Research On Persistent Anomalous Precipitation And Its Propagation Characteristics In Southwest China

This article mainly uses the precipitation data of meteorological stations to analyze the spatial and temporal distribution characteristics of precipitation in the southwestern region,divides it into 4 areas,and defines the continuous abnormal precipitation in 4 areas through calculation and studies the time distribution characteristics.The aggregate empirical mode decomposition method analysis shows that the precipitation in various regions in the southwest region has oscillation periods of 10-20 d and 20-60 d.Secondly,the main oscillation mode method was used to analyze the two main modes of low-frequency precipitation with an oscillation period of 10-20 d in the southwest region.The transitional and peak-type atmospheric circulation fields of precipitation in the two modes were studied respectively.Relevant characteristics and the characteristics of the atmospheric low-frequency field when the two modes have different phases during continuous abnormal precipitation.Finally,a simple atmospheric circulation model can be obtained during the period of persistent abnormal precipitation in Southwest China.In this article,the following conclusions can be drawn:(1)According to the REOF decomposition results,the precipitation field in the southwestern region is divided into four areas,namely the western Sichuan plateau area,the basin area,the Guizhou area,and the Yunnan area.The precipitation in each area of the southwestern area is 10-20 d and 20-60 d.The oscillation period of 10-20 d,and the low frequency oscillation of 10-20 d contribute more to the actual precipitation than 20-60 d.The percentile threshold method is used to define the persistent anomalous precipitation in various regions in the southwestern region.The persistent anomalous precipitation events in the southwestern region mainly occur in summer and autumn,and the specific performance is from April to October.(2)POP analysis Low-frequency precipitation with an oscillation period of 10-20 d yields two main main oscillation modes,POP1 and POP2.The POP1 mode exhibits two main characteristics of precipitation in the southwestern region.One is in eastern Sichuan and Chongqing.The eastward,westward retreat and phase transition of the rain belt position.The second refers to the phase transition of precipitation in the middle of the southwest and south of the southwest.POP2 reflects the transition of rain belts in eastern Sichuan and Chongqing and Guizhou,while in Yunnan and western Sichuan,there are weaker precipitation phase changes.(3)According to the analysis of the associated field: when the POP mode of precipitation in the southwest region is in the transitional type,the outward long wave radiation field in the southwest region is a positive area,the abnormal southwest monsoon is obvious,and the East Asia region is under the control of abnormal high pressure,200 hPa wind field is stable at low and middle latitudes.However,when the POP mode of precipitation in the southwest region is in the peak type,the positive area of the outward long-wave radiation field in the southwest area turns to a negative area or the intensity is weakened.The abnormal north wind is obvious,and the high pressure in the East Asia area is weakened or there is a trough of low pressure.It is conducive to the westward advance of the Western Pacific,which brings about precipitation,and corresponds to an abnormal anticyclone or westerly circulation.(4)Synthesize the different phases of POP1 and POP2 modes for the lowfrequency precipitation and low-frequency circulation field during continuous abnormal precipitation in the southwest region.It can be obtained that in the POP1 mode,the precipitation center of the southwest region first appears in Yunnan,Guizhou area,then moved north to Chongqing area,and finally westward into the Sichuan Basin until disappeared.In the POP2 mode,the precipitation center in the southwest area first appeared in the Guizhou area,then moved north to the Chongqing area,and finally moved westward until it disappeared.At the same time,the precipitation center in the south of the Yunnan area also appeared in phases 4,5,and6.(5)The changes of the two modes of atmospheric circulation provide a favorable circulation background field for the occurrence of persistent anomalous precipitation in the southwest region,thus establishing the atmospheric circulation model during the persistent anomalous precipitation in the southwest region.In the low-frequency OLR field,the POP1 mode has two negative phase regions before the occurrence of persistent abnormal precipitation.In the low-frequency 850 hPa wind field,in the POP1 and POP2 modes,in the fifth phase,cyclones are generated in the Bay of Bengal area,which is 2-5 phases earlier than the occurrence of continuous abnormal precipitation.In addition,whether the southwest region is controlled by the southwest monsoon is an important condition that determines the occurrence of persistent abnormal precipitation.In the low-frequency 500 hPa height field,the southwest region is controlled by the positive potential height,corresponding to the lowfrequency anti-cyclone at 850 hPa in the lower layer,while in the previous two phases,the POP1 and POP2 modes show low grooves in the middle and high latitudes,which is conducive to cold The air is transported to the southwest of China through the low trough.In the low-frequency 200 hPa wind field,the two southwest regions of the two modes are controlled by powerful low-frequency anticyclones.The upper atmospheric dispersion is conducive to the occurrence and development of convection.Previously,there were alternating low-frequency cyclones,The formation of anti-cyclones,and the sequence of eastward movement,gradually developed and become stronger,controlling most of our country.