两类低空急流对华南前汛期暴雨的影响机制

Heavy rainfall often occurs in South China during the pre-summer rainy season and is hard to predict. Low-level jet (LLJ) is one of the most important factors on the heavy rainfall. It is of great scientific significance and social value to study the impact of the LLJ on the occurrence and development of heavy rainfall in South China and the underlying mechanisms. Recent studies have indicated that LLJs could be classified into two types: a boundary layer jet and a synoptic-weather-related LLJ, which exhibit different characteristics and formation mechanisms. The effects of the two types of LLJ on heavy rainfall have not been well understood until now, and need to be further studied. The innovative perspective of this project is the distinction between two types of LLJs. Based on the intensive observations and numerical simulations, the project will explore the fine structure of the two types of LLJ and their impacts on the distribution of heavy rainfall in South China, and further clarify the relevant formation and development mechanisms of heavy rainfall by statistical analysis, dynamic diagnosis and ensemble forecasting. The main research contents include: 1) the relationship between the two types of LLJs and precipitation in spatial distribution and diurnal variation, 2) the interactions between LLJ and terrain, cold pool, frontal systems and gravity waves, and their contributions to the triggering and maintaining of severe convection, 3) the forecasting sensitivities of heavy rainfall on LLJs, and associated key factors.

前汛期华南暴雨频发且预报难度大，低空急流是造成暴雨的重要因素。研究低空急流对暴雨发生发展的影响机制，具有重大科学意义和社会价值。近期研究表明华南存在两类低空急流：边界层急流和与天气系统相关的低空急流，它们具有不同的时空特征和形成原因。目前，这两类急流分别影响华南暴雨的机理尚不清楚，有待深入研究。本项目从区分两类低空急流的新视角出发，基于高分辨率观测数据和数值模式，利用统计分析、动力诊断、集合预报等手段，深入探讨两类急流的精细结构及其对华南暴雨的影响，进一步阐明相关的对流发展机制和关键因素。主要内容包括：1）从统计角度，研究两类急流与降水在空间分布和日变化等方面的对应关系；2）基于云分辨数值模式，研究两类急流的时空配置及其与海陆、地形、冷池等下垫面强迫、锋面系统和重力波的相互作用，揭示它们对暴雨相关的强对流触发和维持的贡献；3）通过集合预报，分析低空急流影响暴雨预报的敏感性及关键因子。

低空急流是影响华南前汛期暴雨发生发展的关键因子之一。华南地区存在两类低空急流：边界层急流和与天气系统相关的低空急流。由于低空急流具有多样性和复杂性，进一步加大了华南前汛期暴雨的预报难度。本项目以区分两类低空急流对华南暴雨的影响机制异同为创新特色，深入探讨两类急流的精细结构和耦合作用，从而厘清了华南低空急流对暴雨影响的区域特色，取得的主要成果如下：（1）提出了“双低空急流耦合”触发华南暖区对流的新概念模型，对华南海岸暖区暴雨的预报具有提供重要理论支撑。（2）对比了两类低空急流的统计特征及其对降水影响的异同，发现急流通过地形作用、天气尺度强迫和水汽输送来影响降水分布（3）建立了1998-2019年华南暖区和锋面两类暴雨和相关两类低空急流的数据集，首度从统计的角度来揭示了不同类型的华南暴雨与不同类型低空急流在日变化、发生频率、空间分布以及相对位置等方面的联系。（4）揭示了南海北部大气边界层急流的形成机制，并进一步阐明了不同强度的南海边界层急流对华南降水影响的非线性关系。（5）定量揭示了南海边界层急流、海陆差异、沿海地形、冷池等不同尺度因子的相对贡献，及其它们耦合作用对华南海岸夜间对流触发和维持的多尺度影响机制。（6）发现模式中不同边界层方案能显著影响南海边界层急流的模拟强度，过强或过弱的急流都不利于沿海暖区暴雨的模拟与预报。（7）揭示了低空急流影响下低层风切形成的波导和wave-CISK机制是造成华南波状带状对流的主要机制。（8）建立考虑背景风的海陆风理论模型，提出低空急流调制重力波传播是影响沿海降水日变化的关键动力过程。项目资助发表论文24篇，均发表在Nature Communications, Monthly Weather Review, Journal of the Atmospheric Sciences, Journal of Geophysical Research和Journal of Climate等国际权威期刊上，多个关键成果入选ESI高被引论文或期刊封面文章，获广东省气象科技杰出青年奖。