Local Torrential Rainfall Event within a Mei-Yu Season Mesoscale Convective System:Importance of Back-Building Processes

An extreme rainfall event occurred over Hangzhou,China,during the afternoon hours on 24 June 2013.This event occurred under suitable synoptic conditions and the maximum 4-h cumulative rainfall amount was over 150 mm.This rainfall event had two major rainbands.One was caused by a quasi-stationary convective line,and the other by a backbuilding convective line related to the interaction of the outflow boundary from the first rainband and an existing low-level mesoscale convergence line associated with a mei-yu frontal system.The rainfall event lasted 4 h,while the back-building process occurred in 2 h when the extreme rainfall center formed.So far,few studies have examined the back-building processes in the mei-yu season that are caused by the interaction of a mesoscale convergence line and a convective cold pool.The two rainbands are successfully reproduced by the Weather Research and Forecasting(WRF)model with fourlevel,two-way interactive nesting.In the model,new cells repeatedly occur at the west side of older cells,and the backbuilding process occurs in an environment with large CAPE,a low LFC,and plenty of water vapor.Outflows from older cells enhance the low-level convergence that forces new cells.High precipitation efficiency of the back-building training cells leads to accumulated precipitation of over 150 mm.Sensitivity experiments without evaporation of rainwater show that the convective cold pool plays an important role in the organization of the back-building process in the current extreme precipitation case.

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.