The Numberical Study On The Formation Mechanism And Suppression Of Hailstorms In Tibet Plateau

In order to abate the blindness of hail suppression and improve its effect in Tibet Plateau, two hailstorms occurred in Lahsa on Aug. 7th, 2004 and Naqu on Jul. 7th, 2005 are simulated using a three-dimensional compressible nonhydrostatic cloud model with two-moment bulk microphysical parameterization to investigate the mechanism of hail formation, growth and the effect of AgI seeding on hail suppression under low supercooled liquid water content conditions.Results show that hailstone embryos are mainly graupels, the hailstones grow through hailstones collection cloud water, similar to those of Maqu in Qinghai and High Plains in America. The dominant formation mechanism for graupel is autoconversion of snow and ice to hail. Once formed, accretion of cloud water becomes the dominant growth mechanism for graupel. The hailstorms in the Plateau don't exist accumulation zones of supercooled raindrops. Melting of graupel is the main source of rain. With the limit of vapour the up-draft airflow is weaker and cloud develops shallowly. The convective clouds have latent ability of hail suppression. The different seeding scheme has different the effect of hails suppression. The optimal seeding position is located at 4kin height in the updraft area. Seeding AgI leads to the decrease of hail fallout and rainfalls. A series of seeding experiments proved that the earlier the seeding time, the less the hail. Seeding produces significant effects, resulting in more cloud ice, more snow but fewer cloud water. These lead to the decrease of graupel and the weakening autoconversion of graupel to hail. Accretion of cloud water by hailstones is decreased after seeding, resulting in hail amount decrease. On the condition of the same seeding time and height, if the amount of AgI is less 100g, the more AgI, the less hail, the amount of AgI is more than 150g, the more AgI, the more hail.