华南地区对流触发的高分辨率观测分析及物理机制

South China has high-frequency occurrence of severe convective weather, while it remains challenging to skillfully forecast the convection. The relatively low forecast skill is partly due to the limited understanding of the statistical characteristics and physical mechanisms of convection initiation (CI). The quality control for the radar mosaics has been significantly improved in China in recent years. Such a radar dataset can provide reliable precipitation echoes with a resolution of several kilometers and minutes, which greatly aids in the CI research. This project aims to reveal the climatological characteristics of CI in South China, and to investigate the typical CI mechanisms. To address these issues, a long-term CI dataset with a high spatio-temporal resolution will first be established by developing objective identification methods based on the historical radar mosaics. We further perform the constructed CI dataset to quantitatively describe the detailed observational features of CI events, including the spatial distribution and temporal evolution. Various of CI-active scenarios are diagnosed to investigate the impacts of the underlying surface and multi-scale atmospheric systems on CI by using the station observations and reanalysis data. A series of cloud-resolving numerical simulations are conducted to evaluate the proposed physical mechanisms of CI. Finally, this study summarizes the conceptual models of the typical CI events in South China. The results of this project will be helpful to improve the scientific understanding of the convective activity.

华南地区的强对流灾害频发且预报难度大。目前，对流触发的观测统计特征和机制认识尚不完善，一定程度制约了灾害性天气的预报预警能力。近年我国雷达拼图质控效果显著提升，可提供数公里和数分钟级分辨率的可靠回波信息，为研究对流触发活动规律提供支撑。由此，本项目以构建对流触发数据集为切入点，对华南地区对流触发的观测特征进行气候统计，并深入探讨影响对流触发的物理机制。主要内容包括：1）基于雷达拼图历史资料，发展对流触发的客观识别方法，建立长时间序列和高时空分辨率的对流触发数据集，进而揭示华南地区对流触发的详细空间分布和时间演变特征；2）结合站点观测和再分析资料，诊断分析对流触发活跃的多种类型，揭示其与下垫面强迫、多尺度天气系统等因素的联系机理，并进一步利用云分辨数值模拟来检验评估关键因素影响对流触发的物理机制，初步总结华南对流触发的主要概念模型。研究结果将有助于提高对强对流发生发展的科学认识。

中国南方地区的强对流天气频发且预报难度大。把握这些对流风暴何时何地启动（即对流触发），是成功预报后续强对流天气的关键。当前，华南对流触发的观测统计特征和机理认识尚不完善，一定程度制约了该地区灾害性天气预报预警能力的提高。近些年来，我国区域雷达拼图的质控效果显著提升，其数公里和数分钟级分辨率的可靠反射率信息，可为研究对流触发的特征规律提供支撑。本文基于华南雷达拼图历史资料（2013−2019年），利用图像处理技术和客观识别方法，构建了长时间序列和高时空分辨率的对流触发数据集；揭示了华南地区对流触发的时空分布和演变特征；阐述了该地区对流活动与季风气流、海陆风环流及海岸复杂地形的相互作用关系密切。主要成果如下：.（1）对流触发频发于距海岸线一百公里范围内，受海陆风环流影响显著，且在夏季风爆发后尤为活跃。海南岛与雷州半岛是对流触发的热点区域。此外，华南海岸地形附近也是对流触发的高频地区，如防城港、阳江等地。白天对流触发主要出现在海岸山脉迎风面，夜间对流触发则频发于这些海岸地形的临近海面，表明新生对流受到季风气流、海陆地形和局地环流的显著影响。海岸夜间对流触发表现出向海传播特征，且超前于海岸夜间降水的离岸传播现象2−3小时。.（2）基于该高分辨率对流触发数据集，本文进一步探讨了海南岛上的对流触发关键物理机制。研究发现，大尺度低层西南风的风速对海南岛的对流触发空间模态影响显著。在低层西南风较强的情形下，海岛山脊通过对背景气流的扰动，在午后的海岛下风向平原处激发出有组织化的边界层水平对流卷。这类辐合边界再与海陆热力差异造成的海风锋发生相互作用，进一步增强低层辐合抬升，使得海南岛东北部平原成为午后对流触发的热点区域。而在低风速情形，山地的热力作用和其北侧的海风锋入侵，是海南岛西南山区对流触发活跃的主要原因。本文对大尺度季风环流与局地环流、复杂地形等跨尺度作用机制的阐释，可能对华南区域强对流天气预报提供科学参考。