定量诊断华南春旱的形成机理

华南地处气候年际变化很大的东亚季风区, 近年来秋冬春干旱灾害频繁, 尤其自20世纪90年代末以来的干旱灾害给华南工农业生产和人民日常生活造成严重的影响。华南地区春季降水多是偏南暖湿气流在华南抬升的结果, 为了全面而客观地查找华南春旱的主要成因, 本研究从建立在流体所有守恒定律的华南 (局地纬向平均) 经向环流数学模式入手, 在成功地模拟了导致华南春旱的华南地区(105°E～120°E纬向平均)经向环流后, 定量地诊断分析了各动力热力因子对该经向环流的贡献。结果表明, 激发华南地区下沉和30°N附近上升的距平经向环流的因子主要有: 反映位置偏北的锋面对流活动的潜热加热(由其激发的经向环流下沉支对华南地区总下沉运动贡献占56％)、反映西风带扰动的经向温度平流(占25％)和反映西风急流的纬向西风动量平流(占20％）。为了了解使锋面位置偏北的大气环流统计特征, 本研究还分析了华南春旱年(涝年)500 hPa位势高度距平(相对于1980～1999年平均)和华南春旱严重的1991年旱期(雨期)500 hPa位势高度场, 发现华南春旱(涝)时乌拉尔山阻塞高压偏强(弱）, 副热带高压偏西偏北 (偏东偏南）。对华南春旱严重的1991年乌拉尔山阻高与500 hPa位势高度距平场作相关分析发现, 西太平洋副高变化滞后乌拉尔山阻高变化2～6天。

A Quantitative Diagnosis for the Mechanisms of Spring Droughts in South China

Since spring droughts in South China(SC) are mainly characterized by the absence of uplifting of moist southerly,it is necessary to carry out an overall quantitative investigation for the forced-local-meridional circulation over SC.With a complete(including dynamic and thermodynamic mechanisms) linear-diagnostic model for the local-meridional circulation and the NCEP/NCAR reanalysis data, the present numerical study show that the contributions to the lower layer subsidence in SC are mainly from the forcing processes associated with the latent heat in the baroclinic troughs located to the north of SC(the compensatory subsidence accounting for 56% of the lower-layer divergence in SC),meridional temperature advection(25%) and the zonal advection of zonal momentum(20%).All of them point to the abnormal northward shift of the baroclinic troughs embedded in the general circulation characterized by the presence of blocking highs around the Ural Mountains and the sinificant northwestward extension of the western Pacific subtropical high(WPSH) for the composite spring droughts in SC.For an extrem dry case(i.e.the 1991 spring drought in SC),the variation of the Ural high was 2-6 days prior to that of the WPSH.