南海夏季风爆发前后华南前汛期降水年际变异的特征和成因研究

The first rainy season (FRS), also known as the pre-summer rainy season, is the most important rainy season for South China. The accumulated precipitation during FRS accounts for approximately 40% of the annual mean over South China. The FRS precipitation shows clear interannual variation and often causes severe floods or droughts. Therefore, it is an important issue to reveal the variations and mechanism of the FRS precipitation over South China not only for the atmospheric science community, but also for the disaster prevention community. The onset of the South China Sea summer monsoon (SCSSM) usually occurs during the FRS. Before the onset of the SCSSM, the FRS precipitation is mainly caused by mid-latitude front and characterized by stable stratification. After the onset of the SCSSM, in contrast, the FRS precipitation is mainly caused by deep convection and characterized by unstable stratification. The distinct different nature of the two types of precipitation suggests that it is very likely unsuitable to regard the precipitation during the FRS as a whole and to investigate its interannual variations. Meanwhile, the start time and end time of the FRS also vary year by year, so it is not suitable to use a fixed time period of the year to represent the FRS. In this project, the FRS will be defined as the period between the start and end date of the FRS based on daily data. In addition, the FRS will be divided into two sub-periods according to the onset date of the SCSSM, i.e., the frontal precipitation period (FPP) before the onset of the SCSSM and the monsoonal precipitation period (MPP) after the onset of the SCSSM. After that, the interannual variations of the precipitation during FPP and MPP, their linkages, and the involved mechanism will be investigated based on data analysis, dynamical diagnostics, and numerical experiments. Especially, the roles of atmospheric dynamical processes and the atmospheric external forcing will be examined. It is expected that the characteristics and mechanism of the interannual variations of the FRS precipitation can be revealed after the abovementioned studies under the support of this project.

华南前汛期是华南地区最重要的雨季，期间降水量可达全年的40%并存在显著的年际变化，常引发严重的洪涝灾害。揭示华南前汛期降水发生的规律和机制不仅对认识我国汛期降水的变异具有重要科学意义，而且对防灾减灾具有重要实际意义。华南前汛期期间南海夏季风会爆发，前汛期的降水在季风爆发前主要为锋面性质，而在季风爆发后主要为季风性质，二者有显著差异。因此，将华南前汛期的降水作为一个整体来进行研究可能会掩盖其年际变异的真正成因。同时，前汛期每年的开始和结束日期也存在变化，因此不宜用固定的时间段来代表前汛期。本项目拟利用日分辨率的资料，在考虑华南前汛期开始和结束日期的基础上，把前汛期分为南海夏季风爆发前的锋面降水阶段和爆发后的夏季风降水阶段，通过统计分析、动力诊断和数值模拟，揭示这两个阶段中各自降水的年际变化特征、大气内动力过程、大气外强迫因子以及它们之间的联系，从而深刻揭示华南前汛期降水年际变异的特征和成因。

华南前汛期是每年我国汛期开始之后的第一个雨季，也是华南地区最重要的雨季，前汛期的降水变异经常会引发严重的旱涝灾害。前汛期期间南海夏季风会爆发，前汛期降水在季风爆发之前和之后的性质有本质差异，以往工作将前汛期作为一个整体来研究有可能掩盖前汛期降水年际变异的真正成因。同时，每年前汛期的起止日期也存在明显变化，用固定的时间段来代表前汛期也是不准确的。因此本项目利用逐日分辨率的资料，将每年的华南前汛期划分为南海夏季风爆发之前和之后两个阶段，并通过资料分析、动力诊断等方法，揭示出前汛期两阶段降水年际变异的时空特征、关键影响系统、大气内部动力过程和外强迫因子等方面都存在明显的差异，主要研究结论可概括为以下三方面：（1）平均而言，第2阶段持续性强降水过程发生的频次明显高于第1阶段，且两阶段降水量的年际变异均与该阶段持续性强降水过程发生次数的年际变异密切相关；（2）第1阶段降水的年际变异主要受到高原南侧西风、东亚中纬度经向风和西太平洋副高等多个环流系统的共同影响，上述关键环流系统通过改变华南地区大气涡度平流的垂直分布（动力作用），触发降水与垂直运动之间的正反馈作用，有利于持续性强降水过程的发生，从而导致该阶段降水总体偏多，而第2阶段降水主要受到西南季风强弱和副高位置的影响，上述环流异常会通过影响华南地区的西南风改变局地温度平流的垂直分布（热力作用），触发降水与垂直运动之间的正反馈作用，有利于第2阶段持续性强降水过程的发生和降水量的增加；（3）热带太平洋东西海温差对第1阶段降水的年际变异具有显著影响，而对第2阶段降水的影响并不显著。研究结果对于深入认识华南前汛期降水年际变异的特征和机理、对于认识我国主雨带变异的特征和成因具有重要的科学意义，对于提高华南前汛期降水的预测水平和国家防灾减灾具有重要的社会意义。