Prediction and Predictability of a Long-Lasting Severe Hailstorm and of Hailfall at Ground over Eastern China

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 seven hours. Due to their convective nature and uncertainties with important microphysical processes, the forecasting of hailstorms 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 rain gauges, 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. Neighborhood-based objective verification of maximum estimated hail size (MESH) is conducted, and the surface accumulated hail mass and number concentration with different hailstone size thresholds are examined. Results indicate that the three-moment scheme has the best skill in predicting hail size, mass and number distribution that quantitively agree with available observations.

Microphysical budget analyses are further performed to investigate the differences and causes of hail prediction within various microphysics schemes. Results suggest 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 predicting the main processes contributing to hail growth. Hailstone size sorting rates, which are against the shape parameters from various microphysics schemes, should also be partly responsible for the hail size distribution discrepancies.

Additional sensitivity experiments are performed to understand the initiation and development processes of the hailstorm. A convective system proceeding the hailstorm is found to have a significant impact on the mesoscale circulations and on the 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.