Circulation Background And Water-vapor Transformation For Xinjiang Rainfall

In 1970s, the atmospheric circulation has significant interdecadal changes which lead to some unavoidable variations in most of regional climates. In this background, one of the most obvious changes is the climate shifts from warm-dry pattern to warm-wet in Xinjiang area. In these years, many notable works have been done to discuss the climatic phenomenon. These works' results are available but also limited to further disclose the complex climatic issue. The main reason is that most of the works are focus on limited meteorological station observations which are effective to depict the climate change but not available to learn the further reason. This paper focus on studying the relationship between the atmospheric general circulation and the augment of Xinjiang rainfall. Besides, the characteristics of water vapor transport in more and less rainfall years in Xinjiang have also been discussed in detail. The data we use are the precipitation records from 85 meteorological stations in Xinjiang, ERA-40 and NCEP reanalysis data sets.The diagnostic analysis shows that the rainfall has a considerable increase all over Xinjiang area since 1987, about 10 years later than the abrupt change of the general circulation in late 1970s. The distribution of the rainfall departures shows that the biggest departures meet in July and August. As for the percentage the summer and winter rainfall increased most significantly in comparison with spring and autumn in the north of Xinjiang, and the rainfall augment in the south of Xinjiang mainly occurred during spring throughout summer. The composition analysis for 500 hPa height field shows a relative big departures at the east side of Xinjiang and a relative small or negative ones at its west side, i.e., Central Asia. Such a distribution is propitious to the rainfall of Xinjiang. In additional, the Ural blocking high conduces to more rainfall in Xinjiang .The 700hPa wind filed shows that the stronger west windcorresponds more rainfall in Xinjiang and the strong east wind from the Hexi Zoulang to Xinjiang is also of a positive impact on atmospheric possible precipitation augment in Xinjiang, it is because that the concentration of moisture on Xinjiang is due to the intensifying of the southwest wind in Central Asia and westward wind anomaly covering from Hexi Zoulang to Xinjiang. It is a rainy pattern for Xinjiang in summertime that there is an anticyclone of wind anomaly over the Ural Mountain and a cyclone on its south in 700 hPa. This dipole structure joints the northern cold flow and the southern wet flow together on the area west to Xinjiang. In a word, the increase of the moisture around Xinjiang is in favor of the development of synoptic systems with more moisture before entering Xinjiang, which is propitious to the large-scale rainfall of Xinjiang.The water vapor transport of rainy years in Xinjiang area is different from rainy years. In the wet years, the west wind is more power in the mid-latitude .The vapor flux on the west boundary of the northern Xinjiang is greater in the rainy years compared to the dry years. Furthermore, along with the increasing blow from the north to the south area, the vapor contents in the south basin also becomes richer. Through analyzing the divergence field of water vapor transport, we could see that along with the zone of water moisture transport, there will be a convergence belt of moisture flux. In spring, the largest convergence area of moisture is nearby Kashi, while in summer the largest convergence area is in West Tianshan Mountain.