Analysis of a sustained extreme rainstorm event in western Sichuan Basin by using multi-source data

Based on the conventional observation data, FY-4A TBB data, and ERA5 reanalysis data, a sustained extreme rainstorm event occurred in the western Sichuan Basin from August 14 to 18, 2020 was analyzed from the aspects of circulation background, mesoscale characteristics, water vapor and dynamic conditions, energy and instability conditions, etc. The results are as follows. The rainstorm was caused by the stable and limited movement of the western Pacific subtropical high, which blocked the eastward movement of the plateau low-trough and kept it stable in the western basin. At the same time, the shear line and jet stream were maintained in the middle and lower levels, and the surface cold air moving southward provides a favorable circulation background for the continuous extreme rainstorm. The formation, merging, and strengthening of mesoscale convective cloud clusters were affected by the topographic barrier, peripheral airflow of subtropical high, the shear line, and mesoscale low vortex. The strong center of the convective cloud cluster was maintained in the western part of the basin, which was the direct influence system of the rainstorm event. The rainstorm water vapor originated from the Bay of Bengal and the western Pacific Ocean. Under the influence of the topographic convergence uplift, a strong water vapor convergence center was formed in the western part of the basin. The each water vapor physical quantity in the rainstorm event presents extreme characteristics, providing sufficient water vapor conditions for the generation of continuous extreme rainstorm. Convergence and positive vorticity were developed throughout the rainstorm over the rainstorm area, which favored the enhancement of vertical upward movement. The rainstorm area was located in the frontal area of dense pseudo-equivalent potential temperature, and the dynamic forcing of the frontal area was beneficial to the upward transport of energy and water vapor, which made the rainstorm area maintain a high-energy and high-humidity environment continuously. The generalized wet potential vorticity anomalies were also found over the rainstorm area. The instability of the rainstorm area potential was mainly caused by its divergence component, which represented the coupling effect of horizontal divergence and potential stability. The positive divergence of potential was determined by the divergence component and the vertical wind shear component. The 850 hPa area with a high potential divergence value corresponded well with the heavy precipitation area.