中文摘要：

利用NCEP/NCAR再分析资料,用拟牛顿法通过菱形15波截断的谱方法对无强迫无耗散的正压涡度方程进行数值求解,得到1987—2006年共20a夏季的逐日定常自由模。由此诊断分析得到,自由模态中副高的第一次北跳日期一般超前于实际流场中副高的第一次北跳日期,而且超前于实际的入梅日期;丰梅年,自由模态中副高的第一次北跳比实际副高的第一次北跳平均提前9.3d,比实际入梅日期提前8～14d,平均提前10.3d;枯梅年,自由模态中副高的第一次北跳比实际副高的第一次北跳平均提前2d,比实际入梅日期提前2～5d,平均提前3.6d。这对长江中下游地区的梅雨预报有较好的参考作用。

英文摘要：

Based on NCEP/NCAR reanalysis data,by using 15-wave rhomboidal truncation spectral model procedure and numerically solving the unforced and inviscid nonlinear barotropic vorticity equation in quasi-Newton method,the daily steady free modes from 1987 to 2006 are obtained.The results show that the first northward jump of the western Pacific subtropical high（WPSH） in the free modes is generally ahead of that in observation,and is ahead of the beginning date of Meiyu consequently.In plentiful Meiyu years,the first northward jumps of WPSH in the free modes are averagely 9.3 days ahead of those in observation,8—14 days ahead of the beginning dates of Meiyu,10.3 days ahead averagely.In dry Meiyu years,the first northward jumps of WPSH in the free modes are averagely 2 days ahead of those in observation,2—5 days ahead of the beginning dates of Meiyu,3.6 days ahead averagely.It is helpful for the forecast of the onset of Meiyu over the mid-lower reaches of the Yangtze River.