Research On Characteristics And Causes Of Low-level Wind Shear’s Evolution Over The East Of Northwest China In Recent60Years

In this paper, the global reanalysis wind data and the mean monthly reanalysis data of NCEP/NCAR from1953-2012are used to analyze the characteristics and causes of low-level wind shear’s evolution over the east of northwest China, with the method of correlation analysis,variation rate analysis,and Mann-Kendall test and wavelet analysis. The results are as follows:The low-level cyclonic shear in the east of northwest China is given priority to with horizontal shear.Horizontal shear points and total shear frequency changes in the monthly international synchronization,which curves were unimodal type,the curves of vertical shear point were multiple peaks type.The interannual variability of low-level cyclonic wind shear frequency decrease significantly in recent60years,and there is an abrupt change point in1976.Through the analysis of wavelet transform coefficient of real part and the time-frequency distribution of wavelet power spectrum,it proved that there is an3-7a period of oscillation during1960-1970which the3a and7a period shows the most remarkable.The distribution situation of cyclonic shear point’s500hPa height anomaly field for many years in the four seasons is west low and east high,and the intensity of negative anomaly center is stronger than years of average. This feature showed that abnormally low geopotential height field is easier to induce the generation of cyclonic wind shear.In the summer,the intensity of the western Pacific is weaken.In the winter,East Asia deep trough more shallow.Four seasons in the eastern mountainous area of northwest China is highly negative anomaly field control before and after the break. The negative center position in addition to the winter in western Inner Mongolia autonomous region, the remaining three season in Mongolia,and negative anomaly center intensity of the strongest in summer and weakest in winter.The western Pacific subtropical high strength weaker before than after the break.850hPaUV synthetic wind anomaly reflects low-level circulation characteristics well between the more wind shear years and the less wind shear years. In the more wind shear years,there is a closed cyclonic circulation over the northwest China and Mongolia as well as the Caspian sea, west Pacific Ocean has an anticyclonic circulation.In the less wind shear years,the cyclonic circulation over the northwest China turned into the anticyclonic circulation.The west Pacific subtropical high becomes stronger.Observations suggested that when the geopotential height field lower than the average level, will lead more wind shears in700hPa generated.Low-level cyclonic wind shear interannual variability in the east of northwest China may be affected by various circulation factors together.Further research shows that main reason of the low-level cyclonic wind field evolution is the significant adjustment of atmospheric circulation in the same period.North Africa subtropical high area index, north Africa subtropical high strength index, north Africa and the Atlantic subtropical high area index of North America, the subtropical high area index of the south China sea has a significantly negative correlation with the cyclonic wind shear interannual variability;the Atlantic subtropical high in the north subtropical high ridge line, the northern hemisphere, north Africa, the Atlantic subtropical high in North America north boundary and the cyclonic circulation factors has a significantly correlation with the cyclonic wind shear interannual variability.