Extended-range Forecast Of Rainy Season Precipitation In Southwest China Based On Low-frequency Oscillations

Firstly,the spatial and temporal distribution characteristics of rainy season precipitation are analyzed by using daily precipitation station data in Southwest region,the beginning date,end date,length and precipitation of rainy season in the past 37 years are delineated by the standard which is proposed by the CMA.Secondly,using the reanalysis data from the European center,the evolution characteristics of the upper and lower atmospheric circulation and the water vapor transport field around the beginning and end of the rainy season in the southwest region are explored by the method of composite analysis.Then,the low-frequency evolution characteristics of the precipitation series in southwest region are analyzed,and eight phases are divided according to the life cycle of 20-60-day filtered precipitation,and the phase composition of low-frequency water vapor transport and low-frequency atmospheric circulation field are composited for those phases.Finally,two methods are used to predict the precipitation in southwest region over the rainy seasonal at the extended range.The main conclusions are as follows:(1)Generally speaking,5-10 is the rainy season in southwest region.The spatial and temporal difference of precipitation in the whole rainy season is obvious,the first three modes of EOF respectively show the distribution characteristics of precipitation in the Southwest region: the consistent across the region,the eastwest reverse and the north-south reverse;The first six modes of REOF divide southwest region into six precipitation sub-regions;Apart from Guizhou and western Sichuan,The long-term change of precipitation in the six regions shows a decreasing trend.According to standard proposed by the CMA,the beginning and end dates of the rainy season in southwest region are determined,the average beginning date is 29 th pentad and the average end date is 57 th pentad.But the time of entering and exiting the rainy season are different from place to place: The rainy season in the western Sichuan Plateau begins early and ends late,lasting for a long time;But there is less precipitation in the rainy season;In other areas,the beginning and end dates of the rainy season in the east are earlier than those in the west.The rainy season precipitation in southwest Yunnan,Guizhou and the west of Sichuan basin is obvious.The long-term changes of beginning and ending dates in the rainy season tend to evolve earlier,but fail to the significance test;The rainy season precipitation shows a significant down trend.(2)Before the beginning of the rainy season in the southwestern region,There is a high-value OLR band in the range of 10°-30°N;Easterly winds prevail in the lower latitudes of the upper Northern Hemisphere at 850 h Pa;South Asian high is located in the Pacific Ocean east of 150°E.At the beginning of the rainy season,the high-value OLR band is fractured nearby the Indochina Peninsula,the south and north low-value zones are opened in the southwest.At 850 h Pa,southerly winds begin to appear in the Gulf of Bangladesh,then Somali cross-equatorial air flow is established,and westerly winds prevail in the lower latitudes of the northern hemisphere.Water vapor channels from the southern hemisphere and the Bay of Meng are opened;Pseudo-equivalent temperature high value region,that is,warm and humid air flow begin to affect the southwest region;At the same time,the South Asian High at 200 h Pa also begin to gradually leap onto the Indochina Peninsula and the Qinghai-Tibet Plateau.At the end of the rainy season,the OLR high-value area in the western Pacific and northern Indian Ocean is opened in the southwest,and the high-value OLR band is reconstructed.The easterly wind over 850 h Pa is also reconstructed in the area south of 20°N;Water vapor channel closed;The South Asian High begin to depart from the QinghaiTibet Plateau and return to the Pacific Ocean.(3)The average precipitation in southwest region obviously shows unimodal characteristics,its peak value mainly in July and August.The precipitation is decomposed by EEMD,the first two modes respectively manifest seasonal changes of 10-20 d and 20-60 d,and they are the important contributors for the time evolution of precipitation series in southwest region.According to the 20-60 d oscillation of precipitation,eight phases are divided,the low-frequency circulation field and water vapor transport field are further synthesized and analyzed.It is found that: The evolution of low-frequency OLR field is related to the seasonal oscillation of precipitation in southwest region which is mainly manifested by the development of positive and negative low-frequency regions,as well as their sequential northward movement.The evolution of the lowfrequency water vapor flux field is mainly manifested from the alternating evolution of cyclonic circulation and anticyclonic circulation in the Arabian sea,the bay of Meng and the northwest Pacific region.The evolution of 850 h Pa lowfrequency wind field is similar to the whole layer water vapor flux field.The timedelay correlation analysis shows that being the maximum correlation when the latitudinal wind in the north of the Indian Ocean leads the low-frequency precipitation in the southwest for about 20 days.The 500 h Pa low-frequency circulation mainly affects the development and interruption of low-frequency precipitation in southwest region through the distribution of trough ridges.The peak precipitation corresponds to the circulation pattern of "two grooves and one ridge",and the valley precipitation corresponds to the distribution pattern of "two ridges and one groove".At 200 h Pa,the low-frequency precipitation peak period is under the control of low-frequency anticyclone in southwest region;The trough period is under the control of low-frequency cyclone.Further more,when MJO is in phase 1 and phase 2,the overall precipitation in southwest region is more than the normal;And when it in the 4th or 5th phases,the overall precipitation in southwest region is less than the normal.(4)Two methods are came into use and build an extended period on forecast model of precipitation for rainy season in southwest region.Model ? is a regression equation based on the low frequency signal of precipitation sequence itself,which can mete out a more accurate forecast about half a month in advance;Model ? is a low-frequency sequence with eight influence factors added into the original equation;It can mete out a more credible prediction conclusion of 20-25 days in advance;In addition,the prediction results of the two models make good performance in forecast from the years of more abnormal precipitation rather than that from the years of less abnormal precipitation.