Relationship Between Heavy Rainfall Process In The Middle-lower Reaches Of The Yangtze River And Low-frequency Rainfall And Their Possible Mechanism

Using daily observed precipitation data from753stations of China from1981to2010, NCEP/NCAR daily reanalysis data, real-time weather map, weather report and hourly observed data from11automatic stations of Shanghai from2004-2012, heavy rainfall processes of different time scales (large-scale persistent heavy rainfall processes and short-time heavy rainfall) in the middle-lower Reaches of the Yangtze River are analyzed. According to the difference of low-frequency oscillation period or affecting system, we discuss the relationship between each type of heavy rainfall process and low-frequency rainfall, and their possible forming mechanisms, in order to provide reference for the extended period forecast of heavy rainfall process in the middle-lower Reaches of the Yangtze River based on low-frequency oscillation.The results suggest that:(1) Large-scale persistent heavy rainfall processes in the middle-lower Reaches of the Yangtze River have close relationship with low-frequency rainfall,62%of the heavy rainfall processes are at the peak stages of low-frequency rainfall. Heavy rainfall processes associated with10-30d low-frequency rainfall mainly occur in Meiyu flood period(Jun and July) and typhoon flood period(August and September), while Heavy rainfall processes associated with30-60d low-frequency rainfall are mainly centered on Meiyu flood period. At the different stages of flood period or different frequencies associated with low-frequency rainfall, the low-frequency affecting systems of heavy rainfall process have different evolution characteristics.(2) For the heavy rainfall process associated with10-30d low-frequency rainfall, allocation of low-frequency circulation is different between Meiyu flood period(Jun and July) and typhoon flood period(August and September). In the Meiyu flood period, by the joint effect of low-frequency system from high and low latitudes, middle-lower Reaches of the Yangtze River form the distribution of "south high north low" in LFO geo-potential height field, which is in favor of south and north low-frequency air convergence. While in the typhoon flood period, the heavy rainfall processes are mainly affected by tropical low-frequency systems. LFO wave train from the tropical west Pacific to the Japan Sea moves towards northwestwards, contributing to the severe convergence and resulting in the heavy rainfall process. In addition, no matter Meiyu and typhoon flood period, Positive anomalous convection over east of Philippines and Indian ocean key region can be used as previous tropical signals of the heavy rainfall process, which is in advance of rainfall peak phase about10days.(3) The heavy rainfall processes associated with30-60d low-frequency rainfall are mainly caused by the convergence of low-frequency geo-potential height low value system from northeast Asia and SCS. The meridional distribution of "north-south high, middle low" in low-frequency geo-potential height and two reversed low-frequency vertical circulation are in favor of severe low-frequency air convergence and the development of upward movement. According to the advanced low-frequency signal, we construct the heavy rainfall process forecast index, HRPI(heavy rainfall process index)=ζ’*NEA-ζ’*YR-OLR’*S-WP. In the significant years of30-60d oscillation, if HRPI achieve the maximum, and HRPI is greater than or equal to4, it is easy to occur heavy rainfall process10-25d later.(4) Using the forecast results of CFSv2(Climate Forcast System Version2) further verify the relationship between heavy rainfall process and low-frequency rainfall, and taking two heavy rainfall processes occurred in the middle-lower Reaches of the Yangtze River in2013summer for example,evaluate the forecast results of the model. CFSv2has certain prediction ability to the low-frequency oscillation characteristic of heavy rainfall process in the middle-lower Reaches of the Yangtze River. The forecasting techniques of low-frequency circulation are better than that of low-frequency rainfall. Preliminary studies suggest that, period of validity of low-frequency circulation is up to15to20days, while Period of validity of low-frequency rainfall is only about10days. (5) Except for the Large-scale persistent heavy rainfall process, shorted-time heavy rainfall such as Shanghai’s also has some low-frequency oscillation characteristics. Its significance has relationship with affecting system. Short-time heavy rainfall caused by quasi-stationary front or tropical cyclone has more significant low-frequency oscillation characteristics, and its low-frequency circulation field has advanced signal about10days earlier, While the low-frequency oscillation characteristics of short-time heavy rainfall caused by other affecting systems is not obvious.