渤海湾地区碰撞型海风锋的统计特征和对流触发机理研究

Sea-breeze front (SBF) is one of the atmospheric boundary layer systems closely related to the severe convective weather in the coastal region. Among all the kinds of SBF, the merger-type SBF can more easily trigger new convective systems, or intensify the original convective systems that merge with the SBFs. Consequently, it may threaten the safety of economic constructions and people’s life and property. However, previous studies of merger-type SBF in the Bohai Bay region were almost limited to case studies. The statistical characteristics of merger-type SBF and the corresponding convection initiation mechanisms are thus not well recognized. As such, this project first studies the statistical features of the merger-type SBFs occurred in recent 10 years in the Bohai Bay region, based on the cluster analysis, mathematical statistics and composite analysis. Then, the characteristics of the merger-type SBFs that initiate or enhance the convective systems merged with them are further investigated. Finally, numerical simulations will be performed for typical merger-type SBF cases that trigger new convective systems, to investigate the convection initiation mechanisms. The goal of this project is to get insights into the general statistical characteristics of the merger-type SBFs in the Bohai Bay region, and their favorable environmental conditions for the initiation and enhancement of convective systems. It can help lay a scientific foundation for the prediction and further study of severe convective weather triggered by the merger-type SBFs in this region.

海风锋是与沿海地区强对流天气密切相关的大气边界层系统之一，其中碰撞型海风锋更容易导致触发或增强对流系统，从而给沿海地区经济建设和人民生命财产安全带来重要影响。由于前人对渤海湾地区碰撞型海风锋的研究几乎都局限于较少的个例样本，目前人们对该地区碰撞型海风锋及其导致的对流触发或增强的统计特征和对流触发机理的认识不足。据此，本项目采用聚类分析、数理统计和合成分析等方法，首先对该地区近10年的碰撞型海风锋各种特征进行统计和归纳；其次，针对触发或增强局地强对流天气的个例特征进行进一步探析；最后，对于触发对流的典型海风锋个例进行数值模拟，通过敏感性实验和诊断分析，揭示其对流触发机理。通过上述研究，认识该地区碰撞型海风锋的普遍特征，获取该地区碰撞型海风锋触发或增强对流系统的有利环境条件，揭示典型对流触发机理，为该地区局地强对流天气的深入研究奠定一定理论基础，并为其预报提供科学思路。

利用2009~2018年5~9月的天津多普勒天气雷达观测资料和渤海湾地区地面自动站观测资料识别出的碰撞型海风锋个例（与阵风锋和对流系统碰撞）约为240个，其中与阵风锋碰撞的个例约为163个，与对流系统碰撞的个例约为57个。总体而言，碰撞型海风锋碰撞位置主要分布在天津雷达中心北侧和西侧区域，其中在西北方向区域的个例最多，南侧区域很少。根据0.2°×0.2°的方形区域内出现的总碰撞位置频次情况大致可以识别出几个碰撞高频区，其中最强的碰撞高频区出现在天津市中心附近约20~40 km区域范围内。相对较弱的一个碰撞高频区出现在唐山市和廊坊市附近约20~40 km范围内，而且其北侧和西侧偏多一些。在天津市附近出现的海风锋与对流系统发生碰撞的位置较多其密集，而其他区域分布较零散。通过一次典型对流触发个例研究发现，对流触发主要是由于动力强迫占主导作用提供垂直加速度所导致，并且在对流触发前大约10分钟时间内的空气粒子快速抬升过程中，动力加速度中的垂直扭转作用起到决定性作用。在前沿的向前倾斜的上升气流和来自前方的另一支气流在阵风锋上方附近辐合并受到垂直风切变的影响形成了垂直扭转现象。