Mechanism Analysis of a Sudden Rainstorm Triggered by the Coupling of Gravity Wave and Convection in Mountainous Area

Based on the ERA5 reanalysis data from ECMWF (European Centre for Medium-Range Weather Forecasts), the China Merged Precipitation Analysis data from stations and CMORPH (CPC (Climate Prediction Center) MORPHing technique, and the FY-2G satellite cloud image data from NSMC (National Satellite Meteorological Centre), the authors conducted a synoptic dynamic analysis on the characteristics of gravity wave during an abrupt rainstorm in the southwest region of Sichuan Basin on 21–22 May 2018. The results indicate that the sudden rainstorm in the mountain area was affected by the gravitational wave activity with a wavelength of 150 km and a period of 5 h. The gravity wave was triggered by the combination of terrain and vertical shear instability. The shear instability appeared in the downstream earlier than the propagation of the gravity wave. The Richardson number was a good indication of the direction of gravity wave propagation and precipitation area. Before the rainstorm, the updraft in the gravity wave transported the water vapor to facilitate the development of the convection, while the downdraft caused the unstable energy in the lower layer to accumulate continuously. With the development of the Northeast low-level jet, a critical layer was formed in the transition zone of the east–west wind shear between 700–800 hPa. The critical layer continuously absorbed the wave energy from the upper air and resulted in the gravity wave energy to be transmitted downward, triggering the release of the unstable energy in the lower layer and stimulating the convection to be strengthened continuously, which finally caused the sudden rainstorm.