Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall

Strong vertical motion(>10 m s^-1) has profound implications for tropical cyclone(TC) structure changes and intensity. While extreme updrafts in the TC are occasionally observed in real TCs, the associated small-scale features remain unclear. Based on an analysis of the extreme eyewall updrafts in two numerical experiments conducted with the Advanced Research version of the Weather Research and Forecasting(WRF) model, in which the large-eddy simulation(LES) technique was used with t...     

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.     

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.     

一种适于较强台风的切向风廓线方案及其对异常台风路径的数值模拟研究

针对现今台风数值预报中强热带气旋切向风廓线设计上的困难，提出了一种利用台风报告中7级风和10级风的半径来确定台风切向风廓线的方法。它既可反映真实台风外围结构又避免了强风速切变问题。该方案有可能改进台风尤其是异常路径台风的预报效果，具有广阔的业务应用前景。     

2015年热带气旋“彩虹”的高分辨率数值模拟与风场诊断分析

利用WRF模式,对热带气旋“彩虹”（1522）登陆过程开展了高分辨率数值模拟,模拟验证表明:模式较成功地模拟出了“彩虹”的移动路径、强度变化和降水分布。利用高分辨率模拟资料,分析了“彩虹”登陆期间的大风分布以及热、动力结构特征,结合切向风动量方程开展了模拟诊断,结果表明:切向风动量方程各项在近地面层和边界层顶代表高度上的平面结构诊断分析显示,近地面（0.4 km）层上,VVOR（角动量径向平流项）和VPGF（切向气压梯度项）是切向风变化的最大贡献项;边界层顶（1.3 km）附近,VPGF和VVA（切向动量垂直平流项）对切向风起到正加速作用,而VVOR则根据入流（出流）特征,起到正（负）加速作用。切向风动量方程各项的轴对称平均结构诊断分析显示,“彩虹”登陆前,VVA和VVOR是决定切向风变化的主要贡献项。     

热带气旋外眼墙形成机制研究综述

介绍了国内外关于热带气旋外眼墙形成和维持过程的相关研究进展,包括大尺度环境场和热带气旋涡旋内部动力学过程,如涡旋罗斯贝波理论、轴对称化过程、涡丝化作用、β-skirt轴对称化外眼墙形成假说和边界层非平衡动力过程等。随着对外眼墙形成机理研究的不断深入,当前存在多种外眼墙形成的机制理论,而这些机制均强调在外眼墙的形成阶段,热带气旋外围有大量对流及位势涡度扰动的发生发展。因此,热带气旋外眼墙的形成很有可能是多种机制相互作用导致的。最后,提出研究多种机制相互作用导致外眼墙处的对流和位势涡度扰动的发生发展过程具有重大意义。     

On the Evolution and Structure of a Radiation Fog Event in Nanjing

An extremely dense radiation fog event during 10-11 December 2007 was studied to understand its macro-micro-physics in relation to dynamic and thermodynamic structures of the boundary layer, as well as its structural evolution in conjunction with the air-surface exchange of heat and water vapor. The findings are as follows. The extreme radiation fog process was divisible into formation, development, mature, and dissipation phases, depending on microstructure and visibility. This fog event was marked by rapid evolution that occurred after sunrise, when enhanced surface evaporation and cold air intrusion led to a three order of magnitude increase in liquid water content (LWC) in just 20 minutes. The maximum droplet diameter (MDD) increased four-fold during the same period. The fog structure was two-layered, with the top of both the surface-layer and upper-layer components characterized by strong temperature and humidity inversions, and low-level jets existed in the boundary layer above each fog layer. Turbulence intensity, turbulent kinetic energy, and friction velocity differed remarkably from phase to phase: these features increased gradually before the fog formation and decreased during the development phase; during the mature and dissipation phases these characteristics increased and then decreased again. In the development and mature stages, the mean kinetic energy of the lower-level winds decreased pronouncedly, both in the horizontal and vertical directions.     

登陆台风等熵面位涡演变的数值模拟研究

选取1997年第11号台风温妮为研究个例,通过中尺度模式MM5模拟再现了该台风登陆后经历初期减弱、变性及变性后再次发展的演变过程.引入Ertel等熵面位涡收支方程,深入分析了登陆台风结构演变的过程中绝热与非绝热作用对对流层低层位涡局地变化的影响.研究表明:台风温妮深入内陆的过程中,对流层低层台风中心西北侧位涡增长,且大值中心不再与台风中心重合;由于摩擦和非绝热加热的存在,对流层低层位涡不守恒,其局地变化主要决定于位涡的水平平流(守恒项)、位涡的垂直平流、加热的垂直微分(非守恒项)的分布;台风温妮变性前后,对流层低层位涡的守恒性逐渐减弱,非守恒项尤其是加热的垂直微分对位涡的局地增长的正贡献不断增强直至占有主导地位.     

