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1.
Long-Term Mean Wind Profiles Based on Similarity Theory   总被引：1，自引：1，他引：0  
We provide general forms for long-term mean wind profiles from similarity-based wind profiles, beginning with a probabilistic adaptation of Monin–Obukhov similarity theory. We develop an analytical formulation for the stability distributions prevailing in the atmospheric surface layer, which in turn facilitates the derivation of a long-term mean wind profile based on Monin–Obukhov similarity theory. The modelled stability distributions exhibit good agreement with measurements from sites having different local conditions. The long-term wind profile formulation is further extended to include the influence of the depth of the atmospheric boundary layer (h), which becomes relevant for heights above h/3, and the resultant long-term ‘tall’ profile form also matches observations.  相似文献   

2.
We present measurements from 2006 of the marine wind speed profile at a site located 18 km from the west coast of Denmark in the North Sea. Measurements from mast-mounted cup anemometers up to a height of 45 m are extended to 161 m using LiDAR observations. Atmospheric turbulent flux measurements performed in 2004 with a sonic anemometer are compared to a bulk Richardson number formulation of the atmospheric stability. This is used to classify the LiDAR/cup wind speed profiles into atmospheric stability classes. The observations are compared to a simplified model for the wind speed profile that accounts for the effect of the boundary-layer height. For unstable and neutral atmospheric conditions the boundary-layer height could be neglected, whereas for stable conditions it is comparable to the measuring heights and therefore essential to include. It is interesting to note that, although it is derived from a different physical approach, the simplified wind speed profile conforms to the traditional expressions of the surface layer when the effect of the boundary-layer height is neglected.  相似文献   

3.
The mean flow profile within and above a tall canopy is well known to violate the standard boundary-layer flux–gradient relationships. Here we present a theory for the flow profile that is comprised of a canopy model coupled to a modified surface-layer model. The coupling between the two components and the modifications to the surface-layer profiles are formulated through the mixing layer analogy for the flow at a canopy top. This analogy provides an additional length scale—the vorticity thickness—upon which the flow just above the canopy, within the so-called roughness sublayer, depends. A natural form for the vertical profiles within the roughness sublayer follows that overcomes problems with many earlier forms in the literature. Predictions of the mean flow profiles are shown to match observations over a range of canopy types and stabilities. The unified theory predicts that key parameters, such as the displacement height and roughness length, have a significant dependence on the boundary-layer stability. Assuming one of these parameters a priori leads to the incorrect variation with stability of the others and incorrect predictions of the mean wind speed profile. The roughness sublayer has a greater impact on the mean wind speed in stable than unstable conditions. The presence of a roughness sublayer also allows the surface to exert a greater drag on the boundary layer for an equivalent value of the near-surface wind speed than would otherwise occur. This characteristic would alter predictions of the evolution of the boundary layer and surface states if included within numerical weather prediction models.  相似文献   

4.
Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50–80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200–300 m and considering only wind speeds above 3 m s?1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale (L MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L MBL , for applied use is suggested, based on measurements from the two sites.  相似文献   

5.
Estimates from semiempirical models that characterize surface heat flux, mixing depth, and profiles of temperature, wind, and turbulence are compared with observations from atmospheric field studies conducted in Colorado, Illinois, Indiana, and Minnesota. Sodar observations are compared with tower measurements at the Colorado site, for wind and turbulence profiles. The median surface heat flux, as calculated using surface-layer flux-profile relationships and an energy budget model, was consistently overestimated by 20 to 80%. Several mixing-depth models were evaluated: (1) integration of the hourly surface heat flux and friction velocity, (2) solving for the time rate of change of profiles of virtual potential temperature, and (3) an interpolation scheme used by the U.S. Environmental Protection Agency in regulatory dispersion models. For the late afternoon, 80 to 90% of the estimates from the first and third models were within 40% of the observed values. For the morning hours after sunrise, all were less accurate. Temperature estimates from surface-layer flux-profile relationships compared well with observations within the mixed layer, but were too low for the inversion layer aloft. Wind profiles were derived using surface-layer flux-profile relationships, a windprofile power-law based on Pasquill stability category, and sodar measurements. The sodar measurements were superior to both types of model estimates. Turbulence profiles were derived from sodar measurements and from semiempirical similarity relationships based on mixing depth and Obukhov length. The scatter in the comparisons with the sodar observations is twice that seen in the comparisons with empirical profile relationships. Overall, it appears that uncertainty of as low as 20 to 30% in the characterization of the diffusion meteorology is the exception rather than the rule.On assignment from the National Oceanic and Atmospheric Administration, U. S. Department of Commerce.Disclaimer: Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.  相似文献   

