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

During a hailstorm event,near-surface meso-γvortices 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-γvortices 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-γvortices than those observed.The model simulation indicates that such low-level meso-γvortices can be used to predict the next 10-min hail fallout zone.The lowlevel meso-γvortices 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-γvortices could help enhance a local updraft.Furthermore,the simulation reveals that the low-level meso-γvortices 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°C isothermal line and the top of the clouds,roughly between the 0 and-20°C isothermal lines.Accordingly,the hailstones grew rapidly.The suitable environment and the positive effect of the meso-γvortices on the updrafts enabled hailstorm formation.