热带气旋海棠(2005)强度数值模拟试验

通过设计台风追随自移动嵌套网格、考虑海洋飞沫作用改进MRF边界层参数化方案、设计垂直-倾斜对流参数化方案等对MM5 V3.7中尺度模式进行改进,以NCEP逐日再分析资料为初始场,对2005年第5号热带气旋"海棠"进行了数值模拟试验。结果表明:通过设计自移动嵌套网格解决了热带气旋业务数值预报模式中细网格区域大小和位置难于确定问题,通过提高模式分辨率能大幅增强数值模式对热带气旋强度的预报能力;考虑海洋飞沫作用后,热带气旋范围内低层热通量明显增强,从而使模式大气低层增温和增湿明显,有利热带气旋内对流发生发展;考虑倾斜对流不稳定作用后,使倾斜对流有效位能得到释放,进一步提高了模式对热带气旋内部物理过程的描述能力。通过改进MM5模式,有效提高了热带气旋强度的数值模拟能力。     

The influence of air-sea interaction on the development and motion of a tropical cyclone: Numerical experiments with a triply nested model

Summary Tropical cyclone (TC)—ocena feedbacks are studied using a coupled tropical cyclone-ocean model consisting of an eightlayer triply-nested movable grid model of a TC and a three-layer primitive equation ocean model. The numerical results indicate that the TC-ocean interaction influences intensities, structures, and the trajectories of tropical cyclones. Two possible mechanisms, barotropic and baroclinic, influencing TC tracks under TC-ocean interaction are suggested. The barotropic mechanism is related to the changes of the vertically averaged TC structure, induced by the TC-ocean coupling. The baroclinic mechanism is related to the asymmetry of the condensation heating within the TC caused by the asymmetry of heat and moisture fluxes at the sea surface. This asymmetry arises due to the asymmetry in sea surface cooling relative to the storm center. The experiments indicate that the influence of TC-ocean interaction on the TC tracks is the greatest for the case of a zero background flow. In the case of a non-zero background flow the sensitivity of storm tracks to the coupling with the ocean decreases. It is found that the influence of the ocean coupling on the TC track is quite sensitive to the method of convective heating parameterization in the TC model. The TC-ocean interaction also results in a change of the amount and spatial distribution of precipitation.     

气溶胶对热带气旋强度及电过程影响的数值模拟研究

将详细的气溶胶活化方案和非感应起电参数化方案耦合到WRF模式的Morrison微物理方案中,在此基础上模拟了理想的热带气旋个例,讨论了气溶胶对热带气旋强度及电活动的影响。研究发现气溶胶对热带气旋强度和电活动的影响在不同发展阶段是不同的。在热带气旋发展阶段,气溶胶增加使云内的云滴数浓度增加,云滴尺度降低,抑制了暖雨过程,未降落的小云滴粒子上升到冻结层之上,冻结形成了更多的冰相粒子,冻结过程释放潜热,激发云系对流发展,使热带气旋强度增强,电活动更为剧烈。而在热带气旋成熟阶段,污染个例中的冰相粒子降落,形成更多的降水,降水粒子在下落过程中对上升气流产生拖曳作用,抑制对流强度。降水粒子在海平面蒸发吸收了大量潜热,使该区域温度降低,这也阻碍了外部暖湿能量向内输送,抑制了对流发展,从而使热带气旋强度降低。而由于污染个例中的冰粒子较少,参与起电过程的冰粒子减少,热带气旋的电活动强度降低。     

Current understanding of tropical cyclone structure and intensity changes – a review

