The feature of 30-60 day low frequency oscillation of heavy rainfall in the flood season of Hunan Province and Extended-Range Forecast index

Based on the daily precipitation data in Hunan Province from 1986 to 2015, reanalysis data of NCEP and NCAR in the same period, the prediction index of heavy precipitation in extended period of flood season in Hunan Province is established by analyzing the characteristics of heavy precipitation in flood season in strong low frequency oscillation year and the influence of low frequency circulation flow field evolution on heavy precipitation. The results are as follows. (1) 33% of the heavy rainfall in the flood season occurs in the years with significant 30-60 d low-frequency oscillation, and most of them are in the peak stage of low-frequency precipitation. (2) It is found that in the active phase, the South Asia high is strong to the East and the subtropical high is strong to the West. This circulation configuration leads to the divergence of the high-level circulation and the convergence of the bottom circulation in most areas of South China, which is conducive to the generation of precipitation. In the phase of interruption, the South Asia high is in the east-west zonal distribution, and its location is to the west and intensity is weak. The subtropical high is to the east and weak, which makes the southwest air flow of water vapor to Hunan area weaken and enter the period of precipitation interruption. (3) Based on the characteristics of different phases of low-frequency divergence field, two key areas related to low-frequency precipitation are selected to establish the extended period forecast index, which can achieve an accuracy of 73% for the return of heavy precipitation in the significant years of low-frequency precipitation. (4) The SSTA of Kuroshio in early April can be used as a prediction index of strong and low frequency oscillation years in Hunan Province.