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1.
Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations.  相似文献   

2.
The boundary layer wind tunnel at the Technische Universit?t München was tested for atmospheric boundary layer (ABL) simulations. The ABLs developing above rural, suburban, and urban terrains were reproduced using the Counihan method, i.e., castellated barrier wall, vortex generators, and a fetch of surface roughness elements. A series of flow-characteristic evaluations was performed to investigate the flow development and uniformity. Experimental results presented as mean velocity, turbulence intensity, integral length scale of turbulence, Reynolds stress, and power spectral density of velocity fluctuations were compared with the ESDU data and/or theoretical models. Generated ABL wind-tunnel simulations compare well with the rural, suburban, and urban ABLs. In the test section area used for experiments on structural models, the ABL simulation is developed and uniform. Results of this study indicate the boundary layer wind tunnel at the Technische Universit?t München can be successfully employed in a broad spectrum of engineering, environmental, and micrometeorological studies, where it is required to accurately reproduce ABL characteristics.  相似文献   

3.
Forcing relationships in steady, neutrally stratified atmospheric boundary-layer (ABL) flow are thoroughly analyzed. The ABL flow can be viewed as balanced between a forcing and a drag term. The drag term results from turbulent stress divergence, and above the ABL, both the drag and the forcing terms vanish. In computational wind engineering applications, the ABL flow is simulated not by directly specifying a forcing term in the ABL but by specifying boundary conditions for the simulation domain. Usually, these include the inflow boundary and the top boundary conditions. This ‘boundary-driven’ ABL flow is dynamically different from its real counterpart, and this is the major reason that the simulated boundary-driven ABL flow does not maintain horizontal homogeneity. Here, first a dynamical approach is proposed to develop a neutrally stratified equilibrium ABL flow. Computational fluid dynamics (CFD) software (Fluent 6.3) with the standard \(k\) – \(\varepsilon \) turbulence model is employed, and by applying a driving force profile, steady equilibrium ABL flows are simulated by the model. Profiles of wind speed and turbulent kinetic energy (TKE) derived using this approach are reasonable in comparison with the conventional logarithmic law and with observational data respectively. Secondly, the equilibrium ABL profiles apply as inflow conditions to simulate the boundary-driven ABL flow. Simulated properties between the inlet and the outlet sections across a fetch of 10 km are compared. Although profiles of wind speed, TKE, and its dissipation rate are consistently satisfactory under higher wind conditions, a deviation of TKE and its dissipation rate between the inlet and outlet are apparent (7–8 %) under lower wind-speed conditions (2 m s \(^{-1}\) at 10 m). Furthermore, the simulated surface stress systematically decreases in the downwind direction. A redistribution of the pressure field is also found in the simulation domain, which provides a different driving pattern from the realistic case in the ABL.  相似文献   

4.
A new quasi-analytical mixed-layer model is formulated describing the evolution of the convective atmospheric boundary layer (ABL) during cold-air outbreaks (CAO) over polar oceans downstream of the marginal sea-ice zones. The new model is superior to previous ones since it predicts not only temperature and mixed-layer height but also the height-averaged horizontal wind components. Results of the mixed-layer model are compared with dropsonde and aircraft observations carried out during several CAOs over the Fram Strait and also with results of a 3D non-hydrostatic (NH3D) model. It is shown that the mixed-layer model reproduces well the observed ABL height, temperature, low-level baroclinicity and its influence on the ABL wind speed. The mixed-layer model underestimates the observed ABL temperature only by about 10 %, most likely due to the neglect of condensation and subsidence. The comparison of the mixed-layer and NH3D model results shows good agreement with respect to wind speed including the formation of wind-speed maxima close to the ice edge. It is concluded that baroclinicity within the ABL governs the structure of the wind field while the baroclinicity above the ABL is important in reproducing the wind speed. It is shown that the baroclinicity in the ABL is strongest close to the ice edge and slowly decays further downwind. Analytical solutions demonstrate that the \(\mathrm{e}\)-folding distance of this decay is the same as for the decay of the difference between the surface temperature of open water and of the mixed-layer temperature. This distance characterizing cold-air mass transformation ranges from 450 to 850 km for high-latitude CAOs.  相似文献   