6.
We present an analysis of data from a nearly 1-year measurement campaign performed at Høvsøre, Denmark, a coastal farmland area where the terrain is flat. Within the easterly sector upstream of the site, the terrain is nearly homogenous. This topography and conditions provide a good basis for the analysis of vertical wind-speed profiles under a wide range of atmospheric stability, turbulence, and forcing conditions. One of the objectives of the campaign was to serve as a benchmark for flow over flat terrain models. The observations consist of combined wind lidar and sonic anemometer measurements at a meteorological mast. The sonic measurements cover the first 100 m and the wind lidar measures above 100 m every 50 m in the vertical. Results of the analysis of observations of the horizontal wind-speed components in the range 10–1200 m and surface turbulence fluxes are illustrated in detail, combined with forcing conditions derived from mesoscale model simulations. Ten different cases are presented. The observed wind profiles approach well the simulated gradient and geostrophic winds close to the simulated boundary-layer height during both barotropic and baroclinic conditions, respectively, except for a low-level jet case, as expected. The simulated winds are also presented for completeness and show good agreement with the measurements, generally underpredicting the turning of the wind in both barotropic and baroclinic cases.  相似文献   

7.
The basic numerical air-sea boundary-layer model described in Pandolfo (1969a, b) was varied to produce a set of models with differing atmospheric boundary-layer formulas, four of which are discussed here. Model I is the basic model itself, with stability and sea-state dependent eddy viscosity, conductivity and diffusivity which may, in certain ranges ofRi, be unequal. This model is applied on a relatively fine grid. Model II, applied on the same grid, uses formulas which yield equal eddy conductivity, diffusivity, and viscosity. The calculated eddy coefficients depend only on the height and wind shear. Model III uses the same exchange coefficient formulas as Model II. However, the surface-layer eddy flux in Model III is calculated by assuming that logarithmic profiles of the transported variables are present in this layer. Model IV is the same as Model III in these respects, but employs a relatively coarse vertical grid. This model, therefore, includes boundary layer formulas most like those conventionally used in large scale atmospheric models (e.g. Miyakoda, 1969).The four models were integrated numerically with identical inputs of initial, boundary, and auxiliary data prepared from observations made over the eastern half of the BOMEX observational area during June 21–25, 1969.Models I and IV are, in general, in better agreement with each other than either is with Model II. This is true for the model-generated upper and lower boundary fluxes of mean momentum and latent heat; and for the internal boundary layer production of mean kinetic energy by the cross-isobaric flow component. Model I agrees, on balance, about as well with Model IV as does Model III. The solutions for Models I, III, and IV are also, in general, more consistent with observed data, viz. 5-day average temperature profiles in the layer from the surface to 1000 meters, and 5-day averages of sea surface temperature and of surface-layer atmospheric humidity. Solutions for Model I are in better overall agreement with the observed data, and with the average observed surface-layer wind.The results show that, under the limitations implicit in these preliminary experiments, accurate simulations of observed data are possible with boundary-layer formulas of the type used in Model IV, and even more accurate simulation with the modest refinements represented by Model I. Piecemeal imposition of such refinements could, however, lead to models, like Model II, with significantly different energetic properties and less simulative accuracy. Specifically, the results support the speculation (Miyakodaet al., 1969) that the shallowness of the simulated Trades noted in some large-scale models is due to deficiencies in the boundary-layer eddy stress formulations used.  相似文献   

