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1.
Andrey Sogachev 《Boundary-Layer Meteorology》2009,130(3):423-435
The note presents a rational approach to modelling the source/sink due to vegetation or buoyancy effects that appear in the
turbulent kinetic energy, E, equation and a supplementary equation for a length-scale determining variable, φ, when two-equation closure is applied to canopy and atmospheric boundary-layer flows. The approach implements only standard
model coefficients C
φ1 and C
φ2 in the production and destruction terms of the φ equation, respectively. Numerical tests illustrate the practical applicability of the method, where, for example, simulations
with the E–ω model (where is the specific dissipation and is the dissipation rate of E) properly reproduce both the surface-layer wind profile estimated from the Monin-Obukhov similarity theory and the mixing-height
evolution observed above forested terrain in Southern Finland. 相似文献
2.
A modification of the most popular two-equation (E–φ) models, taking into account the plant drag, is proposed. Here E is the turbulent kinetic energy (TKE) and φ is any of the following variables: El (product of E and the mixing length l),
(dissipation rate of TKE), and ω (specific dissipation of TKE,
). The proposed modification is due to the fact that the model constants estimated experimentally for ‘free-air’ flow do not allow for adequate reconstruction of the ratio between the production and dissipation rates of TKE in the vegetation canopy and have to be adjusted. The modification is universal, i.e. of the same type for all E–φ models considered. The numerical experiments carried out for both homogeneous and heterogeneous plant canopies with E–φ models (and with the E–l model taken as a kind of reference) show that the modification performs well. They also suggest that E– and E–ω schemes are more promising than the E–El scheme for canopy flow simulation since they are not limited by the need to use a wall function.In addition, a new parameterization for enhanced dissipation within the plant canopy is derived. It minimizes the model sensitivity to C
μ, the key parameter for two-equation schemes, and whose estimates unfortunately vary considerably from experiment to experiment. The comparison of results of new modified E– and E –ω models with observations from both field and wind-tunnel experiments shows that the proposed parameterization is quite robust. However, because of uncertainties with the turbulence Prandtl and Schmidt numbers for the E– model within the canopy, the E–ω model is recommended for future implementation, with the suggested modifications. 相似文献
3.
4.
The turbulence closure in atmospheric boundary-layer modelling utilizing Reynolds Averaged Navier–Stokes (RANS) equations
at mesoscale as well as at local scale is lacking today a common approach. The standard kɛ model, although it has been successful for local scale problems especially in neutral conditions, is deficient for mesoscale
flows without modifications. The k–ɛ model is re-examined and a new general approach in developing two-equation turbulence models is proposed with the aim of
improving their reliability and consequently their range of applicability. This exercise has led to the replacement of the
ɛ-transport equation by the transport equation for the turbulence inverse length scale (wavenumber). The present version of
the model is restricted to neutrally stratified flows but applicable to both local scale and mesoscale flows. The model capabilities
are demonstrated by application to a series of one-dimensional planetary boundary-layer problems and a two-dimensional flow
over a square obstacle. For those applications, the present model gave considerably better results than the standard k–ɛ model. 相似文献
5.
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
6.
A. M. Reynolds 《Boundary-Layer Meteorology》2012,142(2):193-205
Second- and third-order turbulence closure models have met with mixed success when applied to the prediction of turbulent
flows within and above plant canopies and model predictions are typically no better than those obtained using simpler two-equation
(k-e){(k-\varepsilon)} models. This is because the local gradient diffusion approximation (a crucial model requirement) cannot represent accurately
turbulent transport that is dominated by the presence of ejections and sweeps whose length scales are comparable with the
canopy height. To make progress, turbulent transport must be treated without approximation, as in Lagrangian probability density
function (PDF) models. This study is the first to develop and to validate a PDF model of horizontally-homogeneous canopy flow.
The model relies upon a prescribed length-scale that has been used elsewhere in the modelling of turbulent flows. Model predictions
compare favourably with measurements of neutrally stratified turbulent flows within and above canopies of mature corn and
forested eucalypt trees. 相似文献
7.
