The role of gravity wave in a nocturnal mountain rainstorm in the northeastern Sichuan Basin

Mountain gravity wave characteristics are important for analyzing the nighttime heavy rainfall events occurring in Sichuan. Based on the GPM satellite precipitation data, FY-4A satellite cloud data, and ERA5 reanalysis data, we analyze the mesoscale characteristics of a nighttime rainstorm with a gravity wave that occurred on 10-11 July 2018 in the northeastern Sichuan Basin mountains. The results are as follows. (1) The oscillatory feature of cloud water content and the wavelike variation of the potential temperature field reflect, to some extent, the influence of the fluctuations on the mesoscale system. (2) The fluctuation trends of wind vertical shear index, thermal helicity, and moisture helicity appear during the development of waves, indicating the characteristics of gravity waves. (3) The areas with large values of wind vertical shear index, the extreme center of thermal helicity, and moisture helicity are consistent well with the heavy precipitation regions. (4) The gravity wave may be formed by the interaction of topographic disturbances, shear instability, and non-geostrophic equilibrium, which enhance the rainstorm. The regions with a small value of the Richardson number and a nonzero value of the nonlinear equilibrium equation can indicate the location and movement direction of the rainbands. Diagnosis of Richardson number shows that the vertical shear instability is formed before the mesoscale wave is excited, but it decreases with increasing fluctuations, which indicates that part of the energy of the waves comes from the unstable flow.