Dynamical effects of environmental vertical wind shear on tropical cyclone motion, structure, and intensity

Summary A series of numerical experiments on an f plane are conducted using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, version 3 (MM5) to investigate how environmental vertical wind shear affects the motion, structure, and intensity of a tropical cyclone. The results show that a tropical cyclone has a motion component perpendicular to the vertical shear vector, first to the right of the shear and then to the left. An initially axisymmetric, upright tropical cyclone vortex develops a downshear tilt and wavenumber-one asymmetry when embedded in environmental vertical wind shear. In both small-moderate shears, a storm weakens slightly compared to that in a quiescent environment. The circulation centers between 300 hPa and the surface varies from 20 km to over 80 km. The secondary circulation becomes quite asymmetric about the surface cyclone center. As a result, convection on the upshear-right quadrant diminishes, limiting the upward heat transport in the eyewall and thus lowering the warm core and leading to a weakening of the storm. In strong vertical shear (above 12 m s⁻¹), the vertical tilt exceeds 160 km in 48 h of simulation and the secondary circulation on the upshear side is completely destroyed with low-level outflow. The axisymmetric component of eyewall convection weakens remarkably and becomes much less penetrative. As a result, the warm core becomes weak and appears at lower levels and the storm weakens rapidly accordingly. This up-down weakening mechanism discussed in this study is different from those previously discussed. It emphasizes the penetrative role of eyewall convection in transporting heat from the ocean to the mid-upper troposphere, maintaining the warm core structure of the tropical cyclone. The vertical shear is found negative to eyewall penetrative convection.     

Fine-resolution regional climate model simulations of the impact of climate change on tropical cyclones near Australia

Fine-resolution regional climate simulations of tropical cyclones (TCs) are performed over the eastern Australian region. The horizontal resolution (30 km) is fine enough that a good climatological simulation of observed tropical cyclone formation is obtained using the observed tropical cyclone lower wind speed threshold (17 m s^–1). This simulation is performed without the insertion of artificial vortices (bogussing). The simulated occurrence of cyclones, measured in numbers of days of cyclone activity, is slightly greater than observed. While the model-simulated distribution of central pressures resembles that observed, simulated wind speeds are generally rather lower, due to weaker than observed pressure gradients close to the centres of the simulated storms. Simulations of the effect of climate change are performed. Under enhanced greenhouse conditions, simulated numbers of TCs do not change very much compared with those simulated for the current climate, nor do regions of occurrence. There is a 56% increase in the number of simulated storms with maximum winds greater than 30 m s^–1 (alternatively, a 26% increase in the number of storms with central pressures less than 970 hPa). In addition, there is an increase in the number of intense storms simulated south of 30°S. This increase in simulated maximum storm intensity is consistent with previous studies of the impact of climate change on tropical cyclone wind speeds.     

Projected future changes in tropical cyclone-related wave climate in the North Atlantic

Tropical cyclones are a major hazard for numerous countries surrounding the tropical-to-subtropical North Atlantic sub-basin including the Caribbean Sea and Gulf of Mexico. Their intense winds, which can exceed 300 km h⁻¹, can cause serious damage, particularly along coastlines where the combined action of waves, currents and low atmospheric pressure leads to storm surge and coastal flooding. This work presents future projections of North Atlantic tropical cyclone-related wave climate. A new configuration of the ARPEGE-Climat global atmospheric model on a stretched grid reaching ~ 14 km resolution to the north-east of the eastern Caribbean is able to reproduce the distribution of tropical cyclone winds, including Category 5 hurricanes. Historical (1984–2013, 5 members) and future (2051–2080, 5 members) simulations with the IPCC RCP8.5 scenario are used to drive the MFWAM (Météo-France Wave Action Model) spectral wave model over the Atlantic basin during the hurricane season. An intermediate 50-km resolution grid is used to propagate mid-latitude swells into a higher 10-km resolution grid over the tropical cyclone main development region. Wave model performance is evaluated over the historical period with the ERA5 reanalysis and satellite altimetry data. Future projections exhibit a modest but widespread reduction in seasonal mean wave heights in response to weakening subtropical anticyclone, yet marked increases in tropical cyclone-related wind sea and extreme wave heights within a large region extending from the African coasts to the North American continent.

     

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...     

