Numerical Simulation of a Hailstorm Event of April 2015 over Eastern China: Hailstone Size Forecast, Microphysical Budgets and Convection Initiation Mechanism

In the afternoon of April 28, 2015, severe hailstorms swept north-south through most of the western part of Jiangsu Province, China, producing golf-ball sized hailstones on the ground, also affecting its capital city Nanjing. Hail, strong winds and lighting produced by the storm lasted as long as seven hours. Due to their convective nature and uncertainties with important microphysical processes, the forecasting of hail storms remains a significant challenging; forecasting of the numbers and sizes of hailstones falling to the ground represents an even bigger challenge.

This hailstorm event is simulated by the Advanced Regional Prediction System (ARPS) at 1 km grid spacing throughout its life cycle, employing different microphysics schemes predicting single-, double-, and triple- moments of hydrometeors. Results are verified against available observations, such as surface station, radar and satellite observations. Multi-moment schemes are found to better predict the general evolution of the hailstorm than single-moment schemes, with the three-moment scheme performing the best. Based on the predicted hydrometeor moments, the accumulated number of hailstones falling to the ground are also calculated for different hailstone size thresholds. Results suggest that the three-moment scheme has the best skill in predicting hail sizes and numbers at the ground that qualitatively agree with available observations.

Microphysical budget analyses are further performed to determine the main processes responsible for hail growth in simulations with different microphysics schemes. The budget calculations show that the dominant source terms contributing to hail growth are hail collections of rain and cloud water, and the main sink term is hail melt to rain. Different microphysics schemes exhibit significant discrepancies.

In addition, to better understand the initiation and development processes of the hailstorm, several sensitivity experiments are performed. It is found that a convective system proceeding the hailstorm has a significant impact on the mesoscale circulations and in turn on the initial and later evolution of the hailstorm. The rearward spread cold pool from the proceeding convection generated a vertical vortex couplet from tilting of baroclinically generated horizontal vorticity, and the southern component of the couplet sets a strong convergence zone between a vortex located to its northwest, causing deep upwelling of moist air and the initiation of the hailstorm. The cold pool-induced vortex also has a significant impact on the later evolution of the hailstorm system.