Hailstorm Formation Enhanced by Meso-γ Vortices along a Low-Level Convergence Line

During a hailstorm event, near-surface meso-gamma vortices along a convergence line interact with hail cells. Herein we investigate this interaction by using observational data and a high-resolution simulation of a hailstorm that occurred over Taizhou (Zhejiang, China) on 19 March 2014. The 10-m surface wind data from automatic weather stations show that several meso-gamma vortices or vortex-like disturbances existed over the convergence zone and played a vital role in the evolution of the hailstorm and the location of the hail. The model results agree with the observations and present a closer correlation between the hail and the low-level meso-gamma vortices than those observed. The model simulation indicates that such low-level meso-gamma vortices can be used to predict the next 10-min hail fallout zone. The low-level meso-gamma vortices originated over the convergence zone and then fed back into the convergence field and provoked a stronger updraft. Vorticity was initiated primarily by stretching and was extended by tilting. A three-dimensional (3-D) flow analysis shows that the existence of low-level meso-gamma vortices could help enhance a local updraft. Furthermore, the simulation reveals that the low-level meso-gamma vortices formed in the bounded weak echo region (WER) at the front of the hail cell, enhancing convergence and strengthening updrafts. Graupel was broadly located between the 0 degrees C isothermal line and the top of the clouds, roughly between the 0 and -20 degrees C isothermal lines. Accordingly, the hailstones grew rapidly. The suitable environment and the positive effect of the meso-gamma vortices on the updrafts enabled hailstorm formation.