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1.
The parameterization of the non-neutral atmospheric surface layer has been reexamined using the basic principles of small-scale energetics and thermodynamics. On the basis of this more complete treatment, the K- parameterization has been reformulated. It is found that the linear regression laws between fluxes and driving gradient forces of the turbulent heat and humidity exchanges in the surface layer can be derived in a much more comprehensive manner than by using the commonly used K-theory. With respect to stationary conditions and in the context of similarity concepts, a system of algebraic equations has been formulated which provides reasonable estimates of the distributions of the dimension-less rates of viscous energy dissipation as well as turbulent kinetic and thermal-diffusive energy transport as functions of the variable z/L. Quantitative calculations have been performed using the scaling height formulations of Takeuchi and Yokoyama, Prandtl, and von Kármán as closure conditions of the equations.  相似文献   

2.
The budget equations of turbulent kinetic energy and shear stress contain interaction terms of velocity-pressure and velocity-pressure gradient. These terms were estimated in the surface layer using the air pressure observed at the surface and wind velocity components over plant canopies. The magnitude of the pressure interaction terms was significantly large; it was not negligible compared with the production terms in each budget equation. The present results obtained over a rough surface also confirmed previous results that pressure terms play an important role in the turbulent kinetic energy budgets and the shear stress budget. The height dependency of nondimensional pressure terms versus (z - d)/z 0 was not clear.  相似文献   

3.
A new approach to investigations of the structure of the boundary layer above waves is discussed. The approach is based on direct numerical simulation of wave motions in the boundary layer produced by a moving curved surface. Model equations are derived, which are the Reynolds equations in a curvilinear nonstationary system of co-ordinates, evolution equations for turbulent kinetic energy, and Kolmogorov's approximate similarity formulae relating the coefficient of turbulent viscosity to the dissipation of turbulent energy; the length scale is assumed to grow linearly with increasing distance from the surface. Principles of constructing the model numerical scheme are described. Results are given of modelling the structure of the boundary layer above a nonsteady surface, which, in a general case, is a superposition of progressive waves with assigned dispersion relations and amplitudes. Mechanisms of energy and momentum transfer to the surface, effects of density stratification and energy structure in the boundary layer are studied. Merits and demerits of the approach are discussed.  相似文献   

4.
Aircraft, surface, upper air and satellite measurements have been used to observe the evolution and growth of the convective Marine Atmospheric Boundary Layer (MABL) offshore of North Carolina in close proximity to the Gulf Stream, during the intense cold air outbreak of 28 January 1986 and the moderate event of 12 February 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the ocean-atmosphere exchanges of surface turbulent sensible and latent heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed near-surface total heat fluxes were 1045 and 811 W·m–2 over the core of the Gulf Stream, for 28 January and 12 February 1986, respectively. The observed changes in the overlying air mass occurred almost instantaneously as the ambient flow traversed different underlying SST conditions.The turbulent structure showed a buoyancy-dominated MABL below approximately 0.8z/h. However, shear was also observed to be an important production term above 0.8z/h and below 0.1z/h for the 28 January 1986 event. Dissipation of turbulent kinetic energy was the dominant destruction term in the budgets, but vertical transport of energy was a strong contributor below 0.5z/h, above which this term became a source of turbulent energy. Additionally, the normalized standard deviations of the horizontal velocity components showed a near-equal contribution to the turbulence, while the vertical velocity components displayed the characteristic mid-layer maximum profile observed for a convective, well-mixed boundary layer.  相似文献   

5.
We have developed a simple, steady-state, one-dimensional second-order closure model to obtain continuous profiles of turbulent fluxes and mean concentrations of non-conserved scalars in a convective boundary layer without shear. As a basic tool we first set up a model for conserved species with standard parameterizations. This leads to formulations for profiles of the turbulent diffusivity and the ratio of temperature-scalar covariance to the flux of the passive scalar. The model is then extended to solving, in terms of profiles of mean concentrations and fluxes, the NO x –O3 triad problem. The chemical reactions involve one first-order reaction, the destruction of NO2 with decay time τ, and one second-order reaction, the destruction of NO and O3 with the reaction constant k. Since the fluxes of the sum concentrations of NO x = NO + NO2 and O3 + NO2 turn out to be constant throughout the boundary layer, the problem reduces to solving two differential equations for the concentration and the flux of NO2. The boundary conditions are the three surface fluxes and the fluxes at the top of the boundary layer, the last obtained from the entrainment velocity, and the concentration differences between the free troposphere and the top of the boundary layer. The equations are solved in a dimensionless form by using 1/(kτ) as the concentration unit, the depth h of the boundary layer as the length unit, the convective velocity scale w * as the velocity unit, and the surface temperature flux divided by w * as the temperature unit. Special care has been devoted to the inclusion of the scalar–scalar covariance between the concentrations of O3 and NO. Sample calculations show that the fluxes of the reactive species deviate significantly from those of non-reactive species. Further, the diffusivities, defined by minus the flux divided by the concentration gradient may become negative for reactive species in contrast to those of non-reactive species, which in the present model are never negative.  相似文献   

