Analysis of Long-Term Variation Characteristics and Influence Factors of Summer Low Cloud Amount in Xinjiang during 1961-2005

Using the data of 51 weather stations during 45 years (1961〖CD*2〗2005), the long-term temporal variation of summer low cloud over Xinjiang is analyzed by linear trend estimation and Morlet wavelet analysis. The relationship between 500-hPa circulation index and summer low cloud over Xinjiang is examined by means of singular value decomposition，and the impact of polar vortex on the summer low cloud in Xinjiang is discussed. The analysis shows that the summer low cloud in Xinjiang has remarkably increased in recent 45 years, and quasi-two-year, quasi-six-year, and quasi-11-year periodic oscillations were observed in summer low cloud amount. In addition, an abrupt change is found around 1987. The results indicate that the summer low cloud amount in Xinjiang is closely related to the summer subtropical high, polar vortex, and the Qinghai－Xizang Plateau height field index. With weak polar vortex, the upper-level westerly jet in West Asia weakens while the westerly jet in Xinjiang strengthens and correspondingly in lower atmosphere, the south wind increases in Xinjiang accompanied by stronger water vapor transport, which leads to increased low cloud over the area.