Formation Mechanism And Influence Of Penetrating Convection During A Heavy Rain In Southwest China

Overshooting convection is a kind of convection with stronger intensity than the deep convection in the troposphere.Its convection height can pass through the tropopause to the bottom of the stratosphere,which has an important impact on the occurrence and development of rainstorm and the exchange between stratosphere and troposphere(STE).Using ERA5 reanalysis data,meteorological observation data including precipitation and satellite data and WRF simulation results,this study analyzed the formation mechanism of overshooting convection during a rainstorm in the Sichuan Basin located in southwest China on August 192019.This study diagnosed the synoptic background and rotational and divergent kinetic energy budget,then discussed the precipitation mechanism caused by the overshooting convection and the redistribution of ozone,hydrometeor and other matter in the upper troposphere and lower stratosphere(UTLS).The conclusions are as follows:(1)The overshooting convection occurred under the synoptic background that the stratospheric Potential Vorticity(PV)spread down over the east of the Sichuan Basin due to the large-scale tropopause folding in the coastal area of China,so there was strong unstable stratification over the Sichuan Basin.Because of the westward extension of continental high and the development of the Low-Level Jet(LLJ),the cyclonic circulation in the west of Basin strengthened.The cyclonic circulation resulted in the enhancement of the uplifting component flow orthogonal to the large terrain in the southeast of the Plateau.Topographic dynamic uplift superimposed convergence uplift triggered convection in the unstable layer.(2)The stratospheric PV were propagated down in the upper layer over the convective region.The stratiform cloud area of the mesoscale convective system(MCS)developed,which caused strong diabatic heating over the troposphere.The vertical gradient of the net temperature increase in the upper layer was enhanced,which brought about the strengthening of the local positive PV,and then was conducive to the development of the upper ascending movement.The development of stratiform cloud,evaporation of water vapor and melting of ice crystals in dry environment cooled the atmosphere,which led to the descending motion from 300 h Pa to 600 h Pa.The local dry-cold invasion caused by the descending movement intensifies the instability of the middle-lower troposphere.The descending movement of the middle and lower troposphere gradually turned into ascending movement.The ascending movement of the lower troposphere on the west side of the Basin is maintained due to the positive vorticity tilt development.Thus,the ascending movement in the convection region is vertically superimposed and coupled to form overshooting convection.(3)The results of rotational(KR)and divergent(KD)kinetic energy budget diagnosis show that the vertical exchange between KR and KD is strengthened with the development of upper troposphere ascending motion,leading to the conversion of KR to KD.At the same time,the enhancement of ascending motion means that a large amount of available potential energy(A)is released,and the released A is converted into KD and KR through baroclinic process(GD)and barotropic process(GR).Therefore,after the formation of overshooting convection,the values of GD and GR rose sharply.This kinetic energy conversion process has some reference significance for the identification of overshooting convection.(4)The overshooting convection transported cloud ice to the upper layer,and the strong ascending movement also brought water vapor into the middle troposphere,forming a relatively wide supercooled area in the vertical direction.The coexistence of supercooled water droplets,ice crystals and water vapor is conducive to the occurrence and development of precipitation.The transport of water vapor by LLJ supplied the water vapor consumption caused by precipitation in the convective region.Finally,the precipitation can be maintained for a long time.(5)The overshooting convection led to the occurrence of STE process in the UTLS region.Water vapor increased and ozone decreased in the lower the stratosphere.In addition,the vertical flux of PV in the UTLS region decreased,because the ascending movement brought smaller PV into the stratosphere while large PV enters the troposphere.This change means that STE caused by overshooting convection is not unidirectional,but bidirectional transport between upper troposphere and lower stratosphere.After the overshooting convection weakens and then moves out of the convection region,the release of energy caused turbulent activity in the UTLS region.