6.
A bulk model for the atmospheric planetary boundary layer   总被引：1，自引：0，他引：1  
The integrated momentum and thermodynamic equations through the planetary boundary layer (PBL) are solved numerically to predict the mean changes of wind and potential temperature from which surface fluxes are computed using bulk transfer coefficients of momentum and heat. The second part of the study involves a formulation and testing of a PBL height model based on the turbulent energy budget equation where turbulent fluxes of wind and heat are considered as the source of energy. The model exhibits capability of predicting the PBL height development for both stable and unstable regimes of observed conditions. Results of the model agree favourably with those of Deardorff's (1974a) and Tennekes' (1973) models in convective conditions.Contribution number 396.  相似文献   

7.
A one-dimensional numerical model based on the equations of mean motion and turbulent kinetic energy (TKE), with Delage's (1974) mixing-length parameterization has been used to simulate the mean and turbulent structure of the evolving stably stratified nocturnal boundary layer (NBL). The model also includes a predictive equation for the surface temperature and longwave radiational cooling effects.In the absence of advective and gravity wave effects, it is found that the model-simulated structure, after a few hours of evolution, could be ordered fairly well by a similarity scaling (u *0, *0, L 0, and h) based on surface fluxes and the NBL height. Simple expressions are suggested to describe the normalized profiles of momentum and heat fluxes, TKE, eddy-viscosity and energy dissipation. A good ordering of the same variables is also achieved by a local scaling (u *0, * and L) based on the height-dependent local fluxes. The normalized TKE, eddy viscosity and energy dissipation are unique functions of z/L and approach constant values as z/L , where L is the local Monin-Obukhov length. These constants are close to the values predicted for the surface layer as z/L , thus suggesting that the Monin-Obukhov similarity theory can be extended to the whole NBL, by using the local (height-dependent) scales in place of surface-layer scales. The observed NBL structure has been shown to follow local similarity (Nieuwstadt, 1984).  相似文献   

8.
Computations of the buoyantly unstable Ekman layer are performed at low Reynolds number. The turbulent fields are obtained directly by solving the three-dimensional time-dependent Navier-Stokes equations (using the Boussinesq approximation to account for buoyancy effects), and no turbulence model is needed. Two levels of heating are considered, one quite vigorous, the other more moderate. Statistics for the vigorously heated case are found to agree reasonably well with laboratory, field, and large-eddy simulation results, when Deardorff's mixed-layer scaling is used. No indication of large-scale longitudinal roll cells is found in this convection-dominated flow, for which the inversion height to Obukhov length scale ratio –z i /L *=26. However, when heating is more moderate (so that –z i /L *=2), evidence of coherent rolls is present. About 10% of the total turbulent kinetic energy and turbulent heat flux, and 20% of the Reynolds shear stress, are estimated to be a direct consequence of the observed cells.  相似文献   

9.
Ramp features in the turbulent scalar field are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. Although finer scale ramp-like shapes embedded within larger scale ramp-like shapes can readily be perceived in turbulent scalar traces, their presence has largely been overlooked in the literature. We demonstrate the signature of more than one ramp scale in structure functions of the turbulent scalar field measured from above bare ground and two types of short plant canopies, using structure-function time lags ranging in scale from isotropic to larger than the characteristic coherent structures. Spectral analysis of structure functions was used to characterize different scales of turbulent structures. By expanding structure function analysis to include two ramp scales, we characterized the intermittency, duration, and surface renewal flux contribution of the smallest (i.e., Scale One) and the dominant (i.e., Scale Two) coherent structure scales. The frequencies of the coherent structure scales increase with mean wind shear, implying that both Scale One and Scale Two are shear-driven. The embedded Scale One turbulent structure scale is ineffectual in the surface-layer energy and mass transport process. The new method reported here for obtaining surface renewal-based scalar exchange works well over bare ground and short canopies under unstable conditions, effectively eliminating the α calibration for these conditions and forming the foundation for analysis over taller and more complex surfaces.  相似文献   