Summary Current understanding of tropical cyclone (TC) structure and intensity changes has been reviewed in this article. Recent studies in this area tend to focus on two issues: (1) what factors determine the maximum potential intensity (MPI) that a TC can achieve given the thermodynamic state of the atmosphere and the ocean? and (2) what factors prevent the TCs from reaching their MPIs? Although the MPI theories appear mature, recent studies of the so-called superintensity pose a potential challenge. It is notable that the maximum intensities reached by real TCs in all ocean basins are generally lower than those inferred from the theoretical MPI, indicating that internal dynamics and external forcing from environmental flow prohibit the TC intensification most and limit the TC intensity. It remains to be seen whether such factors can be included in improved MPI approaches.Among many limiting factors, the unfavorable environmental conditions, especially the vertical shear-induced asymmetry in the inner core region and the cooling of sea surface due to the oceanic upwelling under the eyewall region, have been postulated as the primary impediment to a TC reaching its MPI. However, recent studies show that the mesoscale processes, which create asymmetries in the TC core region, play key roles in TC structure and intensity changes. These include the inner and outer spiral rainbands, convectively coupled vortex Rossby waves, eyewall cycles, and embedded mesovortices in TC circulation. It is also through these inner core processes that the external environmental flow affects the TC structure and intensity changes. It is proposed that future research be focused on improving the understanding of how the eyewall processes respond to all external forcing and affect the TC structure and intensity changes. Rapid TC intensity changes (both strengthening and weakening) are believed to involve complex interactions between different scales and to be worthy of future research.The boundary-layer processes are crucial to TC formation, maintenance, and decaying. Significant progress has been made to deduce the drag coefficient on high wind conditions from the measurements of boundary layer winds in the vicinity of hurricane eyewalls by Global Positioning System (GPS) dropsondes. This breakthrough can lead to reduction of the uncertainties in the calculation of surface fluxes, thus improving TC intensity forecast by numerical weather prediction models.     

A mechanism for long-term changes of Atlantic tropical cyclone intensity

Although previous studies reported upward trends in the basin-wide average lifetime, annual frequency, proportion of intense hurricanes and annual accumulated power dissipation index of Atlantic tropical cyclones (TCs) over the past 30?years, the basin-wide intensity did not increase significantly with the rising sea surface temperature (SST). Observational analysis and numerical simulation conducted in this study suggest that Sahel rainfall is the key to understanding of the long-term change of Atlantic TC intensity. The long-term changes of the basin-wide TC intensity are generally associated with variations in Sahara air layer (SAL) activity and vertical wind shear in the main development region (MDR), both of which are highly correlated with Sahel rainfall. The drying Sahel corresponds to an equatorward shift in the African easterly jet and African easterly wave activity, introducing the SAL to lower latitudes and increasing the MDR vertical wind shear. As a result, Atlantic TCs are more vulnerable to the suppressing effects of the SAL and vertical wind shear. Since the SST warming, especially in the tropical Indian Ocean, is a dominant factor for the Sahel drying that occurred over the past 30?years, it is suggested that the remote effect of SST warming is important for the long-term change of Atlantic TC intensity. Although influence of the AMO warm phase that started in the early 1990s alone can provide a favorable condition for TC intensification, its influence may have been offset by the influence of the ongoing SST warming, particularly in the Indian Ocean. As a result, there was no significant trend observed in the basin-wide average and peak intensity of Atlantic TCs.     

Development of an eye-wall like structure in a tropical cyclone model simulation

Structural changes during the intensification of a tropical storm into a hurricane in a numerical simulation are examined. A 10 layer primitive equation model that employs a horizontal grid spacing of 20 km over 4400 × 4400 km area is integrated. An elongated band in vertical motion over the storm area intensifies slowly during the first few hours. In the upper troposphere high pressures arise due to condensational heating. Between 8–12 h strong outflow winds develop in the upper troposphere due to the increased pressure gradients. Strong divergence occurs in the outflow wind region, and a large increase in the vertical motion, condensational heating and intensification rate of the storm ensues. Between 12–24 h the elongated band of the storm stage transforms into an eye-wall like structure, and the tropical storm intensifies into a hurricane. Regions with negative moist potential vorticity appear in the high troposphere. Widening of area of condensation and slanting of the convergence area occurs with height in the high level negative moist potential vorticity regions. Results suggest that the formation of anvil clouds in some cases may be due to the development of slantwise convection on the outer periphery of a hurricane's eye-wall.     

