Application Of Low-frequency Variation To Range-extended Forecast For Severe Rainfall Process Of Hunan In Rainy Season

Using the NCEP/NCAR reanalysis daily date and the daily rainfall data from97stations in Hunan, basing on the band-pass filter, EOF analysis and low-frequencyweather map method, we study on the rainy season precipitation features in Hunan,theactive characteristics of the low-frequency system and its impacts on the severe rainstormprocess, then develop a model to forecast the range-extended severe rainfall process ofHunan in rainy season. The main results are as follows:(1) Three models are presented in rainy season precipitation-consistency inprovincial range, north to south and both sides to the center. There are64rainfall process，19stronger rainfall process which account for29.7%. The rainfall process, mostly fromnorth to south, last3or4days, sometimes up to9days.(2) There are7key regions affecting rainfall process in Hunan rainy season. The firstis from the east of Ural mountains to Lake Baikal. The second is from Xinjiang to the northof Tibet Plateau. The third is from Hetao to North China. The forth is from the south ofTibet Plateau to the bay of Bengal. The fifth is from the Yunnan-Guizhou Plateau to SouthChina and south of the Yangtze River district. The sixth is the Western Pacific region. Theseventh is the south China sea region.(3) The characteristics and evolvement of the low-frequency system in key regions areshown below. In the horizontal direction, the low-frequency cyclone and anticyclonepresent in turns，and there are two teleconnection horizontal wave in positive and negativechange. In the vertical direction, the low-frequency atmosphere in the key region4th and6th are baroclinicity, while others are barotropy. The low-frequency atmosphere in region1th,4th,6th and7th varied periodically at30~35days, while others varied periodically at20~25days. The obvious tracks of low-frequency system are east or southeast in region1th,east or northeast in region4th, west or northwest in region6th, and stationary in other regions.(4) The rainfall process are poorly related to low-frequency wave at500hPa butapparently related to low-frequency cyclone (anticyclone) at700hPa. The rainfall mayhappen when at least a low-frequency cyclone occur in region1th,2th,3th and7th, at leasta anticyclone occur in region5th and6th, a anticyclone or cyclone in region4th andlow-frequency wave go down in region1th. The forecast flow is listed as below. Confirmthe period based on the current low frequency system in the seven regions. Get the lowfrequency system in extended-range according to period extrapolation. Make forecast if thelow frequency system in extended-range is match to rainfall. (5) Use the rainy season rainfall data from2006to2010to validate the forecast model.The average fitting rate is64.4%in April,54.9%in May and50.7%in June. Among theforecast results of10d,15d,20d,25d and30d, the fitting rate is highest when it forecasts30days in advance. Take the rainy season rainfall data in2011and2012as independentsample to test the forecast model, the result shows that the accuracy rate reaches70%in2011but56%in2012. The proportion of empty quote is high.