8.
The Weibull distribution is commonly used to describe climatological wind-speed distributions in the atmospheric boundary layer. While vertical profiles of mean wind speed in the atmospheric boundary layer have received significant attention, the variation of the shape of the wind distribution with height is less understood. Previously we derived a probabilistic model based on similarity theory for calculating the effects of stability and planetary boundary-layer depth upon long-term mean wind profiles. However, some applications (e.g. wind energy estimation) require the Weibull shape parameter (k), as well as mean wind speed. Towards the aim of improving predictions of the Weibull- \(k\) profile, we develop expressions for the profile of long-term variance of wind speed, including a method extending our probabilistic wind-profile theory; together these two profiles lead to a profile of Weibull-shape parameter. Further, an alternate model for the vertical profile of Weibull shape parameter is made, improving upon a basis set forth by Wieringa (Boundary-Layer Meteorol, 1989, Vol. 47, 85–110), and connecting with a newly-corrected corollary of the perturbed geostrophic-drag theory of Troen and Petersen (European Wind Atlas, 1989, Risø National Laboratory, Roskilde). Comparing the models for Weibull-k profiles, a new interpretation and explanation is given for the vertical variation of the shape of wind-speed distributions. Results of the modelling are shown for a number of sites, with a discussion of the models’ efficacy and applicability. The latter includes a comparative evaluation of Wieringa-type empirical models and perturbed-geostrophic forms with regard to surface-layer behaviour, as well as for heights where climatological wind-speed variability is not dominated by surface effects.  相似文献   

9.
Data from low-level soundings over Cambridge, U.S.A. were selected on the basis of an Ekman-like variation of the wind vector with altitude combined with evidence of a barotropic atmosphere. The method of geostrophic departure was used to determine the shear-stress distribution. The analysis yields the dimensionless properties of the barotropic Ekman layer under neutral and stable stratification. Some important results include: the geostrophic drag coefficient displays no dependence on the degree of static stability; the dimensionless height of the boundary layer decreases with increasing stability in agreement with the prediction of Zilitinkevich; the properties of the urban surface layer, where the roughness elements are multistory buildings, show no dependence on atmospheric stability under the moderate wind conditions which display the Ekman-like wind profile; and the directions of the horizontal shear stress and the vertical derivative of the velocity vector usually tend to be parallel only near the surface layer. Values of the two constants of the Rossby number similarity theory are found for the neutral barotropic Ekman layer at a surface Rossby number equal to 2 × 105. The implications of the work with respect to wind-tunnel simulation of the flow over models of urban areas are discussed.  相似文献   

10.
Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared imagery and sonic anemometer measurements were obtained during the boundary layer late afternoon and sunset turbulence (BLLAST) experimental campaign. Temporal turbulence data in the surface-layer are then analyzed jointly with spatial surface-temperature imagery. The surface-temperature structures (identified using surface-temperature fluctuations) are strongly linked to atmospheric turbulence as manifested in several findings. The surface-temperature coherent structures move at an advection speed similar to the upper surface-layer or mixed-layer wind speed, with a decreasing trend with increase in stability. Also, with increasing instability the streamwise surface-temperature structure size decreases and the structures become more circular. The sequencing of surface- and air-temperature patterns is further examined through conditional averaging. Surface heating causes the initiation of warm ejection events followed by cold sweep events that result in surface cooling. The ejection events occur about 25 % of the time, but account for 60–70 % of the total sensible heat flux and cause fluctuations of up to 30 % in the ground heat flux. Cross-correlation analysis between air and surface temperature confirms the validity of a scalar footprint model.  相似文献   

11.
Simultaneous measurements of the instantaneous values of absolute temperatureat seven heights within the lower 36 m of the atmospheric boundary layer underdifferent stability conditions were carried out, accompanied by measurements ofthe wind velocity components at two levels and of solar radiation flux at the surface.The data obtained allow one to investigate individual convective cells known ascoherent structures (CS). Outside the CS, i.e., during quiet periods, an instanttemperature profile is in close agreement with the dry-adiabatic lapse rate, butwithin CS the temperature changes much faster with height, and the shape ofthe profile varies significantly.A method was developed to transform temperature records from sensors atseveral heights into an other form, namely, into temporal variations of theheights of isothermal surfaces. Since coherent structures were found to advectwith the mean wind velocity, these temporal height variations may be transformedinto the spatial ones, i.e., into the xoz-plane section of the temperature field.In such a pictorial presentation coherent structures look like asymmetric columnsof heat, penetrating the whole atmospheric surface layer.Coherent structures also exist in the stable stratified surface layer, but they have aninverse asymmetry and occupy only the lower several metres. Wavelike activitydominates in the upper part of the stable surface layer.The characteristic time of surface-layer adjustment to the rapid changes of solarradiation (due to cloud shadows or cloud gaps) was found to be on the order ofone minute. Such a time interval is required for coherent structure to reach the topof surface layer.  相似文献   

