A Numerical Study Of Two Hailstorm Clouds In Zhejiang And The Effect Of Ice Nuclei Parameterizations On These Events

Based on WRF model simulation, two severe hailstorm weathers occurred in Zhejiang Province are investigated and analyzed using the conventional and Doppler radar observational data in 2009(case09) and 2014(casel4).The differences of macro and micro characteristics of hailstorm and hail growth mechanism are discussed from background field, occurrence conditions and temporal and spatial variation of hailstorm structure and variant microphysical quantities. In addition, the influence of six ice nuclei parameterization schemes on the macro and micro structures of hailstorm and precipitation are compared. Results show that:Simulated atmospheric stratification, precipitation and radar reflectivity offer a better representation of two hail weathers, indicating that the simulation results are trusted. Two severe hailstorm weathers both occurred in large scale environment field of dynamic and thermal instability, with a very unstable stratification. The differences of vapor, heat and dynamic are direct factors which cause the differences of the evolution process and microphysical structure of hail cloud. Case 14 is a strong single hailstorm which is a part of mesoscale convective system (MCS). The ascending velocity, vertical wind shear and high echo top of case 14 are greater than caseO9 which is a multicell hailstorm. And the life history of case 14 lasts 4 hours, longer than caseO9.There are some differences between hail growth mechanism of these two cases. As for caseO9, whose updraft center which under the supercooled raindrop center nears 0℃ layer, hail growing up relys on accumulation zone of supercooled raindrop. And for case 14, whose hail forms along the right side of slantwise updraft which is called hail emybro curtain. Because updraft center (8km) much higher than caseO9, hail grows up by accretion of cloud water, ice and snow.By comparing the simulation results in different ice nuclei parameterization schemes, it can be concluded that the effects of ice nuclei on hail falling are greater than precipitation. There are obvious differences on hail area, intensity, hailfalling times and distribution. The changes of ice nuclei concentration result in different degrees of variation of hydrometeors in cloud. Ice nuclei impact the temporal and spatial distribution of other hydrometeors and heat, dynamic structure of cloud by changing ice first. The more ice nuclei, the more number concentration and mixing ratio of ice. Once ice nuclei number concentration changes, the impacts of different stage of hailstorm are different. In the development and early mature stage, ice nuclei concentration increased, a large number of ice competing water vapor to grow up. At the same time, the consumption of water vapor inhibits the increase of supersaturation in cloud so that the growth of ice crystals is prevented. This indirect effect isn’t conducive to the formation and growth of snow and hail emybro. In the later mature stage, ice nuclei are advantageous to the growth of hail because of cloud convection enhanced. However, when the intensity of hailstorm is very strong, and there are plenty of water vapor and ice nuclei, the indirect effect would weaken relatively.