The Analysis Of Relation Between Atmospheric Free Mode And Meiyu Over The Mid-Lower Reaches Of The Yangtze River

In this paper, firstly, the relationship between different levels of geopotential height fields, the previous winter SSTA and Meiyu is analyzed. Results show that, in plentiful Meiyu years, high latitude is double blocking type. The WPSH (western Pacific subtropical high) extends to west and appears strong, while the SAH (South Asian High) extends to east and appears strong. The previous winter SSTA is cold in west, warm in east. In dry Meiyu years, high latitude is single blocking type. The WPSH extends to east and appears weak, while the SAH extends to west and appears weak. The previous winter SSTA is warm in west, cold in east.In the second place, based on NCEP/NCAR reanalysis data, using 15-wave rhomboidal truncation spectral model procedure, and by numerically solving the unforced and inviscid nonlinear barotropic vorticity equation through the quasi-Newton method, the daily steady free modes of June and July from 1982 to 2006 are obtained. It is shown that the first northward jump of WPSH in the free modes is generally ahead of the beginning date of Meiyu, averagely 10.6 days ahead in plentiful Meiyu years, 3.8 days ahead in dry Meiyu years, and 6.8 days ahead in general years. The second northward jump of WPSH in the free modes is generally ahead of the beginning date of the ending Meiyu years, averagely 8.6 days ahead in plentiful Meiyu years, 4.2 days ahead in dry Meiyu years, 6.0 days ahead in general years. It is good for the forecast on Meiyu over the Mid-Lower Reaches of the Yangtze River.Finally, normal mode instability analysis is performed to examine observed stability of free modes. The results suggest that all the free modes are unstable. The frontal fastest growing free modes appear strong local character and low-frequency variation. The growing free modes which has longest unstable period is the leading role, the circulation pattern of which makes it maintain some time on observation.Therefore, we come to the conclusion that the free modes are respectively 3-14, 2-13days ahead of the beginning and the ending date of Meiyu. The growing free mode which has longest unstable period is of great reference to the observation circulation pattern.