Research On Characteristics Of Precipitation Days And Its Possible Causes In The Eastern Of Northwest China In Recent50Years

The NCEP/NCAR reanalysis data (2.5°*2.5°,1dy) and the observed data from1960to2009which has743stations are used to investigate the climatic characters of precipitation days during50a in the eastern of Northwest China. The effect of the wind shear in700hPa was studied considering the characters of precipitation days. Finally, several affecting factors was proposed, which included the atmospheric circulation index and water vapor, when the paper reveals the probable reason which lead to changes of precipitation days in the recent fifty years, and reached some conclusions:The analysis results have indicated that the precipitation days and the rainfall have a significantly decreasing trend (that is far under the average decreasing trend level of China.), it is the reveal of regional climate change behind in the eastern of Northwest China, which is happened in1980s. Form the geographic distribution of the climate trend coefficient the annual precipitation days is ascend remarkable at the edge of the western part, and is downward trend at the edge of the southeastern part. The geographic distribution of precipitation days in eastern of Northwest China is Northwest-southeast type, there is a larger exceptionally region over the Qilian Mountain area, because of the orographic effect. The geographic distribution in spring and autumn are similar to the average of China and the intensity of precipitation cell reach the strongest in summer, and break into two centre in winter. The paper chose200mmrainfall as the demarcation value, the northern of the zone is the arid region and the southern of the zone is the semi-arid region. The trend of the semi-arid region is decreased, which is the major contribution of the region, and keep stable in arid region. The annual precipitation days have mutant points such as1995and2007in eastern of Northwest China. The spring and summer has remarkable mutant between1970s-1980s, and the mutant in autumn happened in1967. Before1980,the precipitation concentration period of eastern of Northwest China is centralized distribution, and more average distribution after1980.The precipitation days and the wind shear in700hPa are strongly coupled, annual wind shear in700hPa also has a mutant point in1972, just the same as the annual precipitation days, The complete agreement of the phase of precipitation days with wind shear in700hPa accounted over60%. The relationship have sharp seasonal differences, it is separate in summer and correlated significantly in the other three seasons. The wind shear in700hPa is a very important precipitation condition in eastern of Northwest China.In the more wind shear years, the atmospheric circulation meridional and zonal wave train anomalies of the500hPa geopotential height field in the Northern Hemisphere, that is beneficial to the transmission of energy and the occurrence of the blocking situation. The west Pacific subtropical high becomes stronger; in the less wind shear years, an obvious negative anomaly exists in eastern Asia. Ural high ridge is weaker than normal year. The annual average of the500hPa geopotential height field is negative anomaly, it indicates that more trough line and low-pressure system are generated. In the more wind shear years, the annual average of the500hPa geopotential height field is lower than that in the less wind shear years. When the geopotential height field lower than the average level, will lead more wind shears in700hPa generatedThe paper introduces the precipitation efficiency of wind shear in700hPa (PEWS in700hPa) to represent the relationship between the precipitation days and wind shear in700hPa,the correlation coefficient is0.704,which is marked positive correlation. The index is more has more advantages than the single data precipitation days and wind field. The index can represent the temporal and spatial variation characteristics of system rainfall. The format of the data is easy to combine of multi metadata and to analysis how the wind field effects the precipitation condition with the other factors. The grid of wind data is big, so the result is no good in small scale the wind shear in700hPa, it is a limitation in application.The correlation coefficient between Vertical integration of water vapor flux and precipitation efficiency of wind shear in700hPa reaches over0.5and reduces from eastern to western, water vapor is the key determinants of precipitation efficiency of wind shear in700hPa.Water vapor play an important role in the relationship between low-level wind shear and precipitation days, especially in spring and summer. The strength of monsoon directly determines the water vapor, the conveyor belt of water vapor over Tibetan Plateau and its surroundings decided the relationship between the low-level wind shear and precipitation days, especially in autumn.