The results of computer simulation for an atmospheric boundary layer (ABL) structure, inversion development and scattering of a passive impurity from a continuous source during a critical meteorological period (weak wind, stable atmospheric stratification) above a thermally heterogeneous surface are presented with condideration for urban roughness phenomenon. The influence of urban roughness is not explicitly resolved, but its effect on the wind and temperature fields is parameterized. The Euler model of atmospheric diffusion is based on a three-parameter RANS turbulence model and includes differential transport equations for the mean concentration and the correlation between turbulent concentration and temperature fluctuations. A completely explicit anisotropic algebraic model of gradient type is formulated for the turbulent impurity flow vector. The numerical simulation results show clearly the penetration of impurity into the inversion layer above the mixed layer and the gravitational propagation of impurity inside this layer.
An explicit anisotropic algebraic model of the Reynolds stresses and the turbulent heat flux is tested in a neutrally stratified and stably stratified atmospheric boundary layer (ABL) over a homogeneous rough surface. To construct the algebraic model a three-parameter turbulence model is used, with the help of which the efficiency of eddy mixing of momentum in a neutrally stratified ABL and the efficiency of eddy mixing momentum and heat in a stable stratified atmospheric boundary layer is studied. The being version of the algebraic model is based on the physical principles of the RANS (Reynolds Average Navier Stokes) approximation for describing of stratified turbulence. The model includes the effect of gravitational waves, which allows taking into account the momentum maintenance under strong stability conditions. A comparison of the calculation results with the observational data and other numerical models available in the literature shows that the turbulence model is capable of reproducing the most important structural features for both the neutrally stratified ABL and stably stratified ABL and shows good agreement with the results of LES modeling.
Two types of vertical structure of turbulence are identified in the atmospheric boundary layer. One is the “traditional” boundary layer, in which turbulence is generated near the surface and is transported upward, in contrast to the second type, where turbulence is transported downward to the surface from a source aloft in the boundary layer. The latter can be called as the “upside-down boundary layer”. In this case, the turbulence can be transferred downward to the surface, e.g., as random process. In this study, the boundary layer is upside down if turbulence increases with height and the transport of turbulence energy is downward toward the surface. Presumably, a similar structure can develop within the flow of air over a surface with large-scale roughness (city), when the horizontal temperature gradient between the heated air above the city and the air on the adjacent colder surface of the environment generates turbulent thermal circulation. If this turbulence generation is substantially larger than the generation of turbulence due to surface processes (wind shear), then the vertical transport of turbulence is downward toward the surface. In the present study, the vertical profiles of wind speed, temperature and turbulent quantities in the nocturnal urban boundary layer, calculated by means of the improved меsoscale model, are analyzed to understand the vertical structure of this layer.
Modeling turbulence is an important object of environmental sciences for describing an essential turbulent transport of heat and momentum in the boundary layer of the atmosphere. The many turbulence model used in the simulation of flows in the environment, based on the concept of eddy viscosity, and buoyancy effects are often included in the expression for the turbulent fluxes through empirical functions, based on the similarity theory of Monin-Obukhov, fair, strictly speaking, only in the surface layer. Furthermore, significant progress has been made in recent years in the development broader than standard hypothesis turbulent viscosity models for the eddy diffusivity momentum and heat, as result of the recording of differential equations for the Reynolds stresses and vector turbulent heat flux in a weaklyequilibrium approximation, which neglects advection and the diffusion of certain dimensionless quantities. Explicit algebraic model turbulent Reynolds stresses and heat flux vector for the planetary boundary layer is used in the stable atmospheric boundary layer, the upper troposphere, and the lower stratosphere. The present algebraic model of turbulence built on physical principles RANS (Reynolds Average Navier Stokes) approach for stratified turbulence uses three prognostic equations and shows correct reproduction of the main characteristics of the stably stratified boundary layer and evaluated the vertical eddy diffusivities of momentum and heat in the upper troposphere and the lower stratosphere.
