Simulated Sensitivity of the Tropical Cyclone Eyewall Replacement Cycle to the Ambient Temperature Profile

In this study, the impacts of the environmental temperature profile on the tropical cyclone eyewall replacement cycle are examined using idealized numerical simulations. It is found that the environmental thermal condition can greatly affect the formation and structure of a secondary eyewall and the intensity change during the eyewall replacement cycle. Simulation with a warmer thermal profile produces a larger moat and a prolonged eyewall replacement cycle. It is revealed that the enhanced static stability greatly suppresses convection, and thus causes slow secondary eyewall formation. The possible processes influencing the decay of inner eyewall convection are investigated. It is revealed that the demise of the inner eyewall is related to a choking effect associated with outer eyewall convection, the radial distribution of moist entropy fluxes within the moat region, the enhanced static stability in the inner-core region, and the interaction between the inner and outer eyewalls due to the barotropic instability. This study motivates further research into how environmental conditions influence tropical cyclone dynamics and thermodynamics.     

A Numerical Investigation of the Eyewall Evolution of a Tropical Cyclone

In this study, a high-resolution numerical simulation is conducted to investigate the eyewall evolution of Typhoon Imbudo (2003). The eyewall contraction, breakdown, and reformation are successfully simulated by the model. The eyewall accordantly shrinks throughout the whole troposphere prior to landfall, while it exhibits di erent variations after landfall in the lower and upper troposphere, respectively. It is found that the dry air advected into the storm inner core through a low-θe channel, the reduced surface latent heat transfer, and the increased in ows in the coastal region are associated with the eyewall contraction.Accompanied with the high-to-low wavenumber change in the vortex Rossby waves, the initial polygonal eyewall transforms to an elliptical one. Such a wavenumber change is likely associated with the change of interaction between the rainbands and the eyewall. The corresponding features of the time-averaged and vertical dynamic and thermodynamic structures are also examined during the storm passage. A tangential wind budget analysis indicates that a strong acceleration due to the contributions of both the eddy and the mean circulation is located in the lower layer in the eyewall during pre-landfall, and the mean circulation contribution to the change in the tendency of the azimuthally averaged tangential wind counteracts the eddy contribution.     

Characteristics of the Asymmetric Flow of Tropical Cyclone Shanshan (2006) During Its Turning and Intensification Period

In this paper,characteristics of the asymmetric flow of Tropical Cyclone (TC) Shanshan (2006) during its turning and intensification period over the oceanic area east of Taiwan are investigated,based o...     

Effect of the Initial Vortex Structure on Intensity Change During Eyewall Replacement Cycle of Tropical Cyclones: A Numerical Study

This study investigates the effect of the initial tropical cyclone (TC) vortex structure on the intensity change during the eyewall replacement cycle (ERC) of TCs based on two idealized simulations using the Weather Research and Forecasting (WRF) model. Results show that an initially smaller TC with weaker outer winds experienced a much more drastic intensity change during the ERC than an initially larger TC with stronger outer winds. It is found that an initially larger TC vortex with stronger outer winds favored the development of more active spiral rainbands outside the outer eyewall, which slowed down the contraction and intensification of the outer eyewall and thus prolonged the duration of the concentric eyewall and slow intensity evolution. In contrast, the initially smaller TC with weaker outer winds corresponded to higher inertial stability in the inner core and weaker inertial stability but stronger filamentation outside the outer eyewall. These led to stronger boundary layer inflow, stronger updraft and convection in the outer eyewall, and suppressed convective activity outside the outer eyewall. These resulted in the rapid weakening during the formation of the outer eyewall, followed by a rapid re-intensification of the TC during the ERC. Our study demonstrates that accurate in- itialization of the TC structure in numerical models is crucial for predicting changes in TC intensity during the ERC. Additionally, monitoring the activity of spiral rainbands outside the outer eyewall can help to improve short-term intensity forecasts for TCs experiencing ERCs.     

弱环境流场中台风运动的若干问题

概述弱环境流场中台风运动研究的若进展，包括对称结构理论，非线性作用强迫影响和准均匀流理论等４个方面，并初步讨论了理论结果的应用前景。     

1980—2014年中国台风大风和台风极端大风的变化

基于1980—2014中国670站日最大风速资料,利用改进的客观天气图分析法（OSAT）分离出中国陆地的台风大风（6级以上,≥10.8 m/s）,并定义了台风极端大风,进而研究了台风大风和台风极端大风的变化特征。分析表明：在地理分布上,台风大风年均日数和占比均自海岸线向内陆迅速减小,在海南、华南和东南沿海省份以及江苏南部,台风大风占比一般为30%～70%;台风极端大风年均日数大值主要分布在沿海省市（除河北和天津）,特别是华东和华南沿海,局部地区台风极端大风日数占比达100%。从季节变化看,在台风活跃的7—9月,中国台风极端大风频次总体上超过了季风极端大风;就全国而言,当阈值从最低值（11.5 m/s）提升至12级（32.7 m/s）时,台风极端大风频数占比则从12%急剧攀升至77%。1980—2014年,中国台风大风和台风极端大风年日数均显著减少,而台风极端大风年平均强度增强;这期间引起中国台风大风和台风极端大风的台风频数均显著减少,但引起台风极端大风的台风在生命期和影响期的平均强度均显著增强,这可能是上述显著变化特征的主要原因。     

