The sudden turn of drought and flood in spring in northern Qinghai Province,2013: Characteristics and cause of formation

Based on the daily precipitation,temperature and geopotential high data in spring from 1961 to 2013,the series of precipitation,air temperature and meteorological drought index were calculated. Then after analyzing the characteristics and cause of sudden turn of drought and flood with the climate diagnosis methods. It is summarized as follow s: from 2013. 3. 1 to 2013. 4. 27 greater part of the province had less rainfall and higher temperature,and then appeared a large range meteorological drought with different degree,and in greater part of Xining and Haibei appeared the meteorological drought with frequencies of 1-in-25 years and 1-in-50 years,respectively. Then from 2013. 4. 28 to 5. 20 greater part of the province had more rainfall,and the higher temperature shifted to low er one gradually,so the drought relieved and waterlogging appeared in a large range with different degree. There were 21 stations had the records of sudden turn of drought and flood. Composition analysis shows that when the polar vortex area is smaller than ordinary,the central and west Asia trough has longer duration,the cold air cumulates in eastern Europe and western Asia regions,the cold air into China inclines to the west,the plateau potential height field inclines to low,the east Asian trough inclines to east and the western Pacific subtropical high inclines to north,Qinghai Province will have more precipitation and more prone to waterlogging,and vice versa. The drought in April and the waterlogging in May is under the climate background of long-term variation in Qinghai plateau,characterized by the Tibetan Plateau surface heating field with high intensity in the early stage of January to March. The transient time from spring to summer in advance will be conducive to abnormal precipitation in May in the province.