Research On The Impact Of Low-frequency Atmospheric Disturbances In Different Latitudes On Persistent Abnormal Precipitation In Southwest China

In this paper,the low-frequency characteristics of persistent extreme precipitation in Southwest China and the characteristics of Atmospheric Low-Frequency Oscillation in different latitudes and altitudes affecting precipitation are discussed by REOF decomposition,wavelet analysis,synthetic analysis and Butterworth filtering with precipitation data from meteorological stations and NCEP/NCAR reanalysis data.The configuration relationship of low-frequency systems in three-dimensional space and its influence on the persistent extreme precipitation in southwest China are shown,which provides a theoretical basis for the forecast of the extended period of persistent abnormal precipitation in southwest China.The results show that:(1)Southwest China can be divided into five regions according to precipitation characteristics.The persistent extreme precipitation in each region is concentrated in summer and has the characteristics of 15-60 days low frequency oscillation.The variance of 15-30 days low frequency oscillation contributes more,which is closer to the actual precipitation variation.(2)In the low-frequency circulation filed,the Ural Mountains,east of Lake Baikal,the Sea of Okhotsk-Japan,the Western Pacific subtropical high control area,the South China Sea,the Indian Peninsula and the Bay of Bengal are the key areas affecting precipitation.15-30 day oscillation can better reflects the original circulation characteristics at all latitudes of each layer,that is,15-30 day oscillation will affect precipitation at all altitudes and latitudes,while 30-60 day low-frequency oscillation is more obvious at low latitudes of the lower layer,but weaker in the middle and upper layer.(3)In the 15-30 day low-frequency circulation field,the low-frequency atmosphere has strong baroclinicity during the continuous precipitation period,which provides energy conditions for precipitation.The northerly wind moves from high to low,as well as from high latitude to low latitude to rainy area,and the southerly wind moves from low to high northward to converge with the northerly wind in mid-latitude.The vorticity is positive in the lower layer and negative in the upper layer,which is conducive to the convergence of airflow in the lower layer and divergence in the upper layer.At the lower level,different low-frequency cyclones and anticyclones move to the mid-latitude,bringing cold air to the South and warm air to the north,and converge in different forms in the precipitation area.In the middle layer,the low-frequency highpressure centers and the low-pressure centers in the middle and high latitudes move southward,finally forming a circulation situation of High West and low east.At the upper level,the rainy area is controlled by low-frequency anticyclone and divergence centers from different regions,forming high-altitude divergence field.(4)Vertical baroclinicity is weak in 30-60 day low frequency circulation,and precipitation in basin and Eastern Sichuan is more affected by 30-60 day low frequency oscillation than in other areas.At the Lower level,low-frequency systems in lowlatitude are stronger,resulting in strong south wind.On the contrary,the low-frequency systems in the middle and high latitudes are weak,leading to the weak northerly wind.In the middle layer,only the west of Sichuan and the basin are affected by the lowfrequency systems in the middle and high latitudes.In the upper layer,the basin,East Sichuan and Yunnan are low-frequency divergent areas,which are conducive to precipitation generation,while the western Sichuan and Guizhou correspond to lowfrequency convergence areas,which are not conducive to precipitation generation.