10.
Neutrally buoyant atmospheric surface-layer flow over a thin vertical wall has been studied using a turbulence closure scheme designed specifically to address flow problems containing high shears. The turbulent flow model consists of a general solution of the time averaged, steady state, twodimensional Navier-Stokes equations, where theE- turbulence model has been used to close the system of equations. Model output compares favorably with measurements made in both a full-scale field study and in an atmospheric wind tunnel. In the simulation of flow over a solid wall, two recirculation eddies are produced. The smallest eddy is located windward of the wall with a separation point located atx/h=–0.8, and the largest is located in the lee of the wall atx/h=5.8. Immediately downwind of the wall top, the turbulent kinetic energy, the energy dissipation rate, and the momentum flux all reach a local maximum. These peak values generally maintain their height positionz/h=1.0, but decrease progressively downwind. The turbulent viscosity is strongly modified under the influence of the wall, with a local maximum forming in the lee of the wall top, and a local minimum forming at a heightz/h=2.0 above the lee recirculation eddy. The surface momentum flux reduction due to the presence of the wall begins atx/h=–10.0. Minimum zero fluxes occur at the surface separation points, and a local peak in momentum flux is produced at the centers of each recirculation eddy. Downwind of the wall, the modeled surface flux reaches an equilibrium at roughlyx/h=30.0.  相似文献   

11.
The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, turbulent kinetic energy, turbulent exchange coefficient, local turbulent length scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers.The numerical results show that the temperature inversion in the atmosphere and density jump in the ocean have strong influences on turbulent structure [especially on the turbulent exchange coefficient (TEC) and turbulent kinetic energy (TKE)] and on air-sea interaction characteristics. Maxima of TKE and TEC strongly decrease with increasing strength of the inversion layer, and they disappear for strong inversions in the atmosphere. Certain strengths (density differences between the upper and the lower layers) of the density-jump layer in the ocean (2 0.1 g/cm3) produce double maxima in TEC-profiles and TKE-profiles in the ocean. The magnitudes of air-sea interaction characteristics such as geostrophic drag coefficient, and surface drift current increase with increasing strength of the density-jump layer in the ocean. The density-jump layer plays the role of a barrier that limits vertical mixing in the ocean. The numerical results agree well with available observed data and accepted quantitive understanding of the influences of a temperature inversion layer and a density-jump layer on the interacting atmospheric and oceanic boundary layers.  相似文献   

12.
We report on a novel approach for the Reynolds-averaged Navier-Stokes (RANS) modelling of the neutral atmospheric boundary layer (ABL), using the standard k-ek-{\varepsilon} turbulence model. A new inlet condition for turbulent kinetic energy is analytically derived from the solution of the k-ek-{\varepsilon} model transport equations, resulting in a consistent set of fully developed inlet conditions for the neutral ABL. A modification of the standard k-ek-{\varepsilon} model is also employed to ensure consistency between the inlet conditions and the turbulence model. In particular, the turbulence model constant C μ is generalized as a location-dependent parameter, and a source term is introduced in the transport equation for the turbulent dissipation rate. The application of the proposed methodology to cases involving obstacles in the flow is made possible through the implementation of an algorithm, which automatically switches the turbulence model formulation when going from the region where the ABL is undisturbed to the region directly affected by the building. Finally, the model is completed with a slightly modified version of the Richards and Hoxey rough-wall boundary condition. The methodology is implemented and tested in the commercial code Ansys Fluent 12.1. Results are presented for a neutral boundary layer over flat terrain and for the flow around a single building immersed in an ABL.  相似文献   

13.
应用含湍流频散效应的近地面层的运动方程求解了不同层结下的风速廓线，着重分析了湍流频散效应对近地面层平稳运动的影响。分析指出：湍流的频散效应对经典的幂律廓线一对数修正，该修正在不稳定层结时比稳定层时明显；利用相似理论也得到了该常数。  相似文献   

14.
The atmospheric boundary layer (ABL) model of Weng and Taylor with E−ℓ turbulence closure is applied to simulate the one-dimensional stably stratified ABL. The model has been run for nine hours from specified initial wind, potential temperature and turbulent kinetic energy profiles, and with a specified cooling rate applied at the surface. Different runs are conducted for different cooling rates, geostrophic winds and surface roughnesses. The results are discussed and compared with other models, large-eddy simulations and published field data.  相似文献   

