Formation mechanism of overshooting convection in the southwest vortex circulation under the influence of mesoscale gravity wave

Abstract

Using ERA5 reanalysis data, GPM satellite precipitation products, and radar mosaic combination reflectivity (RMCR) data, the formation mechanism of overshooting convection (OC) in the southwest vortex (SWV) circulation under the influence of mesoscale gravity wave (MGW) on 18 Jul 2022 was analyzed on synoptic dynamics to deepen the understanding of the correlation characteristics between the SWV and the MGW, and then to explore the formation mechanism of OC in the Sichuan Basin (SCB). Results showed that the undulating terrain and stable atmospheric stratification generated the MGW. The adjustment of the SWV circulation caused the strong water vapor flux convergence at 850 hPa in the early stage of OC. The change of divergence field caused by MGW promoted the eastward development of the updraft in the SWV circulation, and the SWV center tilted to the southeast. The ascending center separated by the SWV merged horizontally with the updraft phase of MGW, and the merged ascending airflow connected vertically to the ascending center in the upper troposphere. The water vapor convergence and heating center in the upper troposphere and the horizontal wind momentum transporting upward led to OC. The strengthening of the negative vertical vorticity phase of MGW and the movement of dry cold air mass led to the northwestward propagation of negative vertical vorticity, thus forming the difference in the vertical distribution of vertical vorticity, which was negative in the upper and positive in the lower. The stable updraft phase of the MGW and the dry cold air mass propagation cooled the lower atmosphere and increased the convective available potential energy (CAPE). The adjustment of the internal circulation of the SWV extended the range of convective instability from the ground to 600 hPa, which was also conducive to the formation of OC.