Study On Characteristics Of The Precipitation In Qilian Mountains During Recent40Years

Orographic influences on precipitation has been a hot issue, the previous studies focus either on dynamical and thermodynamical effects, and on topographic influences to precipitation intensity and spatial distribution. Orographic influences on precipitation weather systems are reviewed and discussed as follows:Orographic influences on the formation and precipitation of shear line, Southwest vortex and airflows; Orographic influences on the path, vertical motion and frontolysis of cold front.With EOF.REOF,Wavelet analysis and catastrophe analysis methods, the spatial anomaly features and time evolution rule of precipitation in northwest China are studied by using the precipitation data during the period1971-2009of128observational stations. The results show that the first three loading vector field could reflect whole anomaly structures of precipitation in northwest China,i.e.wet or dry in all area pattern,wet (or dry) in south and dry (wet) in north pat tern,wet (or dry) in east and dry (wet) in west pattern. The first seven rotated loading vector fields repersent six principal precipitation anomaly areas:east of the Qinghai-Xizang Plateau, the Weishui River basin, the Qinghai plateau, northern Xinjiang, southern Xinjiang and Hexi corridor. It is evident that the Northwest of china saw fluctuations in precipitation, with lows and highs in recent40years, and there is no obvious trend of becoming drier or wetter. Nevertheless, different anomalies regions showed different trends. Furthermore.The precipitation in northwest China have about3-5,10,20years periodic variations, But the prevailing periods exsit differences among different areas and different times in the same area.Using hourly and daily precipitation data in Qilian Mountains, we studied the various trend and distributional characteristics of precipitation in Qilian Mountains. Moreover, the tree-ring data of spruce growing in the area are used to analyze the response of tree-ring width to precipitation in the mountainous region. The unitary linear regression model is used to develop a transfer equation as to reveal the relationship between the standardized chronology of tree-ring width and the average precipitation at Minle Station during the period from previous August to next July and the precipitation change in recent200years. The results show that precipitation in19th century was more abundant than that in20th century. Drought had been increasingly taking place in20th century. Especially, there was a virtual record of extreme drought in1920s and1930s, which is consistent with the genuine drought occurred in history. Precipitation in Qilian Mountain area is closely associated with the altitude, most of which has been taking place in afternoon and nigh. The flood season in this area is in July and August. Meantime, the frequency of extreme precipitation is increasingly growing in past decades, while the total precipitation stands steadily.