基于耦合动力降尺度模式的台风季节性预报研究

Typhoons are one of the most dangerous natural hazards threatening the lives and livelihoods along the eastern coast of China. Although the tropical cyclone forecasting techniques have been much improved during the last decades, to date, it is still a major challenge to predict the seasonal variation of typhoon activities. This is mainly due to the complexity associated with the typhoon genesis, intensification, and interaction with the general circulations. Here our aim is to test the methodology of nested high resolution coupled regional climate models as a way to reduce forecast uncertainty via a dynamical downscaling approach. To achieve this, we will develop a new regional coupled tropical cyclone modelling system, specially designed for the western North Pacific and the South China Sea. The new regional climate model will be used to dynamically downscale the reanalysis data from a well-calibrated coupled general circulation model. The main focus is on the influence of oceanic processes on the potential predictability of typhoon activities. We will examine the potential predictability of typhoons through the study of ensembles. The potential predictability will be evaluated by calculating both ensemble sensitivity and forecast spread within the ensemble framework. There are two specific objectives of this project: (1) to build a high resolution (4 km) coupled regional climate model of the western North Pacific cyclone region, which can be initialized and forced at the boundary by various reanalysis, operational forecast and climate models; (2) to establish the predictability and uncertainties of one ensemble general circulation modelling system with a particular focus on the ocean state.

台风是严重威胁我国东部沿海及南海战略海域安全的自然灾害之一。由于台风过程自身的复杂性及其与大尺度环流场的相互作用，目前对台风活动的季节性预报依然存在强不确定性。本项目以动力降尺度、高分辨率区域气候耦合模式以及集合预报为基本研究手段，探究动力降尺度模式中上层海洋过程与台风可预报性的关系，定量计算动力降尺度耦合模式较仅含大气模块的动力降尺度模式在台风生成频率、强度和可能路径等季节性预报方面的改进，分析与预报能力提升相关的关键上层海洋过程。本项目的研究目标为：（1）针对西北太平洋和南海海域台风，建立一套可以被全球主流再分析、预报系统驱动的高分辨率（4 km）大气-海洋耦合动力降尺度模式；（2）揭示上层海洋过程对台风季节性预报的重要性。本项目在加深台风与上层海洋相互作用的认识，防灾减灾等方面，具有重要的学术和实践价值，可以为提高西北太平洋和南海海域台风的预报水平提供科学依据。

本研究以热带气旋成熟阶段为主要研究对象，探究了该时段热带气旋的演变特征及其对海表升温的响应。具体为：.（1）以热带气旋成熟阶段为研究对象，探究热带气旋强度和尺寸在成熟阶段的变化特征。热带气旋成熟阶段被定义为从生命周期最大强度至生命周期最大空间尺寸的一段时间。观测统计表明，热带气旋在成熟阶段有三个特征：中心强度降低，眼壁外扩，外围初级环流的切向风增加。尽管中心强度和空间尺寸反向变化，但是以平均累积动能为代表的热带气旋毁坏力却在成熟阶段增加约70%。该发现说明，在热带气旋的成熟阶段，尽管中心强度有所降低，但此时整体的毁坏力却往往大幅增加。.（2）尝试对比分别在进行和不进行辐射对流平衡调整的前提下，热带气旋中心强度、空间尺寸和潜在毁坏力在成熟阶段对海表升温的敏感性。研究表明，当同时考虑海表升温和大气相应的辐射对流平衡调整时，气旋外围空间尺寸对海表升温几乎不作出相应，而气旋中心强度对海表升温的敏感度也降为3%-4% oC-1。这说明气候变化背景下的全球热带气旋平均中心强度变化主要表现为对海表升温的热动力学响应。