| Abstract:Based on the daily precipitation datasets from756stations in China, the data of66stations in the region was carried out, combining with the NCAR/NCEP reanalysis data, characteristics and circulation pattern of rainfall in the Upper and Middle reaches of yellow river, which analyzed by precipitation anomaly percentage reflecting precipitation anomaly characteristics and strong precipitation event concentration period(SCP) and strong precipitation-concentration degree(SCD) reflecting intra-seasonal inhomogeneity characteristics.The results are summarized as follows:1. The precipitation appeared downtrend especially at the middle of80’s in autumn from1960to2008except winter, which shows the reverse trend, and goes beyond95%significance level e in early70s.There are significant differences in the spatial distribution of severe flood and drought in seasons. However, Hetao plain in70s and the north shaanxi in00s are the most drought sensitive region in spring, then the most of valley shows flood in60s and inversely in90s in autumn, on the part of winter, there are shows drought in60s and flood in70s and00s.Moreover,the periodic oscillation of rainfall variation and the points of abrupt change at different time scales along the time series are discovered and the main periods of every serial are confirmed. It was showed that the distribution of rainfall in the upper and middle Yellow River Basin had obviously regional difference during the four seasons, which was influenced not only by the weather system but also by the geographical environment such as topography, etc; there are obvious periodic oscillation of less than10years for spring, summer and autumn, but16years for winter.2. Six subdivisions of spring and summer precipitation in the upper and middle Yellow River Basin are divided by using the rotated empirical orthogonal function (REOF) analysis. They are subdivision Hetao Area, Northern Piedmont of Qin Ling, west Shaanxi,north Shaanxi to east Lanzhou, huang water system and upper reaches of Lanzhou. Though the autumn precipitation is divided into6highly correlated anomaly regions, the most typical region are west Shaanxi and plateau, then south Hetao basin and west Shaanxi for subdivisions of winter. Different subdivisions show different periodic oscillation obviously, and they show the same characteristics in Spatial Distribution with most significant main period for every seasons, and the anomalies of rainfall in these subdivisions change in different ways on inter annual time scale, which influence the spatial distribution characteristics of rainfall probably. 3. The most flood sensitive region in the reaches is located in Hetao plain, and then the most drought sensitive region is located in the northeast of the Loess Plateau; There is little change for precipitation with inter-annual considerable oscillation among the recent50years, and the precipitation program is "normal-partial less-artial more-ormal-artial more-artial less"; Based on the wavelet analysis, significant main period for summer precipitation are2years,9years and15years. The first is the most significant one, the second correlate with drought well, the last correlate with flood well, moreover, spatial distribution of most significant main period have a significant correlation with topography, geomorphology, altitude and distribution of basin;. There exists dramatic on the Circulation form between the flood and the drought year. The characteristics result from monsoon, west pacific subtropical high, extratropical cyclone and Baikal cyclone.4. Most area of Middle and Upper Yellow River depend on strong precipitation should be presented significantly, and the most well-distributed precipitation area located in northern foot Qin Ling and southern Sanxi. Oppositely, the most centralized-precipitation area located in northern Shanxi and ordos region; Average occurrence time of strong rainfall in flood season in plateau area is the earliest, followed by arid area, and in monsoon area is the latest, which decrease by five days. In south of monsoon area and Weihe, Jinghe and Luohe in the east of plateau area where the SCP changes greatly, there is significant variation of occurrence time of strong rainfall. In the1990s, in arid area and monsoon area, there was a significant decrease mutation of SCP, while it occurs earlier in arid area than that in monsoon area. In arid area, the high circulation situation of SCP anomalous years shows that there is strong rainfall before or late in flood season when the500hPa western pacific subtropic high (WPSH) is on the northern side of30°N, around120°E, and the200hPa south Asia high (SAH) is on the eastern side of120°E, around30°N, if they move towards or backwards to the same or opposite direction. In monsoon area, there is the same law as arid area when WPSH located in26～30°N,around120°E,meanwhile the SAH on the western side of120°E,around30°N. |
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