Synoptic, dynamical and microphysical properties for splitting and non-splitting hailstorms

Abstract

Understanding storm splitting is of great importance for improving severe weather prediction and risk management. To further understand the mechanisms that cause hailstorm splitting, synoptic, dynamical and microphysical properties for a splitting hailstorm occurred on 19 June 2017 and a non-splitting hailstorm on 16 July 2014 in Beijing in the middle latitudes are investigated and compared based on synoptic maps, radar and sounding observations, as well as a three-dimensional cloud model with hail-bin microphysics. The results indicate that both hailstorms occurred under conditions with quite similar synoptic patterns, a tilting westerly trough at 500 hPa and a cold vertex at 700 hPa, and a clockwise-curved hodograph. However, the intensities of the westerly trough and low-level vertical wind shear were much stronger for the splitting hailstorm case than those for the non-splitting hailstorm case, so that the interactions between dynamical and microphysical processes in the splitting storm are also much stronger. It is found that for the splitting storm case the strong low-level vertical wind shear can cause large asymmetric pressure perturbation, pressure gradient forces and selective development in the storm and induce an obvious main updraft core splitting in the developing stage. The descending of high graupel/hail loading in the stable tropopause and lower stratospheric layer can excite strong downward propagating gravity waves (GWs) that induce a rapid storm splitting in the mature and decaying stages. In contrast, for the non-splitting storm, the relevant processes in the developing stage and the GWs excited by the descending of graupel/hail in the mature and decaying stages are too weak to cause an obvious splitting in the storm.