Analysis on causes of the extreme warm-sector short-time heavy rainfall during non-flood season in Nanning
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.
extreme short-time heavy rainfalldouble low-level jetsmesoscale dew-point frontweak forcing environment
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