TRMM/TMI资料在热带气旋强度估计中的应用研究

利用2007—2009年热带降雨测量卫星（TRMM）的微波成像仪（TMI）观测到的亮温资料,计算9个通道（10、19、37、85 GHz的水平和垂直极化通道及21 GHz的垂直极化通道）的亮温和极化修正温度（PCT）在不同范围内的最大值、最小值、平均值和区域阈值与热带气旋强度之间的关系。结果表明：亮温信息可较好地反映热带气旋的强度,单个参数与热带气旋最大风速的相关性最好可达到0.83,线性拟合的均方根误差接近业务误差;低频通道的亮温相对于高频通道可更好地估计海上热带气旋强度;位于台风中心0.5°～1.5°度范围之间的亮温与气旋强度的相关性较好,圆形区域的相关性好于圆环区域;对于位于海上的热带气旋,区域亮温的最小值与热带气旋强度的关系最好;低频通道（除10 GHz外）,阈值位于260～280 K区间的亮温与热带气旋强度的相关性较好。     

A scheme for estimating tropical cyclone intensity using AMSU-A data

Brightness temperature anomalies measured by the Advanced Microwave Sounding Unit （AMSU） on the National Oceanic and Atmospheric Administration （NOAA） polar-orbiting series are suited to estimate tropical cyclone （TC） intensity by virtue of their ability to assess changes in tropospheric warm core struc-ture in the presence of clouds. Analysis of the measurements from different satellites shows that the variable horizontal resolution of the instrument has significant effects on the observed brightness temperature anoma-lies. With the aim to decrease these effects on TC intensity estimation more easily and effectively, a new simple correction algorithm, which is related to the product of the brightness temperature gradient near the TC center and the size of the field-of-view （FOV） observing the TC center, is proposed to modify the observed anomalies. Without other measurements, the comparison shows that the performance of the new algorithm is better than that of the traditional, physically-based algorithm. Furthermore, based on the correction algorithm, a new scheme, in which the brightness temperature anomalies at 31.4 GHz and 89 GHz accounting for precipitation effects are directly used as the predictors with those at 54.94 GHz and 55.5 GHz, is developed to estimate TC intensity in the western North Pacific basin. The collocated AMSU-A observations from NOAA-16 with the best track （BT） intensity data from the Japan Meteorological Agency （JMA） in 2002-2003 and in 2004 are used respectively to develop and validate regression coefficients. For the independent validation dataset, the scheme yields 8.4 hPa of the root mean square error and 6.6 hPa of the mean absolute error. For the 81 collocated cases in the western North Pacific basin and for the 24 collocated cases in the Atlantic basin, compared to the BT data, the standard deviations of the estimation differences of the results are 15% and 11% less than those of the CIMSS （Cooperative Institute for Meteorological Satellite Studies, Univ     

