Roles Of Underlying Surface, Microphysics And Radar Data Assimilation In The Formation, Evolution And Numerical Forecasting Of Hail Cloud

By using NECP/FNL Reanalysis data, Doppler radar data etc.，two hail cloud processeson May31,2005and June23,2008are simulated using weather Research and Forecasting(WRF). The influences of factors such as underlying surface, terrain height and cloud waternumber concentration on the formation and development of the hail clouds are investigated. Inaddition, the preliminary study is made to explore the conditions of the hailstone growth andthe feasibility of radar base data assimilation for the numerical simulation and forecast of hailclouds. The main conclusions are summarized as follows:The sensitivity tests of underlying surface show that the urban surface has larger sensibleheat flux than that of farmland. The sensible heat that added with the urban heat island isfavorable for the development and enhancement of hail cloud and the formation of largehailstones, which may lead to an increase of surface accumulated hailfall. However, it has lesseffect on the moving path of hail clouds. The farmland surface has a larger latent heat flux andsurface evaporation, and it is favor to the formation of smaller-size hailstones and results inless surface accumulated hailfall.The convection initiation of hail clouds is sensitive to the terrain height, at the windwardside of slope, the increase of terrain height can elevate airflow, gather vapor, accumulateunstable energy and invigorate and strengthen convection. Sensitive tests show that the hailfalland precipitation at windward slope increase as the elevation of terrain height, which willcause more unstable energy accumulation and strengthen the development of hail clouds.Sensitivity tests are also conducted and compared for different cloud water (CCN)number concentrations. With the increase of cloud water number concentration in the hailclouds, the precipitation and hailfall amounts are reduced, and also less precipitation andhailfall rates are found; Meanwhile, the results show that the hail mixing ratio is reduced, buta larger number of hailstone concentration can be seen, which causes the mean diameter ofhailstones to decrease. In the polluted environment (high cloud water number concentration),it is unfavorable for the hail cloud growth and large-hailstone formation, but the cleanatmosphere (low cloud water number concentration) is conducive to the development of hailcloud, precipitation and hail fall.There are some differences of microphysics mechanism for hail formation in differentheights. Above the height of4km，the hailstone embryo is mainly formed by collision and freezing coagulation with supercooled cloud water and graupel, while hailstones grow up bycolliding and freezing coagulation with supercooled rain water below4km. The flow fieldformed by horizontal and vertical wind is the key to decide whether hailstone can grow up.Radar data assimilation is conductive to improve the simulation and prediction of hailclouds. The introduction of radar reflectivity information can improve the water vapor of theinitial field significantly, and make it similar to the real atmospheric conditions. Using theimproved initial field, the3-hour forecast of hail cloud intensity and distribution is better inthe mode simulation. The precipitation and hail peak value also occur earlier and the spin-uptime was reduced with the introduction of radar reflectivity information.