Roles of Synoptic to Quasi-Monthly Disturbances in Generating Two Pre-Summer Heavy Rainfall Episodes over South China

Abstract

In this study, power spectral analysis and bandpass filtering of daily meteorological fields are performed to explore the roles of synoptic to quasi-monthly disturbances in influencing the generation of pre-summer heavy rainfall over South China. Two heavy rainfall episodes are selected during the months of April–June 2008–15, which represent the collaboration between the synoptic and quasi-biweekly disturbances and the synoptic and quasi-monthly disturbances, respectively. Results show that the first heavy rainfall episode takes place in a southwesterly anomalous flow associated with an anticyclonic anomaly over the South China Sea (SCS) at the quasi-biweekly scale with 15.1% variance contributions, and at the synoptic scale in a convergence zone between southwesterly and northeasterly anomalous flows associated with a southeastward-moving anticyclonic anomaly on the leeside of the Yungui Plateau and an eastward-propagating anticyclonic anomaly from higher latitudes with 35.2% variance contribution. In contrast, the second heavy rainfall episode takes place in southwest-to-westerly anomalies converging with northwest-to-westerly anomalies at the quasi-monthly scale with 23.2% variance contributions to the total rainfall variance, which are associated with an anticyclonic anomaly over the SCS and an eastward-propagating cyclonic anomaly over North China, respectively. At the synoptic scale, it occurs in south-to-southwesterly anomalies converging with a cyclonic anomaly on the downstream of the Yungui Plateau with 49.3% variance contributions. In both cases, the lower-tropospheric mean south-to-southwesterly flows provide ample moisture supply and potentially unstable conditions; it is the above synoptic, quasi-biweekly or quasi-monthly disturbances that determine the general period and distribution of persistent heavy rainfall over South China.

摘 要

本研究基于对 2008–15 年 4–6 月逐日气象要素场的能量谱分析以及带通滤波, 探讨了天气尺度至准月尺度扰动在华南前汛期强降水产生过程中的作用, 进而具体分析了天气尺度和准两周尺度扰动, 天气尺度和准月尺度扰动在两次强降水过程中的作用. 研究结果表明, 对第一次强降水过程而言, 准两周尺度扰动的贡献率为 15.1%, 降水主要发生在与南海反气旋异常相关的异常西南风中. 天气尺度扰动的贡献率为 35.2%, 降水主要处于异常西南风和东北风的辐合区, 其分别与位于云贵高原下风坡的异常反气旋向东南移动以及来自更高纬的异常反气旋东移有关. 与之不同的是, 对第二次强降水过程而言, 准月尺度扰动的贡献率为 23.2%, 降水主要发生在西西南异常与西西北异常的辐合区, 该异常分别与南海反气旋以及华北气旋东移有关. 在天气尺度上, 降水发生在西南偏南风异常与云贵高原下游的气旋性异常辐合区, 贡献率为 49.3%. 对两个个例而言, 对流层下层平均西南偏南气流提供了充足的水汽和潜在的不稳定条件; 但天气尺度、 准两周尺度以及准月尺度扰动综合决定了华南持续性强降水的大概持续时间和空间分布.