Ensemble Forecasting of Tropical Cyclone Motion Using a Baroclinic Model

The purpose of this study is to investigate the effectiveness of two different ensemble forecasting (EF) techniques–the lagged-averaged forecast (LAF) and the breeding of growing modes (BGM). In the BGM experiments, the vortex and the environment are perturbed separately (named BGMV and BGME). Tropical cyclone (TC) motions in two difficult situations are studied: a large vortex interacting with its environment, and an apparent binary interaction. The former is Typhoon Yancy and the latter involves Typhoon Ed and super Typhoon Flo, all occurring during the Tropical Cyclone Motion Experiment TCM- 90. The model used is the baroclinic model of the University of New South Wales. The lateral boundary tendencies are computed from atmospheric analysis data. Only the relative skill of the ensemble forecast mean over the control run is used to evaluate the effectiveness of the EF methods, although the EF technique is also used to quantify forecast uncertainty in some studies. In the case of Yancy, the ensemble mean forecasts of each of the three methodologies are better than that of the control, with LAF being the best. The mean track of the LAF is close to the best track, and it predicts landfall over Taiwan. The improvements in LAF and the full BGM where both the environment and vortex are perturbed suggest the importance of combining the perturbation of the vortex and environment when the interaction between the two is appreciable. In the binary interaction case of Ed and Flo, the forecasts of Ed appear to be insensitive to perturbations of the environment and/or the vortex, which apparently results from erroneous forecasts by the model of the interaction between the subtropical ridge and Ed, as well as from the interaction between the two typhoons, thus reducing the effectiveness of the EF technique. This conclusion is reached through sensitivity experiments on the domain of the model and by adding or eliminating certain features in the model atmosphere. Nevertheless, the forecast tracks in some of the cases are improved over that of the control. On the other hand, the EF technique has little impact on the forecasts of Flo because the control forecast is already very close to the best track. The study provides a basis for the future development of the EF technique. The limitations of this study are also addressed. For example, the above results are based on a small sample, and the study is actually a simulation, which is different than operational forecasting. Further tests of these EF techniques are proposed.     

热带气旋生成过程的中尺度涡旋活动数值模拟

热带气旋生成过程中包含不同尺度环流及其相互作用。为此,本文将热带气旋生成数值模拟的起点提前到模拟中尺度涡旋(MCV)的生成,从而利用高分辨率数值试验结果,对热带气旋过程中的不同尺度涡旋活动进行分析。模式首先模拟了季风涡旋的东南侧增强的西南气流中出现低形变旋转性扰动,随着扰动的旋转性增强,中层出现水平尺度为200 km左右的MCV。在扰动区内的不同高度上还发现10~20 km尺度不等的中γ气旋性涡旋扰动,其中部分涡旋扰动具有热塔的特征,中γ气旋性涡旋扰动在MCV的旋转环境内不断组织化,低层气旋性涡旋扰动的分布比中层更加集中。模拟表明这些较小尺度的气旋性中尺度涡旋扰动对热带气旋的生成有重要作用。     

热带气旋结构和强度变化研究进展

热带气旋结构和强度变化的物理过程和影响因子十分复杂,其登陆过程中海-陆-气三者之间的复杂作用常使其结构和强度发生突变,给热带气旋预报带来较大困难。近年来外场科学试验及数值模拟等科学方法都取得较大改进,在此基础上对热带气旋结构和强度研究取得了新的进展,如热带气旋强度突变及不同下垫面对热带气旋强度变化的影响等。但对热带气旋非对称结构对其强度的影响及热带气旋内部结构变化与其强度之间的联系等问题的研究仍不全面。本文通过查阅国内外相关研究文献,从环境气流、下垫面及内部结构3个主要方面,总结影响热带气旋结构和强度变化的主要因子,以期为改进热带气旋结构和强度的预报方法提供有意义的理论依据。     

Improved tropical cyclone intensity and intensity spread prediction in bifurcation situations

Bifurcation or bi-modal tropical cyclone intensity forecasts may arise due to uncertainty in the timing of formation, timing and magnitude of rapid intensification periods, or track forecast uncertainty leading to landfall or non-landfall or leading to interaction with warm- or cold-ocean eddies. An objective technique is developed and tested to detect these intensity bifurcation situations in our weighted-analog intensity (WANI) forecasts that are based on the 10 best historical analogs to the Joint Typhoon Warning Center (JTWC) official track forecasts. About 19% of the overall sample of 1136 WANI forecasts in the western North Pacific during the 2010–2012 seasons met the criteria for a substantial intensity bifurcation situation. Using a hierarchical clustering technique, two clusters of the 10 best analogs are defined and separate WANI forecasts and intensity spreads are calculated for the two clusters. If an always perfect selection of the correct cluster WANI forecast of each bifurcation situation is made, a substantial improvement in the intensity mean absolute errors is achieved relative to the original WANI forecasts based on all 10 of the best analogs. These perfect-cluster selection WANI forecasts have smaller bias errors and are more highly correlated with the verifying intensities at all forecast intervals through 120 h. Without further bias correction and calibration, the cluster WANI intensity spreads are under-determined as the Probability of Detections are smaller than the desired 68%. Four examples of WANI cluster predictions of intensity bifurcation situations are provided to illustrate how a correct choice of the intensity forecast and the intensity spread can be the basis for improved warnings of the threat from western North Pacific tropical cyclones.     