Increased hurricane intensities with CO2-induced warming as simulated using the GFDL hurricane prediction system

 The impact of CO₂-induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO₂ increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO₂ conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO₂ storms, with SSTs warmer by about 2.2 °C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3–7 m/s (5%–11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO₂ sample. In terms of storm tracks, the high CO₂ sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO₂ storm than for the control, and the high CO₂ storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins. Received: 20 July 1998/Accepted: 24 December 1998     

Assimilation of the multisatellite data into the WRF model for track and intensity simulation of the Indian Ocean tropical cyclones

Weather Research and Forecasting (WRF-ARW) model and its three-dimensional variational data assimilation (3D-Var) system are used to investigate the impact of the Quick Scatterometer (QuikSCAT) near surface winds, Special Sensor Microwave/Imager (SSM/I)-derived Total Precipitable Water (TPW), and Meteosat-7-derived Atmospheric Motion Vectors (AMVs) on the track and intensity prediction of tropical cyclones over the North Indian Ocean. The case of tropical cyclone, Gonu (June 2007; Arabian Sea), is first tested and the results show significant improvements particularly due to the assimilation of QuikSCAT winds. Three other cases, cyclone Mala (April 2006; Bay of Bengal), Orissa super cyclone (October 1999; Bay of Bengal), and Very Severe Cyclonic storm (October 1999; Bay of Bengal), are then examined. The prediction of cyclone tracks improved significantly with the assimilation of QuikSCAT winds. The track improvement resulted from the relocation of the initial cyclonic vortices after the assimilation of QuikSCAT wind vectors. After the assimilation of QuikSCAT winds, the mean (for four cyclone cases) track errors for first, second, and third day forecasts are reduced to 72, 101, and 166?km, respectively, from 190, 250, and 381?km of control (without QuikSCAT winds) runs. The assimilation of QuikSCAT winds also shows positive impact on the intensity (in terms of maximum surface level winds) prediction particularly for those cyclones, which are at their initial stages of the developments at the time of data assimilation. The assimilation of SSM/I TPW has significant influence (negative and positive) on the cyclone track. In three of the four cases, the assimilation of the SSM/I TPW resulted in drying of lower troposphere over cyclonic region. This decrease of moisture in TPW assimilation experiment resulted in reduction of cyclonic intensity. In three of the four cyclones, the assimilation of Meteosat-7 AMVs shows negative impact on the track prediction.     

强热带风暴“北冕”登陆前后的风场变化

利用Q3000型车载相控阵风廓线雷达,对强热带风暴“北冕”登陆前后的风场情况进行了连续观测,观测资料反映了“北冕”登陆前后观测点上空风场的变化：“北冕”中心临近时风速逐渐增大,而后逐渐减弱,风向在垂直空间及时间上存在有规律的切变。风廓线雷达探测资料对于研究热带气旋内部的平均风特性、垂直结构、湍流输送性质及风压分布等有重要意义,可以弥补地面探空站在台风期间无法获取有效探空资料的不足。     

星载双频风场雷达热带气旋降雨区测风模拟

基于大气辐射传输理论分别建立Ku波段和C波段的降雨模型,模拟热带气旋降雨区洋面的雷达回波并反演了洋面10 m风场,用于研究降雨对测风的影响以及风云三号双频风场雷达 (WFR) 的测风能力。分析表明:回波的衰减或增强取决于降雨衰减项和后向散射项的相对大小;热带气旋的降雨使反演风速偏小,风向精度降低,Ku波段相对于C波段更易受影响,在高风速 (超过30 m·s-1) 条件下,可达5~20 m·s-1的负风速偏差。反演结果表明:双频反演的新方法能够结合Ku波段与C波段的优势,双频最大似然估计 (MLE) 方法在分辨率上优于C波段单频反演,相对Ku单频反演能降低降雨对测风的衰减作用,结合双频MLE方法和C波段单频反演优势的分区反演方法可以显著减小降雨偏差,提高风速反演精度,在有风云三号湿度计同步观测的条件下,是提高热带气旋降雨区测风精度的有效手段。     

Wind forcing due to synoptic storm activity over the North Pacific Ocean

Abstract

Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u³ _* and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u³ _* and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind components. These quantities describe the atmospheric forcing of the ocean that is attributable to (a) synoptic storm activity by itself, (b) synoptic storm activity in the presence of the low‐pass (mean) flow, and (c) the total spectrum of wind forcing, respectively.