Impact of Surface Exchange Coefficients and Sea Surface Cooling on Tropical Cyclone Simulation

The surface flux exchange associated with the exchange coefficients and upper ocean conditions is essential to the development of tropical cyclones (TCs). Using the Weather Research and Forecasting (WRF) model, the present study has investigated the impact of exchange coefficients and ocean coupling during Super Typhoon Saomai (2006). Firstly, two experiments with different formula of roughness are conducted. The experiment with the Donelan formula for drag coefficient (Cd) and ramped formula for enthalpy coefficient (Ck) can simulate stronger intensity compared to other experiments due to the increased surface wind and enthalpy fluxes. That is because the new formulas allows for a smaller Cd and larger Ck in the high wind regime than the former formulas did. Moreover, two coupled simulations between WRF and a one-dimensional ocean model are conducted to examine the feedback of sea surface cooling to the TC. In the experiments with a horizontal uniform mixed layer depth of 70 m, the sea surface cooling is too weak to change the evolution of TC. While in the experiment with an input mixed layer calculated using the Hybrid Coordinate Ocean Model (HYCOM) data, the significant sea surface cooling induces obvious impact on TC intensity and structure. Under the negative feedback of sea surface cooling, the sensible and latent heat fluxes decreases, especially in the right part of Saomai (2006). The negative feedback with coupled ocean model plays a vital role in simulating the intensity and structure of TC.     

热带气旋强度与结构研究新进展

主要回顾热带气旋(TC)强度与结构变化的研究发展近况。以往热带气旋的理论研究认为在给定的大气和海洋热状况下,存在着一个TC所能达到的最大可能强度(MPI)。但实际上,海洋生成的热带气旋达到的最大强度普遍要比由MPI理论计算得到最大强度要低。近几年的研究表明,存在着内部和外部的不利因子通过对TC结构的改变来阻碍其加强,从而限制TC的强度。以往认为在诸多因子中,垂直风切变产生的内核区非对称结构与眼墙区下方海水上涌造成的海面冷却是制约TC达到MPI的主要因子。最新的研究进一步指出,产生TC非对称性的中尺度过程对其强度与结构的变化至关重要。中尺度过程包含有对流耦合的涡旋Rossby波、内外圈螺旋雨带、嵌于TC环流内的中尺度涡旋。外部的环境气流也是通过这些眼墙的中尺度过程影响到TC的强度与结构变化。     

影响广西的热带气旋与热带季风的某些关系

利用常规的热带气旋气候资料和最近（1998）实施的南海季风试验研究成果资料，统计分析影响广西的热带气旋活动与热带季风爆发迟早，强弱等变异的关系，揭示了一些统计特征，并探讨其中的某些规律，初步分析了热带季风对热带气旋的作用，指出热带季风气流进入热带气旋环流对热带气旋的发展有正贡献。     

我国热带气旋灾害的分析研究

本文对１９４９—１９９１年４３年的热带气旋资料，以及１９５６—１９９２年３７年的热带气旋灾情资料进行了分析，说明了热带气旋灾害的主要特征和发展趋势。提出了关于热带气旋灾情评估的３项指标，表明灾情与热带气旋系统强度、大小、天气，登陆率，预报准确率，热带气旋系统影响地区社会经济活动的发达程度，物质财富积累水平，以及防抗措施的水平等有密切的关系，为我国今后防台减灾提供了依据。     

台风异常运动及其外区热力不稳定非对称结构的影响效应

本文数值模拟研究揭示了台风外区热力不稳定非对称结构对其异常路径的影响问题，提出了台风运动非对称结构的影响，不仅表现在台风涡旋动力结构特征上，而且反映在台风外区三维非对称热力结构特点方面，即包括温、湿不稳定层结分布特征及其强弱程度因素。台风外区不同热力非对称分布特征将导致台风移动轨迹的显著差异，且构成各类“旋转”、“打转”、“转向”等复杂异常路径。     

热带气旋北冕暴雨多普勒雷达分析

分析热带气旋北冕在厦门地区造成的暴雨过程中主要降水时段多普勒雷达资料强度场和速度场的特点。分析表明，这次暴雨是多种天气系统相互作用的结果。第三阶段的短时大暴雨是停滞在厦门上空的强单体直接造成的，对应的速度场上有低层东南风和西南风辐合；单体的合并也起了一定的作用，而且单体的分离合并与低空急流的脉动有关。     

