Abstract:Based on the composite analysis of high-level circulation under the situation of severe convective weather in Pingliang of Gansu Province, the typical fieldmodel and environmental field feature at the level of 500 hPa in severe convective weather process were summarized.Meanwhile, the daily unbalanced fields from June to September in 2008 were caculated by usingT213 mathematical forecastproducts, and the relationship between the unbalanced field and strong convective weatherwas discussed. It showed that dynamical interpretation technique has obviously increased the capability ofobjective forecast for severe convectiveweatherevents in actual operation application.

Based on the blown - up theory and the V - 3θ structural prediction method，V - 3θ figures of 36 aerological stations in Northwest China were made. The hail weather course which occurred in August，2004 in west Qinghai，southeast Gansu and south Ningxia was analyzed by the V - 3θ figures. Results show that the prediction tools based on the blown - up theory is capable of forecasting hail weather to some extent in the semiarid and arid region.

The average beginning and ending period of rainy season in Northwest China was analyzed by using the precipitation data of ten
days by ten days from 131 stations in Northwest China and monthly height grid data of 50 x 100at 500 hPa over Northern Hemisphere from
1960 to 2000 and the relationship between precipitation and 500 hPa height field was analyzed by SVD method in rainy season. The re-
sups show that the rainy phase is from May to September and the distribution style of the beginning and ending period of rainy season is
、omplex} but the first ten days in July to the first ten days in September、an represent the rainy season in  main precipitation area.  The
precipitation in rainy season not only has relationship with the same一period 500 hPa(irc;ulation but also with the previous periods 500 hPa
(irc;ulation from March to May and from December to February but the key areas effected by different period(irc;ulation is great different

Based on the domestic current researches, the relations between climatic change and Chinese Loess Plateau evolution during the Quaternary arc summarized，and the main characteristics can be listed as follows:① the crucial factors of Loess Plateau formation arc the serial otherness structural variation of Chinese mainland caused by the Himalayan Syntaxes from the "hernary;② during the Pleistocene the strong wind caused by "hibetan Plateau apophysis brings great deal of power一dust from the inland desert in middle Asia zone to Loess Plateau zone, which was formed accordingly;③present monsoon occurred after "hibetan Plateau had reached certain height during the Pleistocene, then the higher of 'hibetan Plateau is, the stronger of the barrier against summer monsoon is.  "hhe warm and wet climate in Loess Plateau turned into cold and drought，and the types and distrbution of vegetation adjusted accordingly;①the adaptability of vegetation could repeat according to climatic changes, namely the
distributing zone moves from north to south on the basis of climatic changes in temperature and precipitation;③during the Pleistocene natural corrosions played great role in soil and water losses in Loess Plateau，and during the Holocene the soil and water losses accelerated because of human activities;⑥soil and water losses in Loess Plateau arc influenced by the geological condition，precipitation,vegetation and human activity, and its spatial variety is obvious.

Dronght is the most serious meteorological disaster in GanSll  1neSS  pltateall.  It considering meteorological factors comprehensivly because of the complexity Model of America and the  modified Pahner  Drought Severity  Model for of drought .On is difficult to develop a drought index the base of  the Palmer Drought Severity China,  we fLn-ther  modified the model in  stations CalClllatlOn Of pOtentlal eVapOtTanSplTatlOn and TLInOff,  etC.  AS a TPSlllt ,a TPV1Sed m0(lel Of dTOllght SPVPTlty haS been eStahhShed In paper which is expected to he applicable for drought research on this region.