湖南省汛期强降水的30~60d低频振荡特征及延伸期预报指数研究

基于1986—2015年湖南逐日降水资料、同期美国气象环境预报中心(NCEP)和美国国家大气研究中心(NCAR)再分析资料,通过分析强低频振荡年的汛期强降水特征和低频环流场演变对强降水的影响,建立了湖南省汛期延伸期强降水过程预报指数。结果表明:(1)汛期33%的强降水过程均发生在具有显著30~60 d低频振荡的年份中,且大多位于低频降水峰值阶段。(2)通过对强低频振荡年进行合成发现,在活跃位相,南亚高压偏强偏东,副热带高压偏西偏强,这种环流配置导致中国南方大部分地区的高层环流为辐散,底层环流为辐合,有利于降水的产生。在中断位相,南亚高压呈东西带状分布且其位置偏西、强度偏弱,副热带高压偏东偏弱,使得向湖南地区输送水汽的西南气流减弱,进入降水中断期。(3)基于低频散度场不同位相的变化特征,选取了与低频降水相关的两个关键区,从而建立延伸期预报指数,该指数对低频降水显著年的强降水回报准确率能够达到73%。(4)前期4月黑潮的海温异常(SSTA)可作为湖南省强低频振荡年的预测指标。

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.