季风槽环境中暴雨中尺度对流系统的分析与数值预报试验

应用地面降水观测资料、卫星云图、雷达回波以及NCEP再分析资料，对华南沿海受季风槽影响下发生的一次持续性暴雨的中尺度对流系统（MCS）进行分析，并探讨采用数值模式对中尺度对流系统降水进行预报的可能性。分析表明，暴雨由多个相继发展的中尺度对流系统造成。在相似环境中，不同中尺度对流系统发展形态和水平尺度有较大差异，最大可组织发展成α中尺度对流复合体（MCC），但一般为β中尺度线状或带状对流系统。对其中发展形态分别表现为椭圆形中尺度对流复合体（MCS-2）和带状β中尺度对流系统（MCS-4）的对比分析发现，对流的起始发展均发生在夜间，与季风槽中低空急流的南风脉动有良好对应关系。基于临近探空资料的诊断发现，被认为对中尺度对流系统组织发展有指示作用的关键物理量如对流有效位能（CAPE）和风垂直切变难以区分不同中尺度对流系统的发展形态和趋势，探空资料的代表性将影响诸如“配料法”等暴雨客观预报方法的建立和应用。利用华南区域中心GRAPES（GRAPES_GZ）数值模式对两个中尺度对流系统进行的模拟预报结果表明，采用数值模式对中尺度对流系统降水进行显式预报已成为可能。比较而言，3 km水平分辨率模式可以更好地预报出暴雨的发生，但结果对是否调用对流参数化（CP）方案敏感。尽管不依靠对流参数化方案模式能够较好地预报出中尺度对流系统初始降水的发生，但会过度预报发展成熟后的降水。模式中如何描述中尺度对流系统对流的组织发展机制、如何处理对流参数化方案的“灰色区分辨率”问题需要仔细考虑。 

Analysis and numerical prediction experiment of rainstorm-producing MCSs in a monsoon trough environment

Using ground based precipitation data, satellite cloud images, radar echoes and NCEP reanalysis data, this study analyzes characteristics of MCSs(Mesoscale Convection Systems)associated with a continuous rainstorm that occurred over the coastal areas of South China within a monsoon trough, and investigates the possibility of numerical prediction of the MCSs precipitation. The analysis shows that the rainstorm was caused by a number of successive MCSs. In similar environments, these MCSs developed into different modes with different horizontal scales. The maximum could be organized into a meso-α scale MCC, while meso-β scale linear or banded convective systems were more common. Comparative analysis of two MCSs that respectively presented as quasi-circular MCC (MCS-2) and banded MCS (MCS-4) reveals that convection in both of the two MCSs began to develop at the nighttime, showing a good correlation with fluctuations in the southerly low-level jet in the monsoon trough. Examination of the observed proximity soundings shows that it is difficult to distinguish the modes and the evolutionary tendencies of different MCSs using diagnostic variables such as convective available potential energy (CAPE) and vertical wind shear, which are considered to be good precursors for MCSs precipitation prediction. The representativeness of soundings used to derive these quantities will affect the application of various forecasting tools such as that based on the "ingredients-based method". Prediction simulation with the GRAPES_GZ model shows that it is possible to explicitly predict MCSs precipitation by the numerical model. In comparison, the 3 km resolution run can better predict the occurrence of MCSs-induced rainstorm, but are sensitive to the option with or without CP (convective parameterization) scheme. Though running without the CP scheme could better predict earlier stage rainfall in both MCSs, it overpredicted precipitation as the MCSs evolved into their mature stages. How to describe mechanisms for organized convection in MCSs and how to deal with problems of CP schemes at "grey-zone resolution" are two issues that need to be considered carefully.