Study Of Atmospheric Low-Frequency Features On Typical Persistent Heavy Rainfall During Pre-flood Season In Fujian

In this paper,by using the NCEP/NCAR reanalysis daily data and daily railfall data from66stations in FuJian, after surveying and analysing the typical persistent heavy rainfall processes during pre-flood season in FuJian,the multi-scale features of typical persistent heavy rainfall process during pre-flood season are revealed. The atmospheric low-frequency variation and its influence to the persistent heavy rainfall are studied. The atmospheric low-frequency physicial conceptal model for typical persistent heavy rainfall is summarized.According to the typical years,the characters of low-frequency signals in key area and their roles to the persistent heavy rainfall are in-depth analysed.The methods on how to extract key areas and their low-frequency signals for extension period forecast are introduced. The main conclusions are as follows:1、the frequency of persistent heavy rainfall processe has exhibited obvious multi-scale variability.(1) Both the frequency and the intensity of interdecadal and interannual variations of persistent heavy rainfall process are significant,1961-1975and1992-2010for high frequency periods and1976-1991for low frequency period.After1992,the persistent heavy rainfall tends to be stronger in intensity and longer in duration.(2) The inter-monthly change of persistent heavy rainfall process is significant too.About77.8%processes occurred in June especially in Mid-June. The persistent heavy rainfall processes that occurred in June are more strong and durable than occurred in May and July.(3)The daily precipitation in pre-flood season in Fujian has obvious characteristics of low-frequency variation, the significant cycle is quasi-bioweek(BWO) and about44-day (ISO).The correlation between total rainfall and intensity of low-frequency are positive in pre-flood season. Strong low-frequency oscillications are advantageous to typical persistent heavy rainfall when BWO and ISO are both in positive phase at the same time,the duration of persistent heavy rainfall is about15-20days at years when ISO stronger and5-10days when BWO stonger.2、The low-frequency physicial conceptal model for typical persistent heavy rainfall is established.(1)At200hpa(the upper troposphere),there is low-frequency Depression in Korean Peninsula,weak low-frequency High in western Tibetan plateau which indecates Northeast Cold Vortex and South Asia high are steady,then the Subtropical Westerly Jet-core is at east of Yangtze Dleta. Fujian is at the right to the entrance of the low-frequency jet-core which can trigger low-frequency divergency by strong positive vorticity advection and the continuous divergency for persistent heavy rainfall at upper troposphere is prepared.(2)At500hpa(the middle troposphere),there are four low-frequency perturbation situations in middle and high latitude as follows:double blocking high both strengthen,the Bajkal blocking high strengthen, the Okhotsk blocking high strengthen,the Ural block high strengthen.As there are low-frequency perturbations near Ural Moutain, Bajkal Like,and Okhotsk Sea,the blocking high and trough which are favor of heavy rainfall can be sustained and lead cold air frequently moving southwards to Fujian and the continuous cold air be prepared for persistent heavy rainfall.(3)At850hpa(the lower troposphere):There are two types for low-frequency perturbation in the stream and vort fields: South China anticyclone type and South China cyclone. In the both two types there is low-frequency cyclone in Fujian and then persistant air convergence at lower level that favor for persistent heavy rainfall can be formed.(4) Moisture condition:There are low-frequency perturbations in five main areas such as the southwest side of the Pacific Subtropical high,Somali cross-equatorial flow area,South China cross-equatorial flow area; the westerly zone and Bay of Bengal cross-equatorial flow area, especially in the first three areas,then there are vapour’s low-frequency transportation and convergence in Fujian, sufficient vapour for persistent heavy rainfall can be full provided.3、The characters of atmospheric low-frequency signal during persistent heavy rainfall process in pre-flood season.The typical persistent heavy rainfall in1998and2010are both accompanied by once obvious East Asia monsoon surge. There were significant northwardly and westwardly propagation for low-frequency convective system. During the, the configuration and change of the low-frequency system in key areas is very conducive to maintain the long period of heavy rain. The low-frequency systemes are as follows:the South Asia high、the Subtropical High、the blocking high in Ural Mountains、the East Asian trough and moisture transport from the Bay of Bengal and the South China Sea.The persistent heavy rainfall are closely related to ISO and BWO of the systems in key areas.4、The method of how to extract low-frequency key area and its low-frequency signalAs the cycle and the key area are different for every year, timely monitoring and master the variation discipline of the atmospheric low-frequency precursory signals has a very important significance for the extended range forecast of the persistent heavy rain in the future.By analyzing the typical cases,the steps on how to extract precursor low-frequency signals are got as following:After the first persistent rainfall occurred, at first,analyse the low-frequency disturbance graphics carefully and definite key areas in harmony with the process.The second, draw low-frequency-wave graphics of key areas and Fujian.For the third,as low-frequency system has significant persistent and periodic characteristics,extrapolateing the low-frequency-wave may provide basis for typical persistent heavy rainfall.