Maps of the long‐term (10‐year) monthly mean u³ _* calculated from (a) and (b) are coherent across the mid‐latitude North Pacific and appear to coincide with the normal seasonal evolution of synoptic storm activity in that region. In mid‐latitudes, the values of u³ _* calculated from (a) and (b) are 27 and 83%, respectively, of the value of u³ _* itself. Thus, a major fraction of the production of turbulent energy available for mixing in the upper layers of the ocean comes from synoptic disturbances with a period shorter than 10 days. Maps of the long‐term monthly mean wind stress curl are quite different in that the mean wind stress curl calculated from (a) is essentially negligible. However, the mean curl calculated from (b) closely resembles the pattern of total curl (c), but with a magnitude of only 41% of (c). Thus, synoptic disturbances with a period shorter than 10 days are also responsible for a significant fraction of the Ekman pumping of the ocean.

Future studies with these data will attempt to determine whether a relationship exists between synoptic storm activity, as measured by the parameters developed in this study, and large‐scale sea‐surface temperature anomalies.     

阿拉伯海热带气旋生成特征分析

利用印度气象局（India Meteorological Department,IMD）、国际气候管理最佳路径档案库（International Best Track Archive for Climate Stewardship,IBTrACS）提供的1982—2020年阿拉伯海热带气旋路径资料,美国国家环境预报中心（National Centers for Environmental Prediction,NCEP）再分析资料,对近39 a阿拉伯海热带气旋源地和路径特征、活跃区域、频数及气旋累积能量（accumulated cyclone energy,ACE）指数的季节特征和年际变化特征进行分析,并结合环境因素,说明其物理成因。结果表明：阿拉伯海热带气旋多发于10°~25°N,65°~75°E海域,5—6月、9—12月发生频数较高且强度较强,1—4月、7—8月发生频数较低且气旋近中心最大风速均小于35 kn;频数的季节变化主要受控于垂直风切变要素;阿拉伯海热带气旋发生频数和ACE近年有上升趋势,年际变化主要受控于海面温度（sea surface temperature,SST）和850 hPa相对湿度要素。     

A comparison of wind speed shears for frontal and undisturbed synoptic conditions

Wind speed shear parameters derived from data collected at a 481-m TV tower are computed for disturbed conditions (cold frontal passage), all undisturbed winds, undisturbed high winds, and undisturbed low winds. The exponential values of the wind speed power law for the case of disturbed conditions are found to be smaller than those for the case of all undisturbed winds, for practically all atmospheric stabilities; the largest exponential value (0.45) appears in the undisturbed low wind case at Richardson numbers near 0.25. The largest mean and standard deviations of the wind shear at the lowest layer (7–26 m) are 0.129 and 0.066 s ^-1, respectively, which occur for the high wind case at near-neutral stability. The most significant differences between the cases of disturbed and all undisturbed winds are that the former has much larger wind speeds and wind shear standard deviations in atmospherically-stable conditions.Work performed under the auspices of the U.S. Department of Energy.     

利用海-气-浪耦合模式（COAWST）对北印度洋一次强热带风暴“Fani”(2019)过程的数值模拟

利用最新研发的区域海-气-浪耦合数值模式(Coupled Ocean-Atmosphere-Wave-Sediment Transport,COAWST),对印度洋一次超强热带气旋“Fani”(2019)过程进行了数值模拟试验。“Fani”4月26日从热带印度洋面上的低压扰动中生成,在移动北上的过程中逐渐增强为强热带风暴,并于5月3日在印度登陆,造成了严重的破坏。利用COAWST耦合数值模式,在印度洋区域设计了数值模拟试验,分析了“Fani”发展增强到登陆时间段(4月30日12时—5月3日12时)大气物理量场、海洋表面温度及通量场,以及海浪参数的相应变化。通过与International Best Track Archive for Climate Stewardship (IBTrACS)的实测风暴路径、强度数据,以及欧洲中心ERA5再分析资料对比结果表明：模式模拟“Fani”的移动路径与观测较为接近,但是在强度模拟上偏弱;大气地表2 m温度场、潜热和感热通量与ERA5再分析资料存在一定偏差,但模拟的海表面温度场强度和特征与ERA5再分析资料都比较接近;在强烈的热带气旋与海洋相互作用...     

