生成和维持;南支槽则从抬升暖湿气流、槽前正涡度辐合上升及引导暖湿水汽输送三个方面为强降雨的发生提供优越的动力、热力和水汽条件;双低空南风急流为极端短时强降水提供充足的水汽和热力条件,超低空急流出口区的辐合和低空急流入口区的辐散出现的耦合配置,加强中尺度抬升和水汽辐合,促进对流的发生;925 hPa急流顶端的暖湿气团与桂北相对干冷气团形成的露点锋,是对流不稳定能量爆发的触发机制,在925 hPa东南急流顶端,露点锋南侧形成此次过程的最强降雨。

The causes of the formation of extreme short-time heavy rainfall in Nanning on March 25th,2020 was analyzed by using sounding data,satellite cloud images,Doppler radar and encrypt automatic observations.The results show that it is a warm-sector heavy rainfall process under a weak forcing environment without the involvement of shear lines and fronts.There are two upper-air troughs that provide the weather background.The plateau trough moving east makes the air over Nanning in a state of convection instability with dry and cold air above and warm and wet air below,and the confrontation between the plateau trough and the subtropical high promotes the formation and maintenance of the double-low-level southerly jets.On the other hand,the south-branch trough provides excellent power,heat and water vapor conditions for the occurrence of heavy rainfall in terms of lifting up the warm and wet air flow,rising up of positive vorticity in front of the trough,and guiding the warm and wet water vapor transport.The double low-level southerly jets provide sufficient water vapor and thermal conditions for the extreme short-time heavy rainfall.The coupling configurations of convergence in the exit area of the ultra-low-level jet and divergence in the entrance area of the low-level jet strengthen the mesoscale uplift and water vapor convergence,and then promote the occurrence of convection.The dew-point front,which is formed by the warm and humid air mass at the top of the 925 hPa jet and the relatively dry and cold air mass in northern Guangxi,is the triggering mechanism for the convective unstable energy outburst.The heaviest rainfall occurs at the top of the 925 hPa southeasterly jets,where is also at the south side of the dew-point front.