热带气旋过台湾岛后路径偏折与结构变化的机理研究

Previous statistical analysis indicate that about 68% of TCs making landfall over Taiwan Island would move across the Taiwan Strait and make their second landfall over southeastern coastal region of Mainland China. Because of the effect of the Central Mountain Range (CMR) over Taiwan Island, TCs after their departure from the island often experience track deflection, and dramatic structure and intensity changes, making a great challenge to accurate forecasts of their second landfall location and fine distribution of winds and rainfall. This is one of the press issues for disaster prevention and reduction. Previous studies have mainly focused on track and structure changes when TCs approach and make landfall over the island. The issue how track, structure and intensity change after their departure from the island has received few attentions. This project attempts to address this issue based on observational data analysis and numerical simulations. The study will focus on how changes in track, structure and intensity for TCs moving away from the island vary with the TC structure, intensity, and motion direction in their pre-landfall stage as well as the environmental steering flow. Special attention will be given to the roles of the large-scale circulation, the mesoscale mountains, and the TC vortex in causing the different evolutions in asymmetric structure and steering flow and the uncertainty and predictability of their positions and structure and intensity during their post-landfall stage. Results from this project will improve our understanding of the dynamical and thermodynamic processes affecting the track deflection and structure and intensity changes for TCs departing from the Taiwan Island, and thus providing a theoretical basis and technical support to better forecast TC track and the fine distribution of winds and rainfall when a TC makes its second landfall over Mainland China.

据统计，登陆台湾的热带气旋（后称TC）有68%会穿越台湾海峡再次登陆我国大陆东南沿海。由于台湾中央山脉地形的影响，TC离岛后会出现路径偏折和结构及强度较大的变化，使得TC在大陆的二次登陆及风雨精细化预报难以把握，也成为防灾减灾决策的难点。以往的研究多集中在TC靠近和登陆台湾岛阶段，而对离岛后的路径和结构及强度变化未给与足够重视。本项目拟采用观测资料分析和数值模拟等手段，研究不同的环境引导气流情况下，不同结构（含强度和尺度）的TC从不同方位越过台湾岛后路径偏折和结构及强度的变化，重点研究在大尺度环流、中尺度地形和涡旋三者作用下，不同结构TC过岛后非对称结构及引导气流的演变及差异，及过岛后TC中心位置、结构不确定性造成的可预报性问题，揭示造成TC路径偏折和结构及强度变化的动力和热力过程，提高对岛屿地形影响下TC路径、结构和强度变化机理的认识，为过岛TC的路径和风雨精细化预报提供理论和技术支持。

由于台湾岛中央山脉的阻挡，过岛后台风的路径、强度和结构变化情况复杂，为预报带来挑战。本项目旨在考察热带气旋（后称TC）穿越台湾海峡再次登陆我国大陆的路径和强度变化的事实，研究造成路径偏折、强度和结构变化的机理，揭示造成TC路径偏折和结构及强度变化的动力和热力过程，提高对岛屿地形影响下TC路径、结构和强度变化机理的认识，为过岛TC的路径和风雨精细化预报提供理论和技术支持。经过四年的执行，本项目完成了预期目标，在以下三方面取得了重要进展：（1）统计分析发现过台湾岛后TC的路径大多会发生偏折，强度迅速衰减。三个业务中心（CMA，JMA和JTWC）对过岛台风的定位差异大，这个大的差异尤其表现在海峡和二次登陆时。（2）深化了中尺度地形对台风雨带移动影响的及岛屿地形对TC眼墙的破坏和入海后眼墙的重建机理认识。发现造成过岛台风强降水螺旋雨带形成和维持的机制涉及到TC涡旋内涡旋罗斯贝波的活动；山地地形高度及形状在TC结构和强度变化中作用不容忽视，而非绝热加热项对涡旋移动方向和移动的速度起重要作用。发展了采用地形补偿技术的中尺度地形影响下的涡旋动力初始化技术，系统性地解决了数值预报中TC接近台湾岛中尺度地形时的动力初始化问题；（3）在TC强度变化的机理方面，提出了ＴＣ双眼墙形成的新机制，并揭示了TC眼墙替换过程中的暖心演变过程；定量分析了初始涡旋尺度对后期强度变化率的影响，海上TC强度减弱率的影响因子及相对贡献；揭示了大气环境因子对TC最大潜势强度的影响，以及在气候变化背景下，我国登陆TC的潜在危险指数及降水变化趋势。