云迹风资料在热带气旋移向预报中的应用

利用云迹风并结合实测风,从涡度方程出发,对１９９８年南海和西太平洋的几个热带气候进行诊断计算。结果反映,涡度局地变率ｅ↓ζ／ｅｔ对热带气旋移动有指示意义,它既可批示热带气旋正常路径,在引导气流不明显的情况下,也能对气旋的转折路径有较明显的反映,气旋常朝涡度局地变率最在的方向移动。当热带气旋周围有数个ｅ↓ζ／ｅｔ中心时写和提交 矢量合成法确定了气旋的最大可能移动方向。此外,玷如引导气流一样,热带气旋     

A subtropical rainstorm associated with a tropical plume overAfrica and the Middle-East

Summary  Cloud bands that extend from the ITCZ along the subtropical jet toward the subtropics are known as ‘tropical plumes’. At times rainstorms develop at their subtropical edges. One such rainstorm swept eastern North Africa and the Middle East on 23–24 December 1988, with rainfall comparable with the annual averages there. This study examines the storm using the ECMWF initialized data together with surface observations and satellite imageries. The analysis indicates that the storm developed at the inflection region ahead of a pronounced trough in the subtropical jet, with which a mid-latitude trough was merged. Two ageostrophic effects taking place along the jet ahead of the trough contributed to the intensity of the rainstorm. One was associated with acceleration at the jet entrance, located at tropical latitudes, which contributed to the enhancement of both tropical convection and the southerly wind component, which enhanced the moisture tropical transport toward the subtropics. The second was the enhanced near-tropospheric divergence associated with positive vorticity advection at the inflection region itself. Since both effects have a quadratic dependence on wind speed, the observed jet speed, 50% larger than its average value, explains the observed divergence at the inflection point at the 200 hPa level, over 6 × 10⁻⁵ s⁻¹, and the vertical velocity at the 700 hPa level, about 10⁻¹ ms⁻¹. It is suggested here that the merging of a mid-latitude with the trough in the subtropical jet, with which the tropical plume is associated, is the cause for the intensification of the subtropical jet and hence of its related rainstorms. Received May 24, 2000     

台风的螺旋结构

本文从描写台风运动的动力方程组出发,讨论了台风的螺旋结构,给出了这种螺旋结构的图样,指出:(1)台风螺旋结构的形成是台风中重力惯性内波,特别是重力内波的作用所致,(2)台风螺旋结构的形式与波位相速紧密相关,北半球的曳式螺旋结构要求螺旋图样环绕台风眼逆时针且由中心向外运动,其向外运动的速度近于重力内波的传播速度,(3)台风的曳式螺旋倾角一般小于45°,而在台风边缘,倾角约为10°,螺旋臂间距在二旋臂结构中平均为220公里,而在四旋臂结构中平均为110公里,(4)台风螺旋波也有频散作用,在曳式螺旋中c_(gr)与c_r皆为正,且c_(gr)相似文献   

Impacts of the Lowest Model Level Height on Tropical Cyclone Intensity and Structure简

Variable thicknesses in the lowest half-ηmodel level （LML） are often used in atmospheric models to compute surface diagnostic fields such as surface latent and sensible heat fluxes.The effects of the LML on simulated tropical cyclone （TC）evolution were investigated in this study using the Weather Research and Forecasting （WRF） model.The results demonstrated notable influences of the LML on TC evolution when the LML was placed below 12 m.The TC intensification rate decreased progressively with a lowering of the LML,but its ultimate intensity change was relatively small.The maximum 10-m winds showed different behavior to minimum sea level pressure and azimuthally-averaged tangential winds,and thus the windpressure relationship was changed accordingly by varying the LML.The TC circulation was more contracted in association with a higher LML.Surface latent heat fluxes were enhanced greatly by elevating the LML,wherein the wind speed at the LML played a dominant role.The changes in the wind speed at the LML were dependent not only on their profile differences,but also the different heights they were taken from.Due to the enhanced surface heat fluxes,more intense latent heat release occurred in the eyewall,which boosted the storm＇s intensification.A higher LML tended to produce a stronger storm,and therefore the surface friction was reinforced,which in turn induced stronger boundary layer inflow together with increased diabatic heating.     