5.
复杂地形城市冬季边界层对气溶胶辐射效应的响应   总被引：6，自引：3，他引：6  
郑飞  张镭  朱江 《大气科学》2006,30(1):171-179
作者着眼于城市气溶胶辐射效应与大气边界层的相互作用问题,针对地形复杂的兰州市及周边地区,开发应用了WRF(Weather Research and Forecasting,天气研究和预报)模式,使之与包含了大气气溶胶辐射效应和气溶胶粒子扩散的综合大气边界层数值模式嵌套起来.通过个例分析,揭示了冬季气溶胶辐射效应对边界层结构的定量影响.主要特征为夜间气溶胶的长波辐射效应使地面附近的气温增高,增温幅度为0.1～0.3 K/h,使低空(25～300 m)大气层冷却,降温幅度为0.08～0.15 K/h,风速在150 m以下减小;白天气溶胶的短波辐射效应使地面层内明显增温,1 h内升温约0.5 K,增温最大值在混合层顶500～600 m高度.受增温影响,垂直风场和水平风场随之调整,风速在450 m以下增大约0.1 m/s左右,而在450 m以上风速减小0.1 m/s左右.  相似文献   

6.
The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the mixing-length concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited mixing-length already proposed for non-logarithmic wind profiles, while large errors characterize traditional mixing-length formulations.  相似文献   

7.
Reynolds-number dependence of flow fields within a modelled urban area was studied in a wind tunnel. We measured flow around a single model building and around model city blocks at various wind speeds, and studied Reynolds number indices more appropriate than the building Reynolds number. Our results led to the following conclusions. Firstly, the flow around the models in the wind tunnel was roughly divided into three parts according to the intensities of viscous stress and Reynolds stress as follows: (1) the flow in the vicinity of the ground or the surfaces of the model, where viscous stress became dominant under certain conditions; (2) the flow detached from the surfaces of the model, where Reynolds stress was always dominant; and (3) the flow around the separation bubble at the leading edge of the building model, where the influences of both viscous stress near the wall and the Reynolds stress in the separated boundary layer were mixed.Secondly, the critical Reynolds number of the flow in the modelled urban area could be defined by using both the roughness Reynolds number Rez0 (= z0u*/) and the dimensionless height z+ (= zu*/). Reynolds-number independence could be expected for whole flow fields in the modelled urban areas as long as the critical values of Rez0 and z+ were satisfied.  相似文献   

8.
Accurate predictions of turbulent characteristics in the atmospheric boundary layer (ABL) depends on understanding the effects of surface roughness on the spatial distribution of velocity, turbulence intensity, and turbulence length scales. Simulation of the ABL characteristics have been performed in a short test section length wind tunnel to determine the appropriate length scale factor for modeling, which ensures correct aeroelastic behavior of structural models for non-aerodynamic applications. The ABL characteristics have been simulated by using various configurations of passive devices such as vortex generators, air barriers, and slot in the test section floor which was extended into the contraction cone. Mean velocity and velocity fluctuations have been measured using a hot-wire anemometry system. Mean velocity, turbulence intensity, turbulence scale, and power spectral density of velocity fluctuations have been obtained from the experiments for various configuration of the passive devices. It is shown that the integral length scale factor can be controlled using various combinations of the passive devices.  相似文献   

9.
张恩才  杨修群 《气象科学》1996,16(3):206-214
本文利用１９７０至１９８９年共２０年的逐月平均的太平着区的表面风应力和海表温度距平的分析资料，检验了以前设计的热带太平洋和热带大气距平模式的模拟性能，通过使用两组风应力异常场即观测场和热带大气模式对观测海温响应所得的模拟场，重点分析了热带太平洋距平模式对风应力异常的响应特征，结果表明，本文海洋距平模式完全有能力再现ＥＮＳＯ循环折际变化性及其水平结构，且赤道中太平洋区域的低频风应力异常对于ＥＮＳＯ事  相似文献   

10.
水平风作用下雨滴水平速度的数值仿真   总被引：1，自引：0，他引：1  
为了研究雨滴在水平风作用下的水平移动情况,在分析水平风作用下雨滴受力情况的基础上,通过对曳力系数与雷诺数对应关系的研究,采用数值仿真的方法分别对海平面大气条件和不同大气条件下雨滴水平运动速度进行仿真.结果表明:当有水平风作用时,雨滴的水平运动速度不等于风的速度,而是随雨滴直径和水平风速的变化而变化;在水平风作用下,雨滴的水平移动速度可以在较短的时间内(一般小于15 s)达到一个稳定的值;在水平风速相同的情况下,气压越高、温度越低,雨滴达到平衡时的水平移动末速度相对越大,反之则越小.这些结论对基于图像采集原理的光学降水自动观测仪器进行雨滴图像拼合有重要的应用价值.  相似文献   

