Analysis of Biweekly Oscillation and Low-Frequency Circulation Characteristics of Summer Precipitation in Drought-Flood Years over the Qinghai-Xizang Plateau

In order to study the characteristics of low frequency oscillation of summer precipitation over Qinghai-Xizang Plateau， based on the daily precipitation data of 62 meteorological stations over the Qinghai-Xizang Plateau from 1978 to 2017 and the high-resolution daily reanalysis data of ERA5.This paper adopted the collection of empirical mode decomposition （EEMD）， Lanczos filter and synthesis analysis method.Filter out the years of drought and flood， and analyzed the characteristics of low frequency oscillation， variation of meteorological elements and circulation of precipitation in dry and flood years in summer over the Qinghai-Xizang Plateau.Reveals the drought and flood years， the characteristics of low frequency oscillation period of precipitation in plateau area， the circulation characteristics and low frequency and other transmission characteristics of wind field and vorticity field.The results show that： （1） There is an oscillation period of 10~20 days in summer precipitation over the Qinghai-Xizang Plateau.The period frequency of drought and flood years is different， the oscillation frequency of flood years is obviously higher than that of drought years.In flood years， the significant oscillation is 10~20 days and 30-days oscillation， while in drought years， the oscillation is mainly 10~20 days.（2） In flood years， the low-frequency precipitation phase is strong， the high-altitude low-frequency cyclones control the plateau， the high-altitude divergence， the low-altitude convergence.The water vapor supply from the south of the Bay of Bengal， and the low-frequency disturbance is strong， which is conducive to the occurrence of precipitation， there is an abnormal circulation distribution of low-frequency anticyclone- low-frequency cyclone- low-frequency anticyclone from west to east.On the contrary， in the low frequency precipitation weak phase， the high frequency anticyclone controlled the main body of the plateau， high altitude convergence， low altitude divergence， the warm and wet southerly air is weak， the disturbance is weak， not conducive to the occurrence of precipitation.The distribution of low-frequency anticyclone， low-frequency cyclones and low-frequency anticyclone is not obvious in drought years， and the oscillation intensity is not as strong as that in flood years， and the intensity of low-frequency element field is weak.（3） The transmission of low-frequency fluctuation is different in drought and flood years.In flood years， the high and low plateau is the sink area of low-frequency oscillation， and the meridian is also the convergence area.The precipitation in the oscillation sink area is more frequent， so the precipitation in flood years is more.In the dry years， the low-frequency oscillation propagates upward from the plateau to the upper level， so the low-frequency precipitation occurs less in the dry years.