15.
A three-dimensional numerical meteorological model is used to perform large-eddy simulations of the upslope flow circulation over a periodic ridge-valley terrain. The subgrid-scale quantities are modelled using a prognostic turbulence kinetic energy (TKE) scheme, with a grid that has a constant horizontal resolution of 50 m and is stretched along the vertical direction. To account for the grid anisotropy, a modified subgrid length scale is used. To allow for the response of the surface fluxes to the valley-flow circulation, the soil surface temperature is imposed and the surface heat and momentum fluxes are computed based on Monin–Obukhov similarity theory. The model is designed with a symmetrical geometry using periodic boundary conditions in both the x and y directions. Two cases are simulated to study the influence of along-valley geostrophic wind forcing with different intensities. The presence of the orography introduces numerous complexities both in the mean properties of the flow and in the turbulent features, even for the idealized symmetric geometry. Classical definitions for the height of the planetary boundary layer (PBL) are revisited and redefined to capture the complex structure of the boundary layer. Analysis of first- and second-moment statistics, along with TKE budget, highlights the different structure of the PBL at different regions of the domain.  相似文献   

16.
Specification of the eddy exchange coefficients is perhaps one of the most difficult problems in the numerical modeling of the planetary boundary layer. These coefficients have been computed from finite-difference analogs to analytical expressions associated with surface boundary-layer similarity theory, which is based on observations in an equilibrium surface layer. This procedure leads to erroneous results in the region above the surface layer and in a non-equilibrium surface layer. In addition, differencing problems arise in regions of small vertical wind shear. A new turbulence transport model has been obtained through the closure procedures for the transport equations of the Reynolds stress and the turbulent length scale. The new approach could be used to calculate Reynolds stresses and eddy exchange coefficients throughout a non-neutral planetary boundary layer under non-equilibrium conditions.  相似文献   

17.
Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection. At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values of λ/z i (λ is the dominant horizontal scale of the turbulent eddies, z i is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL.  相似文献   

18.
Downward fluxes of turbulent kinetic energy have been frequently observed in the air layer just above plant canopies. In order to investigate the mechanism for such downward transport, analysis of observational data is attempted. Height-dependency of turbulent kinetic energy flux and turbulence statistics including higher order moments is represented as a function of a non-dimensional height z/H, where z is an observational height and H an average height of plant canopies. Downward fluxes and non-Gaussianity of wind velocity fluctuations are predominant just above plant canopies and decrease with increasing height. The downward flux is closely related to the high intensity of turbulence and the non-Gaussianity of wind velocity fluctuations, especially with a positive skewness in the longitudinal wind and a negative skewness in the vertical wind. The analysis method of conditional sampling and averaging is applied to the present observations. The results show that the predominance of the intermittent inrush phase over the intermittent ejection phase leads to the above-mentioned non-Gaussianity. Finally, a simple explanation is given in order to interpret the turbulent flow structure in the air layer near the plant canopies, which is associated with the downward energy transport process.  相似文献   

19.
The influence of an idealized moving wavy surface on the overlying airflow is investigated using direct numerical simulations (DNS). In the present simulations, the bulk Reynolds number is Re = 8000 (; where U0 is the forcing velocity of the flow, h the height of the domain and v the kinematic viscosity) and the phase speed of the imposed waves relative to the friction velocity, i.e., the wave age varies from very slow to fast waves. The wave signal is clearly present in the airflow up to at least 0.15λ (where λ is the wave length) and is present up to higher levels for faster waves. In the kinetic energy budgets, pressure transport is mainly of importance for slow waves. For fast waves, viscous transport and turbulent transport dominate near the surface. Kinetic energy budgets for the wave and turbulent perturbations show a non-negligible transport of turbulent kinetic energy directed from turbulence to the wave perturbation in the airflow. The wave-turbulent energy transport depends on the size, tilt, and phase of the wave-induced part of the turbulent Reynolds stresses.According to the DNS data, slow waves are more efficient in generating isotropic turbulence than fast waves.Despite the differences in wave-shape as well as in Reynolds number between the idealized direct numerical simulations and the atmosphere, there are intriguing similarities in the turbulence structure. Important information about the turbulence above waves in the atmosphere can be obtained from DNS—the data must, however, be interpreted with care.  相似文献   

20.
Our focus is the time evolution of the turbulent kinetic energy for decaying turbulence in the convective boundary layer. The theoretical model with buoyancy and inertial transfer terms has been extended by a source term due to mechanical energy and validated against large-eddy simulation data. The mechanical effects in a boundary layer of height z i at a convective surface-layer height z = 0.05z i are significant in the time evolution of the vertical component of the spectrum, i.e. they enhance the decay time scale by more than an order of magnitude. Our findings suggest that shear effects seem to feedback to eddies with smaller wavenumbers, preserving the original shape of the spectrum, and preventing the spectrum from shifting towards shorter wavelengths. This occurs in the case where thermal effects only are considered.  相似文献   

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