登陆热带气旋研究的进展

随着大气探测技术的发展 ,登陆热带气旋研究已经成为热带气旋研究中一个新的领域。新的探测技术能初步揭示出热带气旋登陆过程中发生的多种改变。近年来 ,国内外科学家实施了一系列外场科学试验 (Fieldscientificexperi ments) ,对登陆热带气旋进行探测和研究 ,旨在提高预报的准确率。登陆热带气旋研究内容包括 :海岸和内陆山脉地形影响 ,结构和强度变化 ,登陆热带气旋的暴雨强度和分布 ,大风强度和分布 ,风暴潮强度和范围 ,登陆热带气旋在陆上的维持机制 ,陆地涡旋的路径和入海加强 ,边界层结构 ,陆面过程和能量交换 ,变性过程等。研究采用外场科学试验与数值模拟相结合的方法。模拟或预报模式中使用同化资料尤其是卫星同化资料来构造初值场 ,取得较好结果。登陆热带气旋的研究目前正在展开 ,并取得了一些重要结果。研究表明 ,潜热释放和斜压位能释放是近海或登陆热带气旋加强或维持的两种主要能源。这两种能量可分别从水汽输送和热带气旋与中纬度环流的相互作用中获得。另外 ,陆面饱和湿地或水面的潜热输送、热带气旋与中尺度涡旋或热带云团的合并以及高空流出气流强辐散也对其加强和维持有利。世界气象组织的热带气象研究计划 (TMRP)正在组织对这一领域的总结和下一步的研究计划。这项研究将对预报和     

Boundary-layer wind structure in a landfalling tropical cyclone

In this study, a slab boundary layer model with a constant depth is used to analyze the boundary-layer wind structure in a landfalling tropical cyclone. Asymmetry is found in both the tangential and radial components of horizontal wind in the tropical cyclone boundary layer at landfall. For a steady tropical cyclone on a straight coastline at landfall, the magnitude of the radial component is greater in the offshoreflow side and the tangential component is greater over the sea, slightly offshore, therefore the greater total wind speed occurs in the offshore-flow side over the sea. The budget analysis suggests that： （1） a greater surface friction over land produces a greater inflow and the nonlinear effect advects the maximum inflow downstream, and （2） a smaller surface friction over the sea makes the decrease of the tangential wind component less than that over land. Moreover, the boundary layer wind structures in a tropical cyclone are related to the locations of the tropical cyclone relative to the coastline due to the different surface frictions. During tropical cyclone landfall, the impact of rough terrain on the cyclone increases, so the magnitude of the radial component of wind speed increases in the offshore-flow side and the tangential component outside the radius of maximum wind speed decreases gradually.     

热带气旋变性过程客观判别方法在“海马”变性过程中的应用

介绍了一种判别热带气旋变性过程的客观方法.此客观方法使用三个判别参数: 对流层低层热力非对称参数B、高低层的热成风、对变性过程进行诊断判别.此方法使用格点的数值预报产品,计算简单、使用方便,便于业务化.同时针对2004年影响我国的0421号热带风暴"海马",使用NCEP再分析资料对其变性过程进行了分析验证,结果表明:此客观方法所定义的三个判别参数B、、在"海马"的变性过程中有很好的指示作用.     

The Impact of Land-Surface Parameter Properties and Resolution on the Simulated Cloud-Topped Atmospheric Boundary Layer

A method to simulate characteristics of wind speed in the boundary layer of tropical cyclones in an idealized manner is developed and evaluated. The method can be used in a single-column modelling set-up with a planetary boundary-layer parametrization, or within large-eddy simulations (LES). The key step is to include terms in the horizontal velocity equations representing advection and centrifugal acceleration in tropical cyclones that occurs on scales larger than the domain size. Compared to other recently developed methods, which require two input parameters (a reference wind speed, and radius from the centre of a tropical cyclone) this new method also requires a third input parameter: the radial gradient of reference wind speed. With the new method, simulated wind profiles are similar to composite profiles from dropsonde observations; in contrast, a classic Ekman-type method tends to overpredict inflow-layer depth and magnitude, and two recently developed methods for tropical cyclone environments tend to overpredict near-surface wind speed. When used in LES, the new technique produces vertical profiles of total turbulent stress and estimated eddy viscosity that are similar to values determined from low-level aircraft flights in tropical cyclones. Temporal spectra from LES produce an inertial subrange for frequencies \(\gtrsim \)0.1 Hz, but only when the horizontal grid spacing \(\lesssim \)20 m.