基于雷达观测的华南前汛期海岸对流日变化研究

The southern coastal region is one of the torrential rainfall centers in China. Affected by the East Asian summer monsoon, severe convection and heavy rainfall frequently occur over the region and often lead to huge social and economic losses. Due to the unique synoptic weather patterns and the complex underlying land surface and orography, the diurnal variations of precipitation over the region is different from the classic oceanic or continental ones, and is extremely difficult to forecast. Based on long-term radar network observations, this project aims to investigate the the diurnal variations of convection over the southern coastal region of China, and to unveil the unique characteristics of diurnal cycles of convection in different sub-regions before and after the onset of the East Asian summer monsoon. Furthermore, advanced radar data assimilation and analysis and numerical simulations will be utilized to investigate key factors including land/sea breezes, mountain-plain solenoid circulations, the summer monsoon environment, and the prevailing synoptic winds. Their individual roles and combined interactions across different spatial-temporal scales will be examined to reveal the physical mechanisms behind their influences on the diurnal cycles of convection. This study will advance the understanding of the physical mechanisms related to coastal convection in South China, and improve quantitative precipitation forecast over the region.

华南海岸地区是我国的暴雨中心。受东亚夏季风影响，每年华南前汛期，海岸区强对流和强降水频发，导致巨大社会经济损失。由于该区域天气背景独特，下垫面和地形复杂，对流和强降水存在不同于海洋和大陆的日变化特征，给预报造成了极大的困难。本项目将利用多年华南天气雷达组网观测，从雷达统计角度开展华南海岸地区对流的日变化特征研究，揭示季风爆发前与爆发后，不同沿岸地形区域的对流日变化特征差异。在观测基础上，结合先进的雷达同化分析系统和数值模拟，深入研究季风爆发前与爆发后，不同盛行风条件下，海陆差异和海岸地形强迫的海陆风、山谷风环流等局地环流对海岸对流日变化影响的物理机制。此研究对入理解华南海岸强降水带的形成和提高定量降水预报能力，具有重要的科学意义和应用价值。

本项目按照计划，利用华南沿岸业务雷达网观测资料，以及自动站与卫星降水产品融合数据等，研究了东亚夏季风背景下华南前汛期海岸区对流和强降水的日变化特征及形成机制。首先，通过客观天气分型方法，识别了影响华南海岸地区暖季对流和强降水的三种主导天气环流型（强、弱西南季风型和低涡型），揭示了不同天气环流型下强降水的空间分布和日变化特征差异，发现低层向岸风的日变化是影响华南海岸区对流和强降水的关键因子，其与沿岸地形、海岸线和海陆风的相互作用过程随天气环流型变化。其次，结合雷达四维变分反演典型个例的动力和热力场，揭示了强、弱西南季风背景下珠三角地形区对流的日变化特征差异，提出了惯性振荡影响下增强的背景西南风、多尺度地形阻挡形成的偏东风和热力驱动的偏北下坡风辐合抬升触发夜间对流的动力机制。第三，基于雷达统计和双偏振雷达反演，揭示了海风环流影响下海岸区飑线的时空分布和演变特征，阐明了海风锋触发对流与飑线合并、组织化的热动力和微物理相互作用过程。最后，通过半理想数值模拟，再现了珠三角城市化导致强降水频次增加和峰值时刻延迟等特征，提出城市热岛是主要的影响因子，其通过增强海风环流和产生城市区低层非对称入流，减缓海风传播速度，并改变降水位置与峰值时间。这些研究成果加深了对华南海岸对流和降水日变化特征及形成机制的认识，为提高该区域定量降水预报能力建立了基础。