11.
Summary The dynamical effect of land surface heterogeneity on heat fluxes in the atmospheric boundary layer (ABL) is investigated using numerical simulations with a non-hydrostatic model over a wide range of grid resolutions. It is commonly assumed that mesoscale or dynamical fluxes associated with mesoscale and convective circulations simulated by a high-resolution model (subgrid (SG) model) on the subgrid scale of a climate model (large-scale (LS) model) represent additional processes in the ABL, which are not considered by the turbulence scheme of the LS-model, and which can be parameterized using the SG-model. The present study investigates the usefulness of this methodology for small-scale and large-scale idealized heterogeneities using a SG-model resolving mesoscale or even microscale circulations to compute the mesoscale fluxes on the scale of the LS-model. It is shown that the dynamical transports as derived from the SG-model should not be used to correct the parameterized turbulent fluxes of the LS-model. The reason is that the subgrid circulations simulated by the SG-model interact with the fields of wind and scalars in the ABL, which results in reduced turbulent fluxes in the ABL. Thus the methodology of previous studies to use mesoscale/dynamical fluxes for the correction of flux profiles simulated by climate models seems to be questionable.  相似文献   

12.
The wind finding equations currently used to retrieve horizontal winds from Global Positioning System dropwindsonde wind measurements are derived based on a point object model in which the drag coefficient of the dropwindsonde is assumed to be a constant. The wind tunnel tests performed as part of this study showed, however, that the dropwindsonde aerodynamic coefficients vary appreciably with angles of attack. To investigate the impact of this finding, the dropwindsonde motion in a pseudo-stochastic wind field has been simulated using a motion model more sophisticated than the point object model. The results showed that, although the constant drag coefficient assumption is not supported by the wind tunnel test results, the wind finding equations still correctly calculate both the mean and the turbulence intensity profiles. In addition, a revised method to calculate the vertical wind was proposed based on the derivation of the improved motion model, which enhanced the accuracy of vertical wind estimates by including the real-time dropwindsonde drag coefficient and the dropwindsonde vertical acceleration into calculation.  相似文献   

13.
深凹地形边界层风场与湍流场结构及扩散规律的数值研究   总被引：1，自引：0，他引：1  
建立了一个模拟小尺度复杂地形边界层平均流场和湍流场结构分布特征的三维非静力细网格高阶矩湍流闭合模式,并将此模式与一个拉格朗日粒子随机游动大气扩散数值模式相联结,以边界层气象模式的输出作为扩散模式的输入,构成了一个大气扩和模拟系统,成功地模拟了某地实际深凹露天矿区和理想凹坑地形两种情况下的平均流场、雷诺应力和湍流通量场,分析了其分布特征,并模拟了矿坑内污染物的散布规律  相似文献   

14.
A Single Column Model(SCM) for Global and Regional Atmospheric Prediction Enhanced System (GRAPES) is constructed for the purpose of evaluating physical process parameterizations.Two observational datasets including Wangara and the third Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study(GABLS-3) SCM field observations have been applied to evaluate this SCM.By these two numerical experiments,the GRAPES_SCM is verified to be correctly constructed.Furthermore, the interaction between the land surface process and atmospheric boundary layer(ABL) is discussed through the second experiment.It is found that CASE3(CoLM land surface scheme coupled with ABL scheme) simulates less sensible heat fluxes and smaller surface temperature which corresponds with its lower potential temperature at the bottom of the ABL.Moreover,CASE3 simulates turbulence that is weaker during the daytime and stronger during nighttime,corresponding with its wind speed at 200 m which is bigger during daytime and smaller during nighttime.However,they are generally opposite in CASE2(SLAB coupled with ABL).The initial profile of the water vapor mixing ratio is artificially increased by the experiment setup which results in the simulated water vapor mixing becoming wetter than actually observed.CASE1 (observed surface temperature taken as lower thermal forcing) and CASE2 have no soil moisture prediction and simulate a similar water vapor mixing ratio,while CASE3 has a soil moisture prediction and simulates wetter.It is also shown that the time step may affect the stabilization of the ABL when the vertical levels of the SCM are fixed.  相似文献   

15.
Systematic errors in the COLA R15 AGCM are analyzed by the SVD technique.In order toremove or reduce this kind of error source and reduce climate drift in coupled runs a way in whichthe wind stress anomalies simulated by an AGCM are reconstructed is proposed by using SVD anal-ysis.Experimental results show that not only wind stress anomalies simulated by an AGCM areobviously improved by reconstructed wind stress anomalies but also this reconstruction has a func-tion of a low-pass time filter,as a result,response of the ZC ocean model to reconstructed windstress anomalies is more realistic than that of simulated wind stress anomalies by an AGCM.Inthis paper,application of the hybrid couple ocean-atmosphere model is further discussed.  相似文献   

