THE RELATIONSHIP BETWEEN LARGE-SCALE PERSISTENT HEAVY RAINFALL PROCESSES AND 30-60 DAY LOW-FREQUENCY RAINFALL AND ITS FORECASTING INDEX
Low-frequency oscillation has been employed effectively to develop extended range forecast.Using daily observed precipitation data from 753 stations of China from 1981 to 2010,NCEP/NCAR daily reanalysis data and real-time weather maps,we analyzed the relationship between large-scale persistent heavy rainfall processes and 30 ～ 60 d low-frequency rainfall.A Heavy rainfall process index (HRPI) is also constructed based on the precedent low-frequency signal.The results have the following suggestions:(1) The heavy rainfall process (HRP) has close relationship with 30 ～ 60 d low-frequency rainfall,which contributes greatly to the HRP.In the years of significant 30 ～ 60 d oscillation low-frequency rainfall,HRP all occurs at their peak stages but only 56％ of the peak stages have HRP.(2) When HRP happened at the peak stage of low-frequency rainfall,low-value areas of low-frequency geopotential height from northeast Asia and South China Sea converge in the middle-lower reaches of the Yangtze River so that the distribution of low-frequency geopotential height being higher in the north and south than in the middle is in favor of severe low-frequency air convergence,formation of two reversed low-frequency vertical circulation,and development of upward movement,thus causing HRP.When the peak stage of low-frequency rainfall is not with HRP,cold air from the north is relatively weak,low-frequency systems at high latitudes are abnormally northward.The middle-lower reaches of the Yangtze River are characterized by the distribution of low-frequency geopotential height being higher in the south than in the north and having one single vertical circulation cell,which is not good for low-frequency air convergence.(3) HRPI is constructed based on precedent low-frequency signals from high,middle and low latitudes,which has value of guidance for the extend-period forecasting of heavy rainfall processes in the middle-lower reaches of the Yangtze River.
heavy rainfall process30 ～ 60 daylow-frequency oscillationforecasting indexmiddle-lower reaches of the Yangtze River
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