中文摘要：

研究低频振荡是目前开展强降水过程延伸期预报的有效途径。利用1981—2010年中国753站逐日的降水观测资料、NCEP/NCAR第二套再分析资料及实况天气图等资料，分析大范围持续性强降水过程与30～60 d低频降水的联系，并根据前期低频信号构造强降水过程预报指数。研究表明，（1） 大范围持续性强降水过程与低频降水紧密相联，低频降水对强降水过程有显著贡献。在30～60 d低频降水显著年，强降水过程均发生在低频降水峰值阶段；但对于低频降水而言，仅有56%的峰值阶段发生强降水过程。（2） 当低频降水峰值阶段发生强降水过程时，来自东北亚和南海的低频位势高度低值系统在长江中下游汇合，“南北高、中间低”的低频位势高度分布有利于低频气流强烈辐合，并在经向上形成两个完整的反向低频垂直环流圈，促进了上升运动发展，导致强降水过程发生；而对于低频降水峰值未发生强降水过程的情况，北方冷空气南下较弱，高纬度低频影响系统位置偏北，长江中下游附近表现为“南高北低”的低频位势高度分布和单圈垂直环流，不利于低频气流强烈辐合。（3） 综合高、中、低纬的前期低频信号构造了强降水过程预报指数，对延伸期（10～25 d）长江中下游大范围强降水过程预报具有参考价值。

英文摘要：

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.