A New Method for Calculating the Wind Speed Distribution of a Moving Tropical Cyclone

Based on gradient wind equations,including frictional force,and considering the effect of the movement of a tropical cyclone on wind speed,the Fujita Formula is improved and further simplified,and the numerical scheme for calculating the maximum wind speed radius and wind velocity distribution of a moving tropical cyclone is derived.In addition,the effect of frictional force on the internal structure of the tropical cyclone is discussed.By comparison with observational data,this numerical scheme demonstrate...     

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...     

台风"云娜"的热动力结构模拟试验研究

对2004年8月登陆我国的台风"云娜"的热动力结构及其演变进行了数值模拟。试验从大尺度环境场、台风中尺度动力和热力场三维结构、台风云雨特征等多角度,对模拟结果与实际观测进行了对比。结果表明,模式不仅较准确地再现了台风登陆前24h移动和演变的全过程,在移向、移速和登陆时间、地点上与实况十分接近,而且对台风系统中尺度结构、云雨特征的模拟与分析场及卫星观测也有较好的一致性。采用的模拟工具是新开发的双重嵌套AREM模式,该模式是静力模式,但对台风仍有较强的模拟能力。此外,本试验无任何人为台风模型的嵌入是其中的一个特点,从而增加了试验的可信度。     

A Numerical Study of the Evolution of a Mesoscale Convective Vortex on the Meiyu Front

The Advanced Research WRF（Weather Research and Forecasting） model is used to simulate the evolution of a mesoscale convective vortex（MCV） that formed on the Meiyu front and lasted for more than two days. The simulation is used to investigate the underlying reasons for the genesis, intensification, and vertical expansion of the MCV. This MCV is of a type of mid-level MCV that often develops in the stratiform regions of mesoscale convective systems. The vortex strengthened and reached its maximum intensity and vertical extent（from the surface to upper levels） when secondary organized convection developed within the mid-level circulation. The factors controling the evolution of the kinetic and thermal structure of the MCV are examined through an analysis of the budgets of vorticity, temperature, and energy. The evolution of the local Rossby radius of deformation reveals the interrelated nature of the MCV and its parent mesoscale convective system.     

近年来关于西北太平洋热带气旋和台风活动的气候学研究进展

本文主要综述和回顾了近年来季风系统研究中心关于西北太平洋热带气旋和台风(TCs)活动的气候学研究进展及有关的国内外研究.文中不仅回顾了最近关于夏、秋季西北太平洋利于TCs生成的大尺度环流型及其与涡旋的正压能量交换、西北太平洋TCs活动的年际和年代际及季节内的变化特征、以及今后全球变暖背景下西北太平洋TCs活动的变化趋势的气候学研究进展,而且综述了西北太平洋季风槽及热带对流耦合波动对西北太平洋上TCs生成的动力作用的研究.此外,文中还指出今后有关西北太平洋TCs活动一些亟需进一步研究的气候学问题.     

Reexamination of the Relationship between Tropical Cyclone Size and Intensity over the Western North Pacific

This study reexamines the correlation between the size and intensity of tropical cyclones (TCs) over the western North Pacific from the perspective of individual TCs, rather than the previous large-sample framework mixing up all TC records. Statistics show that the positive size-intensity correlation based on individual TCs is relatively high. However, this correlation is obscured by mixing large samples. The weakened correlation based on all TC records is primarily due to the diversity in the size change relative to the same intensity change among TCs, which can be quantitatively measured by the linear regression coefficient (RC) of size against intensity. To further explore the factors that cause the variability in RCs that weakens the size-intensity correlation when considering all TC records, the TCs from 2001 to 2020 are classified into two groups according to their RC magnitudes, within which the high-RC TCs have a larger size expansion than the low-RC TCs given the same intensity change. Two key mechanisms responsible for the RC differences are proposed. First, the high-RC TCs are generally located at higher latitudes than the low-RC TCs, resulting in higher planetary vorticity and thus higher planetary angular momentum import at low levels. Second, the high-RC TCs are susceptible to stronger environmental vertical wind shear, leading to more prolific outer convection than the low-RC TCs. The positive feedback between outer diabatic heating and boundary layer inflow favors the inward import of absolute angular momentum in the outer region, thereby contributing to a larger size expansion in the high-RC TCs.     

