The Multiple Time Scales Characteristics Of Precipitation And Its Circulation Anomaly In Midsummer In The Upper And Middle Reaches Of The Yellow River

Based on the yearly and monthly precipitation data at45stations from1961to2011over the upper and middle reach of Yellow River(UMRYR) and NCEP/NCAR reanalysis data, by the method of climate tendency rate, precipitation index, concentration degree and period, skewness and sharpness coefficients method, Pearson-Ⅲ, the precipitation interannual variation and distribution characteristic, its evolution trend and its normality and stability were analyzed, the fitting precipitation from different guarantee rate was calculated, the temporal and spalial variation of midsummer precipitation and its circulation anomaly characteristics were analyzed.The results show that:(1) The annual precipitation rapid decline from southeast to northwest, there was a rain shadow area from Lanzhou to Huining of middle Gansu province. The two climate change sensitive areas were source area of the Yellow River and middle and east of gansu loess plateau. The precipitation increased in source area of the Yellow River, average value of climate tendency rate was9.5mm/10a.The drought trend was obvious in middle and east of Gansu loess plateau, average value of climate tendency rate was-23.4mm/10a. There were significant difference of mobile trend and track of400mm rainfall contour between source area and middle reach of the Yellow River. In source area of the Yellow River the trend of400mm rainfall contour moved north, but in middle reach of the Yellow River the contour obvious moved southeast before2000,and regressed to northwest after2000.(2)The general variation trend of seasonal distribution of precipitation in UMRYR were that the precipitation reduced obviously in spring and summer but increased significantly in autumn, especially after2000.The variation trend of precipitation in each season was inconsistent in different areas, the area which seasonal distribution of precipitation chenged obvious was guanzhong plain. The precipitation concentration period of UMRYR was in July, but concentration period of source area was slightly earlier than middle reach of the Yellow River. The precipitation concentration degree increased from southeast to northwest, the value was between0.45-0.71. The interannual fluctuation of concentration degree were relatively large in the whole region before1998, but it had stabilized and closed to average value since1998.(3)The stations that annual precipitation cannot pass the normality test occupied33.3%. All the stations of monthly precipitation cannot pass the normality test in each month of winter, the normality of each month in summer is better than other month, but the stations precipitation cannot pass the normality test occupied above60%.The normal efficiency reached95.4%that the precipitation was transformed by nonlinear method, the square root was better than cube root method in January, February and March, it was contrary in November and December, the efficiency was general the same in other month.(4)The precipitation stability was worst in winter, the second was spring, the best were in summer and autumn. Seeing from the regional perspective, in east of Qinghai plateau the precipitation stability was best, the one of the worst area was in north of Hetao river band. The precipitation under guarantee rate of P=50%closed to average value, there were21stations that the maximum value of annual precipitation breakthrough not occur even in a century, and it corresponded with the stations that annual precipitation cannot pass the normality test, these stations were mainly in middle and east of Gansu, south of Ningxia, the area was sensitive to climate change and occurred extreme precipitation events frequently.(5) midsummer precipitation of UMRYR was downward trend, at the beginning of the1960’s,late of the1970’s and1990’s were flood period, the others were drought period. The midsummer precipitation can be divided into three kinds of distributed types, there were namely entire regional distribution type, north and south opposite distribution type and west and east opposite distribution type. With REOF the midsummer precipitation can be further divided into east of Qinghai plateau area, the upper reaches of weihe river area, north of Hetao area, the lower reaches of weihe river area and source area of the Yellow River, there were significant difference of precipitation trend and periodic variation of each areas.(6) The variation of drought and flood was closely related with West Pacific Subtropical High, India trough, Okhotsk high pressure, circulation of mid-high latitudes on500hpa and strength and position of the South Asian High on100hpa. The main reason of drought in UMRYR was that Okhotsk high pressure was strong and South Asian High was east extension.