The Characteristics Of Summer Extreme Precipitation In The Tibetan Plateau And Its Relation With Large-scale Circulations

Based on the homogenized basic meteorology historical precipitation dataset in the eastern and central Tibetan Plateau(TP)and the summer extreme precipitation indices,the interdecadal characteristics of the temporal and spatial distribution and variation tendencies of summer extreme precipitation were analyzed during 1961-2014.The results showed as follows:The summer precipitation contributed more than 50%to annual total precipitation with a regional difference and an increase tendency is in the northern TP.Summer extreme precipitation indices show an increase tendency except for the eastern TP.Majority of summer extreme precipitation indices reduced before the 1970s,and turn to increase afterward.Furthermore,the turning point years detected by the Mann-Kendal method and moving t-test of very wet day precipitation(R95)and R99 are appeared at 2006.Then,the two-parameter and three-parameter Weibull distribution,generalized extreme value distribution,Pearson type-III distribution and log-logistic distribution are selected to fit the summer extreme precipitation indices to study the characteristics of the probability distribution of summer extreme precipitation indices.The results are showed that the distribution of extreme precipitation decreases gradually from southeast to northwest of the eastern and central TP,with a maximum center located in the southeast of Sichuan province,while the minimum center is seated at the northwest of Qinghai province.Through the fitting of extreme precipitation indices by the extremum probability function,we found that different probability functions are fitted for different indices,and which probability function to choose is depends on the comparison of a variety probability function models combined with the actual physical sense.Gumbel distribution is used to calculate different return levels of summer extreme precipitation indices,and it has the same spatial distribution of the 50a and 100a return levels that the precipitation.The extreme precipitation increased after 2006 according to the return level values of Gumbel probability distribution for summer extreme precipitation indices.On the basis of the analysis of circulation anomaly,a possible mechanism influence the R95 on the eastern and central TP has been concluded:the strengthening of the North Atlantic storm track intensifies the forcing effect of the transient eddy activity on the basic flow,accelerating the high-latitude westerly.And the barotropic instability at the exit of the North Atlantic jet stream stimulated the high pressure anomalies in the northern or western Europe,which is in turn affected the Asian monsoon by the means of Rossby wave and finally decrease the extreme precipitation in the eastern and central TP.