一次热带气旋过程对出梅影响的数值模拟研究

采用中尺度数值模式WRF对2001年江淮梅雨期热带气旋(Tropical Cyclone,TC)进行数值模拟,研究TC"飞燕"对梅雨结束过程的影响。结果表明,模式能够较好地模拟出"飞燕"的路径与强度演变,并能够较为精确地再现梅雨后期的降水强度和落区。对比分析消除"飞燕"影响的敏感性数值试验结果发现,"飞燕"影响梅雨的机制在于:随着其向东北向移动,西太平洋副热带高压北抬,东亚高、低空急流强度均有所减弱,低空切变辐合变弱,梅雨锋强度减弱,使得来自西南方向的水汽输送减少,造成假相当位温梯度和湿位涡倾斜项减小,垂直上升运动明显削弱,降水中断,导致梅雨结束。而消除"飞燕"影响后,梅雨后期降水仍然较多,梅雨期将有所延长。     

Evaluation of long-term trends in tropical cyclone intensity forecasts

Summary The National Hurricane Center and Joint Typhoon Warning Center operational tropical cyclone intensity forecasts for the three major northern hemisphere tropical cyclone basins (Atlantic, eastern North Pacific, and western North Pacific) for the past two decades are examined for long-term trends. Results show that there has been some marginal improvement in the mean absolute error at 24 and 48 h for the Atlantic and at 72 h for the east and west Pacific. A new metric that measures the percent variance of the observed intensity changes that is reduced by the forecast (variance reduction, VR) is defined to help account for inter-annual variability in forecast difficulty. Results show that there have been significant improvements in the VR of the official forecasts in the Atlantic, and some marginal improvement in the other two basins. The VR of the intensity guidance models was also examined. The improvement in the VR is due to the implementation of advanced statistical intensity prediction models and the operational version of the GFDL hurricane model in the mid-1990s. The skill of the operational intensity forecasts for the 5-year period ending in 2005 was determined by comparing the errors to those from simple statistical models with input from climatology and persistence. The intensity forecasts had significant skill out to 96 h in the Atlantic and out to 72 h in the east and west Pacific. The intensity forecasts are also compared to the operational track forecasts. The skill was comparable at 12 h, but the track forecasts were 2 to 5 times more skillful by 72 h. The track and intensity forecast error trends for the two-decade period were also compared. Results showed that the percentage track forecast improvement was almost an order of magnitude larger than that for intensity, indicating that intensity forecasting still has much room for improvement.     

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.     

热带太平洋地区海气系统的耦合振荡

本文讨论了东太平洋赤道海温和太平洋月平均云量距平的关系,结果表明:(1)东太平洋赤道海温距平和中太平洋赤道云量距平有很好的正相关,而与东、西太平洋赤道云量距平有很好负相关。所以太平洋赤道上空应该存在二个距平的东西向环流。(2)云和海温存在周期为34—38个月的耦合振荡,我们提出了云-辐射-海温机制来作解释。(3)在东太平洋海温暖水月的前12个月到后6个月期间,东太平洋赤道云量是负距平的(即偏少),这表明此期间可能是大气在影响海洋,亦即Walker环流影响海温变化,而不是海温影响了Walker 环流。海温和Walker环流是相互作用的,这种相互作用组成了大气和海洋之间的一种耦合振荡。(4)东太平洋海温、中太平洋云和北半球中、西太平洋信风亦有很好的相关。     

Sensitivity of tropical cyclone intensity and structure to vertical resolution in WRF

