Relationship Between Regional Rainstorm Process In Hainan Province And Atmospheric Low Frequency Oscillation

Atmospheric low-frequency oscillation is considered to be an important breakthrough in the forecast of rainstorm process.Based on daily precipitation data and NCEP reanalysis data during 1961-2014,the climate characteristics and change law of regional rainstorm process were studied,the anomalous characteristics of ocean-atmosphere and its possible influence path in autumn severe regional rainstorm processes were analyzed,and an objective quantitative evaluation index was developed,which could reflect the regional rainstorm process in Hainan province comprehensively.Moreover,by using methods of ensemble empirical mode decomposition(EEMD),wavelet analysis,Butterworth filter and phase synthesis analysis et al.,the low-frequency oscillation characteristics of precipitation in flood period and effects of 10-30 day low-frequency oscillation on severe and above regional rainstorm process in Hainan were analyzed.We found the important influential low-frequency oscillation systems and its oscillation sources.Then,on the basis,the concept graphs of low-frequency impact systems configuration in severe regional rainstorm process were summarized,providing the prediction information and prediction theory for the extended period forecast of heavy rainfall process in Hainan province.The main results are as follows.(1)A composite evaluation index was defined,which could reflect the sustained duration,influence range and extreme intensity during regional rainstorm process comprehensively in Hainan province in an objective and quantitative way,also can make a more reasonable composite intensity grade partition in different heavy rainfall processes.And low-frequency signal in such heavy rainfall processes is more significant than the heavy rainfall processes which defined simply from the rainstorm duration.(2)The regional rainstorm processes in Hainan province have obvious spatial and temporal distribution characteristic.In the temporal distribution,they are more during May to October,most concentrated during August to October.Severe and above regional rainstorm processes appear during May to November generically,are more during July to October and most in October.In the spatial distribution,the total frequencies of regional rainstorm processes,severe and above regional rainstorm processes are higher in central and eastern regions of Hainan Island,and lower in the west coast.(3)The regional rainstorm process,severe and above regional rainstorm process in Hainan province both have obvious inter-annual and inter-generational characteristics,but the trends are not significant.In the seasonal distribution,only the increasing trend of autumn severe and above regional rainstorm process is significant,and,the influence range,extreme intensity and sustained duration time both show significant increased trends.Autumn severe and above regional rainstorm processes are closely related to sea surface temperature anomalies in the tropical central and eastern Pacific,East Asia trough,South Asia high,western Pacific sub-tropical high and tropical low systems in the south China sea.(4)The precipitation during flood season in Hainan has obvious low-frequency oscillation characteristics,variance of 10-90 days low-frequency oscillation accounts for 23.6% in total precipitation variance,and the highest year reaches 46.2%.Thereinto,the 10-30 days low-frequency oscillation is stronger than the 30-60 days obviously.Commonly,the 30-60 days low-frequency oscillation in abnormally more precipitation years is stronger than in abnormally less years.During the severe and above regional rainstorm processes,the significant quasi-biweekly oscillation,or the superposition between the significant quasi-biweekly oscillation and the positive phase of Madden-Julian oscillation appear frequently.(5)The severe and above regional rainstorm processes have close relationship to the 10-30 days atmospheric low-frequency oscillation.Based on 110 cases of the severe and above regional rainstorm processes in the recent 54 years,the atmospheric low-frequency key areas and their configurations can be roughly divided into three broad categories.The first category is typhoon type,it's mainly the tropical low-frequency low systems around the south of Hainan island cooperating with the low-frequency high systems in Pacific northwest regions.For different types of regional rainstorm processes caused by tropical cyclone,the locations of low-frequency low systems and the low-frequency high systems are slightly different,the rainstorm areas are also different,and the low-frequency oscillation sources have slight differences,but there are all in the Pacific Northwest.The second category is cold and warm type in autumn,it's mainly combined action between the tropical low-frequency low systems in the low-latitude of South China Sea,the low-frequency high systems in western Pacific and low-frequency high systems in high-latitude regions,the low-frequency oscillation sources are located in the Pacific Northwest and in western Siberia.The third category is the other type,of which the frontal type is mainly combination between the low-frequency high systems in high-latitude and low-frequency low systems in south of Hainan island,and the low-frequency oscillation sources are located in central Siberia and the southeast of Bay of Bengal.