12.
Validation of ECMWF and NCEP–NCAR Reanalysis Data in Antarctica   总被引：3，自引：0，他引：3  
The European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis (ERA-40) and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) ECMWF (ERA-40) and NCEP-NCAR reanalysis data were compared with Antarctic station observations, including surface-layer and upper-layer atmospheric observations, on intraseasonal and interannual timescales. At the interannual timescale, atmospheric pressure at different height levels in the ERA-40 data are in better agre...  相似文献   

13.
In large-eddy simulations (LES) of the atmospheric boundary layer (ABL), near-surface models are often used to supplement subgrid-scale (SGS) turbulent stresses when a major fraction of the energetic scales within the surface layer cannot be resolved with the temporal and spatial resolution at hand. In this study, we investigate the performance of both dynamic and non-dynamic eddy viscosity models coupled with near-surface models in simulations of a neutrally stratified ABL. Two near-surface models that are commonly used in LES of the atmospheric boundary layer are considered. Additionally, a hybrid Reynolds- averaged/LES eddy viscosity model is presented, which uses Prandtl’s mixing length model in the vicinity of the surface, and blends in with the dynamic Smagorinsky model away from the surface. Present simulations show that significant portions of the modelled turbulent stresses are generated by the near-surface models, and they play a dominant role in capturing the expected logarithmic wind profile. Visualizations of the instantaneous vorticity field reveal that flow structures in the vicinity of the surface depend on the choice of the near-surface model. Among the three near-surface models studied, the hybrid eddy viscosity model gives the closest agreement with the logarithmic wind profile in the surface layer. It is also observed that high levels of resolved turbulence stresses can be maintained with the so-called canopy stress model while producing good agreement with the logarithmic wind profile.  相似文献   

14.
Orthonormal wavelet expansions are applied to surface-layer measurements of vertical wind speed under various atmospheric, stability conditions. The orthonormal wavelet transform allows for the unfolding of these measurements into space and scale simultaneously to reveal the large intermittent behavior in space for the turbulent production wavenumbers. Both Fourier and wavelet power spectra indicated the existence of a –1 power law for the vertical velocity measurements at the production wavenumbers. The –1 power law in the turbulent production range was derived from surface-layer similarity theory. A dimensionless skewness, structure function is applied to the wavelet decomposed vertical velocity field to trace the destruction of the shear-or buoyancy-induced anisotropy under various stability conditions. The structure skewness function revealed shear- or buoyancy-induced eddy asymmetry dependence on stability at each scale within the –1 power-law wavenumber range with more isotropy during propagation from smaller to larger wavenumbers. The asymmetry of these events at the turbulent production wavenumbers appeared very localized in space, as well as in scale, and could be described with a simple eddy-overturning model. It is demonstrated that the wavelet transform is suitable for such analysis.  相似文献   

15.
A simple algorithm is proposed in order to transform routine surface wind speed observations near the coast to a wind at the height of the equilibrium planetary boundary layer as well as to any other height over a relatively flat coastal region. The model is based on the well known internal boundary layer (IBL) concept, Monin-Obukhov similarity theory and the resistance law, and describes the effects of the roughness transition from sea to land as well as the effect of stability on the shape of the profiles and the IBL growth. The required input weather data are no more than surface wind speed, air temperature and total cloud cover. Satisfactory agreement was found between measurements at Hellinikon airport and estimations made with the scheme. The introduction of a transition layer above the IBL did not improve the agreement to any significant extent. Mean values of the estimated wind differed by less than 1 m s -1 from the observed ones, a difference within the accuracy of the reported rawinsonde values. The rms error varied in the range of 17–22% of the observed average value, giving the best agreement under unstable conditions. The correlation coefficient between the observed and the estimated values of the wind, at the height of the equilibrium planetary boundary layer, ranged between 0.74 and 0.90.  相似文献   

