Numerical Simulations And Mechanisms Of Hailstorm System Over Eastern China

Hailstorm is one of the severe convective systems,characterized by strong seasonality,locality,and rapid development.Owning to the lack of high-quality hail observations,understanding of the dynamics and microphysics of hailstorms is rather limited.The prediction of hailstorms using operational numerical weather prediction models remains a challenge.The explicit prediction of hail at the surface is even more challenging,because of the complex microphysics as well as dynamic and thermodynamic processes involved in hail production.Two different types of hailstorms(i.e.,pulse-type hailstorm and multi-cellular hailstorm),are studied in this dissertation.The explicit hail prediction skill of this two types hailstorms using different microphysics schemes are evaluated objectively.A new two-moment scheme with diagnosed shape parameters of particle size distributions(PSDs)is designed,whose performance is close to that of a three-moment scheme while the computational cost is comparable to that of a two-moment scheme.In addition,the initiation and evolution mechanisms of the multi-cellular hailstorm are investigated.For pulse-type hailstorm,a severe hailstorm that swept through eastern central Taizhou city of Zhejiang Province of China on 19 March 2014 is chosen for study.The Advanced Regional Prediction System(ARPS)is used to simulate the hailstorm at a 3-km grid spacing,applying different configurations of a microphysics scheme that predict one,two,or three moments of the hydrometeor particle size distributions.Results suggest that the general evolution of the hailstorm is better predicted with multi-moment microphysics schemes than with the one-moment scheme;the three-moment scheme produces the best forecast.The life cycle of the hailstorm is analyzed,using the most skillful,three-moment forecast.Based upon the tendency of surface hail mass flux,the hailstorm life cycle can be divided into three stages:developing,mature,and dissipating.Microphysical budget analyses are performed to examine microphysical processes and characteristics during these three stages.The vertical structure of the storm and its link to environmental shear conditions are discussed.A conceptual model for the life cycle of the pulse hailstorms is proposed.For multi-cellular hailstorm,a convective system that swept through much of the Jiangsu Province,China on 28 April 2015 is examined.The hailstorm event is simulated using ARPS at 1-km grid spacing.Different microphysics schemes are again used,predicting one,two,and three moments of the hydrometeor PSDs.Simulated reflectivity and maximum estimated size of hail(MESH)derived from the simulations are verified against reflectivity observed by operational S-band Doppler radars and radar-derived MESH,respectively.Comparisons suggest that the evolution of the hailstorm is better predicted by the three-moment scheme,and neighborhood-based MESH evaluation(Fraction skills score)further confirms the advantage of three-moment scheme in hail size prediction.Surface accumulated hail mass,hail number,and hail distribution characteristics within simulated storms using the three-moment scheme are more realistic.Size-sorting is found to play a significant role in determining hail distribution within the storms.Detailed microphysical budget analyses are conducted for each experiment,and results indicate that the differences in hail growth processes among the experiments can be mainly ascribed to the different treatments of the shape parameter within different microphysics schemes.Differences in size sorting and hail growth processes contribute to the differences in simulated hail distribution within storms and at the surface.Furthermore,diagnosed relations between the shape parameter and medium mass diameter of PSDs are obtained by fitting data from a three-moment simulation.A two-moment microphysics scheme using the diagnosed shape parameters is designed.The hail prediction skill of this new two-moment scheme is evaluated using the Jiangsu multi-cellular hailstorm.Comparisons with available observations and other sensitivity experiments using multi-moment schemes indicate that this two-moment scheme can reproduce the storm intensity and hail PSD characteristics more accurately than the original two-moment schemes,and the computational cost remains similar.Finally,the initiation and evolution mechanisms of the long-lasting,severe multi-cellular hailstorm event over Jiangsu is investigated using the high spatial and temporal resolution satellite and radar observations,and high-resolution model simulations and sensitivity experiments.Results suggest that the lifespan of this long-lasting multi-cellular hailstorm system can be characterized by two episodes.In the first episode,a series of multi-cellular hailstorms was initiated and intensified along the northwest border of Jiangsu Province.The storms organized into a northwest-southeast line and moved southeastward.The storm initiation was along a northwest-southeast oriented convergence line between two low-level vortexes,with one over the eastern coast of China,and the other over the East China Sea.In the second episode,the hail-cooled downdrafts generated a gust front along the leading edge of the surface cold pool.Initiation continued ahead of the gust front.The differences between dry and moisture simulation experiments indicate that the rearward spreading cold pool from the preceding convection generated a vortex couplet from the 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 initiation of the hailstorm.The cold pool-induced vortex also has a significant impact on the later evolution of the hailstorm system.