16.
华莉娟  俞永强  尹宝树 《大气科学》2010,34(6):1046-1058
热带印度洋偶极子 (Indian Ocean Dipole) 是印度洋海域内海洋和大气环流年际变化的主要特征模态之一, 在热带海气耦合系统中起到非常重要的作用。同热带太平洋的ENSO现象类似, 热带印度洋偶极子也呈现出显著的不对称性。本文利用中国科学院大气物理研究所发展的全球海洋环流模式, 在观测风应力距平的强迫下, 评估了模式对热带印度洋季节变化、 热带印度洋偶极子 (IOD) 模态及其不对称性的模拟能力, 并且通过数值试验分析了IOD模态不对称性特征及其对气候平均态的影响。对照观测资料, 模式较好地再现了热带印度洋SST在季风驱动下的季节变化特征。在年际时间尺度上, 模式不仅能够再现IOD指数的变化趋势, 而且可以成功模拟出IOD模态的空间分布特征, 即表层和次表层海温在西印度洋表现为正异常, 在东印度洋表现为负异常。可见, 对于热带印度洋而言, IOD模态主要是对风应力异常的响应。热带印度洋海温与Niño3.4指数的相关性分析表明, 模式能够模拟出超前热带太平洋ENSO现象2～4个月时海温的偶极子型分布, 但是不能模拟出滞后ENSO现象2个月左右的全海盆增暖模态, 可能是因为模式试验中没有考虑热通量年际异常的强迫。同时, 模式模拟的IOD模态具有同观测结果相类似的不对称性, 进一步的敏感性试验表明风应力的不对称性对偶极子指数的不对称性贡献较小, 次表层及以下海温的不对称性可能主要受到海洋内部非线性动力过程的影响。通过数值试验, 本文还发现热带印度洋海温的不对称性对气候平均态会有影响, 而这种不对称性长期积累后, 会导致上层热带印度洋温度层结趋于稳定状态。  相似文献   

17.
建立了一个农林复合带地区一维非静力大气边界层能量闭合模式，对1000m以下的大气边界层内的风、温、湿作了24h的预报，并对下垫面3种不同参数化方案(农作物、森林、无植被)的输出结果与实测值进行了分析和比较；同时通过敏感性试验，突出比较了农作物和森林下垫面对大气边界层垂直流场，湍流垂直交换和湍能的影响。结果表明，本模式能改善边界层风速、位温和湿度预报的模拟效果，下垫面植被对边界层气象要素大小和分布有显著的作用，对湍能垂直分布有一定影响。  相似文献   

18.
Miles' (1957) theory of wave generation is modified to include the effects of air turbulence. It is shown that the fluctuating wave-induced Reynolds stresses are important only within a critical layer surrounding the critical height where the mean wind speed matches the wave phase speed. When the critical height is such that the critical layer extends to the air-sea interface, the energy transfer from the wind to the wave is affected greatly by the turbulence.  相似文献   

19.
The temperature and wind profiles in abaroclinic atmospheric boundary layer (ABL) are investigated.Assuming stationary conditions, the turbulent state in the ABL forstable and neutral conditions is uniquely determined by the Rossbynumber, the external stratification parameter and two externalbaroclinic parameters. A simple two-layer baroclinic model isdeveloped. It consists of a surface layer (SL) and overlyingEkman-type layer. The system of dynamic and heat transfer equations isclosed using K-theory. In the SL the turbulent exchangecoefficient is consistent with the results of similarity theorywhile in the Ekman layer it is assumed constant. The universalfunctions in the resistance, heat and humidity transfer laws arededuced from the analytical solutions for the wind and temperatureprofiles. The solutions of the ABL resistance laws for theinternal ABL parameters, necessary for the calculations of the ABLprofiles, are approximated in terms of the external ABLparameters. Favourable agreement of model results with theavailable experimental data is demonstrated.  相似文献   

20.
Turbulence structures of high Reynolds number flow in the near-neutral atmospheric boundary layer (ABL) are investigated based on observations at Shionomisaki and Shigaraki, Japan. A Doppler sodar measured the vertical profiles of winds in the ABL. Using the integral wavelet transform for the time series of surface wind data, the pattern of a descending high-speed structure with large vertical extent (from the surface to more than 200-m level) is depicted from the Doppler sodar data. Essentially this structure is a specific type of coherent structure that has been previously shown in experiments on turbulent boundary-layer flows. Large-scale high-speed structures in the ABL are extracted using a long time scale (240 s) for the wavelet transform. The non-dimensional interval of time between structures is evaluated as 3.0–6.2 in most cases. These structures make a large contribution to downward momentum transfer in the surface layer. Quadrant analyses of the turbulent motion measured by the sonic anemometer (20-m height) suggest that the sweep motion (high-speed downward motion) plays a substantial role in the downward momentum transfer. In general, the contribution of sweep motions to the momentum flux is nearly equal to that of ejection motions (low-speed upward motions). This contribution of sweep motions is related to the large-scale high-speed structures.  相似文献   

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