Northwest Numerical Simulation Of The Formation Mechanism Of Strong Hail Storm Weather

Hail storms occur with high frequency in north-west China since thevarieties of terrain altitude and physiognomy and the combination of climaticregions. Using MM5 model, a regional hail shooting happened on 6th to 8th,July, 2003 on the northeast of Qinghai-Tibet plateau is simulated properly.Then some regional terrain height is modified in the numerical experimentsto find out the relationship of hail storm and terrain altitude. By analyzingstatistic of hail damage and model output some conclusions are drew out andlisted below:Warm humid air from south at lower level and cold dry air from northwestat middle and upper level made this hail storm possible and moist air'sconvergence and lift forced by terrain height made this hail happened. Withlarge grid size MM5 simulates this rainfall process properly. With small gridsize MM5 exhibits the characteristic to convective cells: high vertical windspeed and strong simulated reflectivity.The terrain height of Wushaoling,Liupansan Mountains is one of the importfactors that cause convective cells first form at high altitude spots. Oncethe convective cells happened they spread to south and east with the guideof northwest wind in upper air. When cells spread from high altitude to lower,moist air flows into cells and unstable energy would be touched off, and cellswould grow stronger and harder and then bring us hails. When they first birthedand spread to death cells will cover the distance of 20km or up to 100km.When terrain height modifications are carried out in simulate experiments,the affects of landform on cells are concluded in two aspects: (1) varietiesof terrain cause the asy ㎜ etry of wind, temperature and moisture in lowerair and make the difference of MCAPE and wind shear. A 1km height variationof a ridge may cause a 700J/㎏ CAPE change in different area. (2) Cells oftenoccur at ridges and spread to lower altitude area, the intensity the speedand direction of the spread of cells differ very much with various landforms.A 1km height variation of a ridge may cause the change of maximium verticalspeed of some cell up to 2～3m/s.