In this paper the impacts of vertical resolution on the simulations of Typhoon Talim (2005) are examined using the Weather Research and Forecasting (WRF) model, with cumulus parameterization scheme representing the cumulus convection implicitly. It is shown that the tropical cyclone (TC) track has little sensitivity to vertical resolution, whereas the TC intensity and structure are highly sensitive to vertical resolution. It is partly determined by the sensitivity of the planetary boundary layer (and the surface layer) and the cumulus convection processes to vertical resolution. Increasing vertical resolution in the lower layer could strengthen the TC effectively. Increasing vertical resolution in the upper layer is also beneficial for the storm intensification, but to a lesser degree. In contrast, improving the midlevel resolution may cause the convergence of environmental air, which inhibits the TC intensification. The results also show that the impacts of vertical resolution on features of the TC structure, such as the tangential winds, secondary circulations and the evolution of the warm-core structure, are consistent with the impacts on the TC intensity. It is suggested that in the simulations of TCs, the vertical levels should be distributed properly rather than the more the better, with higher vertical resolution being expected both in the lower and upper layer, while the middle layer should not hold too many levels.     

Numerical simulations of the tropical air-sea planetary boundary layer

A one-dimensional numerical model of the planetary boundary layer was used to investigate thermal and kinetic energy budgets. The simulation experiments were based on two sets of data. The first set was based on a ‘typical’ June with climatological data extracted for the oceanic region slightly northeast of Barbados. The second set used data from the third phase of project BOMEX, for approximately the same area and time of year as the first set. Comparison with observations of three simulated elements (viz., sea surface temperature and wind and humidity at 6 m) which are important in determining the near-interface energy transports shows that:

1. the model is capable of realistic simulations of both ‘typical’ conditions, and conditions for a specific four-day period;
2. the model is capable of realistically simulating the differences between prevailing values of these parameters in the two cases (‘typical’ and specific four-day period).

The simulated interface fluxes are those of incoming and outgoing short- and long-wave radiation; transmitted radiation at -0.5 m in the ocean, sensible heat transfer into the ocean and air, and latent heat flux of evaporation. Comparison with observational analyses shows that the diurnal variations in net radiation and heat storage in the mixed layer are realistically simulated. The simulated values of evaporation are consistent with other estimates for both ‘typical’ conditions and specific conditions during this four-day period. The rate of heat storage varies between +51 and -37 percent of the diurnal maximum incoming radiation, and the evaporation varies between +16% and -13% of this term. The non-dimensional transfer coefficients (C _D, C_T, C_q) computed from the model show general agreement with the coefficients calculated from observations in the simulated region (Pondet al., 1971). The simulated vertical profiles of temperature are in general agreement with observed profiles, except in the uppermost portions of the atmospheric boundary layer where deviations of approximately 1.5C occur. Simulated vertical profiles of wind speed are generally consistent with observed profiles, with the largest deviations appearing to be of the order of 0.5 m s^-1. Simulated vertical profiles of the eddy fluxes of sensible heat, water vapor, and momentum are generally consistent with Bunker's (1970) aircraft-based measurements of these quantities. The time averages of these simulated profiles show regular decreases with height, while simulated profiles for specific hours of the day show intermediate maxima and minima, which are also seen in the measured profiles. The vertically integrated kinetic energy budgets of the modelled atmospheric layer are presented through the four terms of the kinetic energy budget, viz., the upper and the lower boundary drags, dissipation, and potential-to-kinetic conversion. The dominant terms in the atmospheric energy budgets are the production and dissipation terms, with kinetic energy being exported both to the overlying atmospheric layer and to the underlying oceanic layer at rates of about 2 to 6% of the production, respectively. Comparisons between the climatological and BOMEX simulations are presented. The vertically integrated humidity budgets are presented for the two simulation experiments. Under ‘typical’ conditions, the humidity budget reveals an upper boundary flux of about +29% of the lower boundary flux with the vertically integrated advective flux being -59% of the lower flux. For the specific four-day simulation, the upper boundary flux and advection are about +28 and -70%, respectively, of the lower boundary flux.