16.
An Analytical Model for Mean Wind Profiles in Sparse Canopies   总被引：2，自引：2，他引：0  
Existing analytical models for mean wind profiles within canopies are applicable only in dense canopy scenarios, where all momentum is absorbed by canopy elements and, hence, the effect of the ground on turbulent mixing is not important. Here, we propose a new analytical model that can simulate mean wind profiles within sparse canopies under neutral conditions. The model adopts a linearized canopy-drag parametrization and a first-order turbulence closure scheme taking into account the effects of both the ground and canopy elements on turbulent mixing. The resulting wind profile within a sparser canopy appears to be more like a logarithmic form, with the no-slip condition at the ground being satisfied. The analytical solution converges exactly to the standard surface-layer logarithmic wind profile in the case of zero canopy density (i.e., no-canopy scenario) and tends to be an exponential wind profile for a dense canopy; this feature is unique compared with existing analytical models for canopy wind profiles. Results from the new model are in good agreement with those from laboratory experiments and numerical simulations.  相似文献   

17.
Diurnal variations of the vertical profiles of wind and temperature have been surveyed, and the diffusivity and the dimensionless gradient function in the atmospheric boundary layer have been estimated. Even in the middle of the atmospheric boundary layer (e.g., below a height of 442 m), the vertical wind profile normalized by the surface friction velocity has approximately a universal profile function different from that in the surface boundary layer. Under strong stability conditions, the dimensionless gradient function has a value of about 9.  相似文献   

18.
Statistical analysis techniques based on multiplicative cascades are investigated for use with surface-layer wind data sets collected in the atmospheric boundary layer over flat farm land. The data were found to exhibit multiscaling statistics, allowing the surface-layer winds to be simulated with the use of multiplicative random cascades. The study found evidence that, for the surface-layer at least, these cascade models (andhence the methods of multifractal analysis) should be applied in separate ways to the microscale inertial range, and the mesoscale. This is at odds with the view found in the existing literature, which proposes a `universal multifractal' model to replace the widely held view that there exists separate microscale, mesoscale and synoptic scales for which the processes governing each are different. At least two separate ranges of scaling are suggested for surface-layer wind data, corresponding to the microscale inertial range and the mesoscale. For the case of the mesoscale range, a self-similar distribution of weighting factors was found for the wind speed data themselves, rather than for an intermediate (dissipation) field, as is required for themicroscale data.  相似文献   

19.
Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night   总被引：1，自引：1，他引：0  
Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres. In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points, if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed, the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater than 3 m s−1, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that, in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak. Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time need to be carefully treated.  相似文献   

20.
The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation. Idealized diurnal cycle simulations of the atmospheric boundary layer are performed with the geophysical flow solver EULAG over both homogeneous and heterogeneous terrain. Under homogeneous conditions, the diurnal cycle significantly affects the low-level wind shear and atmospheric turbulence. A strong vertical wind shear and veering with height occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas atmospheric turbulence is much larger in the convective boundary layer and in the evening boundary layer. The increased shear under heterogeneous conditions changes these wind characteristics, counteracting the formation of the night-time Ekman spiral. The convective, stable, evening, and morning regimes of the atmospheric boundary layer over a homogeneous surface as well as the convective and stable regimes over a heterogeneous surface are used to study the flow in a wind-turbine wake. Synchronized turbulent inflow data from the idealized atmospheric boundary-layer simulations with periodic horizontal boundary conditions are applied to the wind-turbine simulations with open streamwise boundary conditions. The resulting wake is strongly influenced by the stability of the atmosphere. In both cases, the flow in the wake recovers more rapidly under convective conditions during the day than under stable conditions at night. The simulated wakes produced for the night-time situation completely differ between heterogeneous and homogeneous surface conditions. The wake characteristics of the transitional periods are influenced by the flow regime prior to the transition. Furthermore, there are different wake deflections over the height of the rotor, which reflect the incoming wind direction.  相似文献   

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