9610号热带风暴中尺度涡旋数值模拟

以实测资料作为初始场,用套网格的中尺数值模式分析和模拟与９６１０号热带低压相伴的特大暴雨过程,结果发现模式对本过程的模拟效果较好,不论粗细网格,在揭示中尺度涡旋方面都具有参考价值。     

A Method of Initial Vortex Relocation and Numerical Simulation Experiments on Tropical Cyclone Track

Using the technique of smooth filtering and cylindrical filtering, the initial vortex circulation and large-scale environmental field were separated from the background field. Then the separated initial vortex circulation was translated and reinserted in the location where it was observed. This led to the determination of a method of initial vortex relocation. For seven tropical cyclones at 23 points of measurement time in the years of 2006 and 2007, two schemes, either directly adding a tropical cyclone bogus model in the background or adding it after the relocation of the initial vortex in the background field, were employed. Simulation experiments were compared. The results showed that the mean errors of the simulated tropical cyclone tracks at 24 and 48 hours were both smaller with the scheme of adding tropical cyclone bogus model after the relocation of the initial vortex in the background field. The relocation method of the initial vortex decreases the error caused by the deviation of the initial tropical cyclone location in tropical cyclone models. The relocation method is conducive to improving the track forecast of tropical cyclone models and has a good perspective for operational application.     

The Roles of Barotropic Instability and the Beta Effect in the Eyewall Evolution of Tropical Cyclones

Diabatic heating by convection in the eyewall often produces an annular region of high potential vorticity(PV) around the relatively low PV eye in a strong tropical cyclone(TC). Such a PV ring is barotropically unstable and can encourage the exponential growth of PV waves. In this study, such instability and the subsequent nonlinear evolution of three TC-like vortices having PV rings with different degrees of hollowness on an f-plane are first examined using an unforced, inviscid shallow-water-e...     

Mechanism Study of Tropical Cyclone Impact on East Asian Subtropical Upper-Level Jet: a Numerical Case Investigation

In the case study of this paper, sensitivity experiments are carried out using the mesoscale non-hydrostatic Weather Research and Forecasting (WRF) model to investigate the impact of tropical cyclone (TC) Soudelor (2003) on the East Asian subtropical upper-level jet (EASJ) before TC Soudelor transformed into an extratropical cyclone. The physical mechanism for changes in the EASJ intensity and position caused by TC Soudelor is explored. Results indicate that TC Soudelor would warm the air in the middle and upper troposphere over the Japan Sea and the adjacent areas through stimulating northward propagating teleconnection pattern as well as releasing large amounts of latent heat, which led to increase (decrease) the meridional air temperature gradient to the south (north) below the EASJ axis. As a result, the geopotential height abnormally increased in the upper troposphere, resulting in an anomalous anticyclonic circulation belt along the EASJ axis. Correspondingly, the westerly winds to the north (south) of the EASJ axis intensified (weakened) and the EASJ axis shifted northward by one degree. The case study also suggests that before the extratropical cyclone transition of TC Soudelor, the TC activities had exerted significant impacts on the EASJ through thermodynamic processes.     

A Statistical Analysis on the Effect of Vertical Wind Shear on Tropical Cyclone Development

Using tropical cyclone (TC) best track and intensity of the western North Pacific data from the Joint Typhoon Warning Center (JTWC) of the United States and the NCEP/NCAR reanalysis data for the period of 1992-2002, the effects of vertical wind shear on TC intensity are examined. The samples were limited to the westward or northwestward moving TCs between 5°N and 20°N in order to minimize thermodynamic effects. It is found that the effect of vertical wind shear between 200 and 500 hPa on TC intensity change is larger than that of the shear between 500 and 850 hPa, while similar to that of the shear between 200 and 850 hPa. Vertical wind shear may have a threshold value, which tends to decrease as TC intensifies. As the intensifying rate of TC weakens, the average shear increases. The large shear has the obvious trend of inhibiting TC development. The average shear of TC which can develop into typhoon (tropical depression or tropical storm) is below 7 m s-1 (above 8 m s-1).