A case study of hail formation mechanism and hail suppression technique in Shandong Province

The mechanism of hail formation and hail suppression technique are studied using the three dimensional hailstorm numerical model with fully elastic primitive equations,which is established by Institute of Atmospheric Physics,Chinese Academy of Sciences.The data used here are taken from a severe convective process with hailstorm and thunderstorm accompanied by gale occurred in Shandong Province on 5 July 2006.Simulation results show that 79% hailstone embryos are graupel particles,which are the main hail embryos.The dominant formation mechanism of graupel is the collision freezing of ice crystals,snow flakes with supercooled rain water and the auto conversion of snow.Once formed,the accretion of cloud water and rain water becomes the dominant growth mechanism of graupel particles.And the main growth mechanism of hail particles is the auto conversion of graupel and the accretion of supercooled cloud water.The results of seeding show that the growth of hail particles can be effectively inhibited if the seeding happens between one to four minutes before the hail content reaches 0.1 g·m-3.And the seeding effect is better at 6.5—7.0 km heights than that at the other heights.Generally,the more AgI,the better seeding effect.Artificial ice crystals are produced with the sublimation nucleus AgI,and the sublimation growth of ice crystal results in decrease of the supercooled cloud water content.The number of hailstone embryos especially the graupel particles increases after seeding,and these numerous smaller hailstone embryos compete for the less available supercooled cloud water,so the average size and quality of hailstones decrease.The conversion rate of hailstone embryos to hailstones is reduced.The accretion of supercooled cloud water by hailstones decreases after seeding,resulting in decrease of the hail amount.