The Characteristics And Possible Causes Of Change In Precipitation In Northwest China During The Recent 50 Years

Change in precipitation is a very important aspect of climate change in Northwest China. Recently, studies in change of precipitation have received attention by many scientists. Based on daily precipitation data and surface pressure data in China for the period of 1954-2003 and ECMWF ERA40 reanalysis for the period of 1958-2001. The change of precipitation is studied by dividing Northwest China into two subregions such as Eastern part of NWC (ENWC) and Western part of NWC (WNWC) by using REOF analysis. The characteristics are studied using the statistical methods of grading intensity and the trend is based on Kendall-tau test. Possible causes of change of different intensity precipitation are studied by calculating atmospheric moisture transportation and the atmospheric stability. The results are given as follows:(1) Precipitation has increased in Northwest China during recent 50 years. The trend of change is different in eastern and western parts of NWC with increasing in the west but decreasing in the east. The increase of precipitation total in Northwest China is mainly due to the increase of precipitation in WNWC.(2) By dividing daily precipitation into five intensity categories: heavy, relative heavy, medium, relative weak and weak. The result shows that the variations in precipitation total are primarily reflected by the heavy precipitation.(3) The contribution of precipitation intensity is greater than that of the precipitation frequency in precipitation variations for most intensity categories. Variations in precipitation total are primarily reflected in precipitation intensity over Northwest China.(4) The results by analyzing the trend of change of every intensity rank of precipitation and the contributions of the intensity and frequency are as follows: the increase in total is basically caused by the positive trends of heavy precipitation intensity in summer in WNWC. On the contrary, in ENWC, the negative trends of heavy precipitation intensity in summer result in the decreasing total.(5) Water vapour content has markedly increased in summer in WNWC. In northern and eastern of Xinjiang, northerly intensified due to the cyclonic convergence near to northern Mongolia and the Baikal as well as the low system at 500hPa heightfield. The southerly has also intensified in southern Xinjiang. Heavy precipitation has been becoming easily induced by the convergence.(6) The atmospheric stability has weakened becoming unstable in Xinjiang during rencent 20 years gradually.(7) The changes of every intensity category daily precipitation are associated with the changes of surface pressure. The surface pressure decreases with the increase of precipitation intensity category, and especially in heavy precipitation intensity. It is obvious that one of the causes of the increase of heavy precipitation intensity is due to the decrease of surface pressure and the increase of low-pressure system in summer.