Numerical Simulation of Changes in Tropical Cyclone Intensity Using a Coupled Air-Sea Model

A coupled air-sea model for tropical cyclones （TCs） is constructed by coupling the Pennsylvania State University/National Center for Atmospheric Research mesoscale model （MM5） with the Princeton Ocean Model.Four numerical simulations of tropical cyclone development have been conducted using different configurations of the coupled model on the f-plane.When coupled processes are excluded,a weak initial vortex spins up into a mature symmetric TC that strongly resembles those observed and simulated in prior research.The coupled model reproduces the reduction in sea temperature induced by the TC reasonably well,as well as changes in the minimum central pressure of the TC that result from negative atmosphere-ocean feedbacks.Asymmetric structures are successfully simulated under conditions of uniform environmental flow.The coupled ocean-atmosphere model is suitable for simulating air-sea interactions under TC conditions.The effects of the ocean on the track of the TC and changes in its intensity under uniform environmental flow are also investigated.TC intensity responds nonlinearly to sea surface temperature （SST）.The TC intensification rate becomes smaller once the SST exceeds a certain threshold.Oceanic stratification also influences TC intensity,with stronger stratification responsible for a larger decrease in intensity.The value of oceanic enthalpy is small when the ocean is weakly stratified and large when the ocean is strongly stratified,demonstrating that the oceanic influence on TC intensity results not only from SST distributions but also from stratification.Air-sea interaction has only a slight influence on TC movement in this model.     

热带气旋异常运动可预报性问题的理论研究

简要讨论了３个问题：（１）热带气旋移动与大尺度波动流型演变两者可预报性的差别。（２）热带气旋移动可预报性问题的两类方法。（３）近几年来热带气旋移动可预报性研究的若干结果。     

Importance of Air-Sea Coupling in Simulating Tropical Cyclone Intensity at Landfall

An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones (TCs) along the South China coast is upgraded by including ocean and wave models. A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea. Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size. In particular, interaction with the ocean does not necessarily imply a weakening of the TC, with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC. These results suggest that to simulate the evolution of TC structure more accurately, it is essential to use an air-sea coupled model instead of an atmosphere-only model.     

热带气旋预报方法业务预试验的效果分析

通过对参加１９９４年夏季热带气旋预报方法业务预试验的８个方法进行实例分析，初步指出各方法的成功之处与不足，以期得到改进和提高。     

Azimuthal Variations of the Convective-scale Structure in a Simulated Tropical Cyclone Principal Rainband

Previous numerical simulations have focused mainly on the mesoscale structure of the principal rainband in tropical cyclones with a relatively coarse model resolution. In this study, the principal rainband was simulated in a semi-idealized experiment at a horizontal grid spacing of 1/9 km and its convective-scale structure was examined by comparing the convective elements of the simulated principal rainband with previous observational studies. It is found that the convective scale structure of t...     

城市效应对登陆热带气旋妮妲降水影响的模拟

运用中尺度数值模式WRF耦合城市冠层模式（urban canopy model,UCM）,对2016年登陆深圳的热带气旋妮妲（1604）（以下简称妮妲）进行数值模拟。高分辨率数值模拟较好地再现了妮妲登陆前后的强度、路径和累积降水。利用城市化过程当中城市冠层对热带气旋降水的敏感性试验结果表明:城市冠层会减弱对流运动和水汽的输送,导致热带气旋登陆后珠江口城市群区域累积降水量略减少。应用最新的土地利用资料进行的城市下垫面敏感性试验结果表明:由于城市下垫面粗糙度增加,造成登陆地面风的减速,强度减弱,潜热通量与2 m高度比湿相应减小;城市下垫面粗糙度增加会加强该区域垂直对流运动以及不稳定能量增加,有利于降水增强,尤其在城市化下垫面处,热带气旋登陆后6 h累积降水增加量最大可超过20 mm。总体而言,对登陆热带气旋降水而言,耦合城市冠层使城市区域热带气旋降水减少,但在数值模拟中城市冠层影响作用不显著。城市化下垫面对登陆热带气旋暴雨的增幅作用明显,在登陆热带气旋降水预报中应重视。     

Bias Analysis in the Simulation of the Western North Pacific Tropical Cyclone Characteristics by Two High-Resolution Global Atmospheric Models

This study compares the atmosphere-only HighResMIP simulations from FGOALS-f3-H(FGOALS) and MRIAGCM3-2-S(MRI) with respect to tropical cyclone(TC) characteristics over the Western North Pacific(WNP) for the July–October months of 1985–2014. The focus is on investigating the role of the tropical easterly jet over the Western Pacific(WP＿TEJ) in modulating the simulation biases in terms of their climatological distribution and interannual variability of WNP TC genesis frequency(TCGF) based on the a...