Modeling turbulence is an important object of environmental sciences for describing an essential turbulent transport of heat and momentum in the boundary layer of the atmosphere. The many turbulence model used in the simulation of flows in the environment, based on the concept of eddy viscosity, and buoyancy effects are often included in the expression for the turbulent fluxes through empirical functions, based on the similarity theory of Monin-Obukhov, fair, strictly speaking, only in the surface layer. Furthermore, significant progress has been made in recent years in the development broader than standard hypothesis turbulent viscosity models for the eddy diffusivity momentum and heat, as a result of the recording of differential equations for the Reynolds stresses and vector turbulent heat flux in a weakly-equilibrium approximation, which neglects advection and the diffusion of certain dimensionless quantities. Explicit algebraic model turbulent Reynolds stresses and heat flux vector for the planetary boundary layer is tested in the neutral atmospheric boundary layer over the homogeneous rough surface. The present algebraic model of turbulence built on physical principles RANS (Reynolds Average Navier Stokes) approach for stratified turbulence uses three prognostic equations and shows correct reproduction of the main characteristics of the Ekman neutral ABL: the components average of wind velocity, the angle of wind turn, turbulence statistics. Test calculations shows that this turbulence model can be used for the purposeful researches of the atmospheric boundary layer for solving of various problems of the environment.
The nonlocality of the mechanism of turbulent heat transfer in the atmospheric boundary layer over a rough surface manifests itself in the form of bounded areas of countergradient heat transfer, which are diagnosed from analysis of balance items in the transport equation for the variance of temperature fluctuations and from calculation of the coefficients of turbulent momentum and heat transfer invoking the model of gradient diffusion. It is shown that countergradient heat transfer in local regions is caused by turbulent diffusion or by the term of the divergence of triple correlation in the balance equation for the temperature variance.
The RANS high close approach for the turbulent fluxes of momentum, heat and mass for simulating of the circulation structure and dispersion pollutant over the urban heat island in a stably stratified environment under nearly calm conditions is formulated. The turbulent fluxes of momentum − uiuj , heat −uiθ and mass −uic in this approach determined from the gradient diffusion type models with the turbulent kinetic energy (TKE), its spectral consumption (or dissipation), the temperature variance and the covariance for cθ as parameters which are obtained from transport equations. Such the RANS approach minimizes difficulties in the turbulent transport modeling in a stably stratified environment and reduces efforts needed for the numerical implementation of the numerical model. The simulation results demonstrates that the three-four equations RANS approach is able to predict the structure of turbulent circulation flow induced by the heat island that is in good agreement with the experimental data.
The results obtained from both atmospheric and laboratory experiment and from LES data show that, in the stably stratified flows of the atmospheric boundary layer, turbulent mixing occurs at gradient Richardson number that significantly exceed one: the inverse turbulent Prandtl number decreases with an increase in the thermal stability. The decreasing trend of the inverse turbulent Prandtl number is reproduced in a stably stratified planetary boundary layer in agreement with measurement data with aid of the high closure RANS turbulence scheme, which takes into account the influence of internal gravity waves on the eddy mixing of momentum and heat. Applicability of such RANS turbulence approach for the estimate of eddy diffusivities of momentum and heat in the upper troposphere and lower stratosphere also examined. It is concluded that the high closure RANS turbulence scheme shows the good agreement with the direct measurement data of eddy diffusivities for momentum and heat in the upper troposphere and lower stratosphere during clear-air conditions.
The modified three-parameter model of turbulence for a thermally stratified atmospheric boundary layer (ABL) is developed. The
model is based on tensor-invariant parameterizations for the pressure-strain and pressure-temperature correlations that are more complete
than the parameterizations used in the Mellor-Yamada model. The turbulent momentum and heat fluxes are calculated with explicit
algebraic models obtained with the aid of symbol algebra from the transport equations for momentum and heat fluxes in the approximation
of weakly equilibrium turbulence. The three-parameter model of thermally stratified turbulence is employed to obtain
closed form algebraic expressions for the fluxes. The effects of urban roughness are parameterized. Results of 2D computational test
of a 24-hour of the ABL evolution are presented. Comparison of the computed results with the available observational data and other
numerical models shows that the proposed model is able to reproduce both the most important structural features of the turbulence in
an urban canopy layer near the urbanized PBL surface and the effect of urban roughness on a global structure of the fields of wind and
temperature over a city.
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