Buoyancy and The Sensible Heat Flux Budget Within Dense Canopies 总被引：1，自引：8，他引：1
D. Cava G. G. Katul A. Scrimieri D. Poggi A. Cescatti U. Giostra 《Boundary-Layer Meteorology》2006,118(1):217-240
In contrast to atmospheric surface-layer (ASL) turbulence, a linear relationship between turbulent heat fluxes (FT) and vertical gradients of mean air temperature within canopies is frustrated by numerous factors, including local variation
in heat sources and sinks and large-scale eddy motion whose signature is often linked with the ejection-sweep cycle. Furthermore,
how atmospheric stability modifies such a relationship remains poorly understood, especially in stable canopy flows. To date,
no explicit model exists for relating FT to the mean air temperature gradient, buoyancy, and the statistical properties of the ejection-sweep cycle within the canopy
volume. Using third-order cumulant expansion methods (CEM) and the heat flux budget equation, a “diagnostic” analytical relationship
that links ejections and sweeps and the sensible heat flux for a wide range of atmospheric stability classes is derived. Closure
model assumptions that relate scalar dissipation rates with sensible heat flux, and the validity of CEM in linking ejections
and sweeps with the triple scalar-velocity correlations, were tested for a mixed hardwood forest in Lavarone, Italy. We showed
that when the heat sources (ST) and FT have the same sign (i.e. the canopy is heating and sensible heat flux is positive), sweeps dominate the sensible heat flux.
Conversely, if ST and FT are opposite in sign, standard gradient-diffusion closure model predict that ejections must dominate the sensible heat flux. 相似文献
8.
A mesoscale Planetary Boundary Layer (PBL) model with a simple turbulence closure scheme based on the turbulence kinetic energy (TKE) equation and the dissipation () equation is used to simulate atmospheric flow over mesoscale topography. Comparative studies with different parameterizations suggest that with a proper closure assumption for turbulence dissipation, the E-model can simulate the circulation induced by the mesoscale topography with results similar to those obtained using the E- model. On the other hand, the first-order closure using O'Brien's cubic interpolation for eddy diffusivities (K) generally produces much larger K profiles in the stable and the unstable regions, which is believed to be due to the overprediction of the height of the PBL. All models with the TKE equation yield quite similar ensemble mean fields, which are found to be little sensitive to the closure assumption for turbulence dissipation, though their predicted magnitudes of TKE and K may differ appreciably. A discussion on the diurnal evolution of the mesoscale topography-induced circulation and the spatial variations of the turbulence fluxes in the surface layer is also given based on the E- model results. 相似文献
9.
Local Imbalance of Turbulent Kinetic Energy in the Surface Layer 总被引：1，自引：1，他引：0
We utilize experimental data collected in 2002 over an open field in Hanford, Washington, USA, to investigate the turbulent
kinetic energy (TKE) budget in the atmospheric surface layer. The von Kármán constant was determined from the near-neutral
wind profiles to be 0.36 ± 0.02 rather than the classical value of 0.4. The TKE budget was normalized and all terms were parameterized
as functions of a stability parameter z/L, where z is the distance from the ground and L is the Obukhov length. The shear production followed the Businger–Dyer relation for −2 < z/L < 1. Contrary to the traditional Monin–Obukhov similarity theory (MOST), the shear, buoyancy and dissipation terms were found
to be imbalanced due to a non-zero vertical transport over all stabilities. Motivated by this local imbalance, modified parameterizations
of the dissipation and the turbulent transport were attempted and generated good agreement with the experimental data. Assuming
stationarity and horizontal homogeneity, the pressure transport was estimated from the residual of the TKE budget. 相似文献
10.
S. S. Zilitinkevich T. Elperin N. Kleeorin I. Rogachevskii I. Esau 《Boundary-Layer Meteorology》2013,146(3):341-373
Here we advance the physical background of the energy- and flux-budget turbulence closures based on the budget equations for the turbulent kinetic and potential energies and turbulent fluxes of momentum and buoyancy, and a new relaxation equation for the turbulent dissipation time scale. The closure is designed for stratified geophysical flows from neutral to very stable and accounts for the Earth’s rotation. In accordance with modern experimental evidence, the closure implies the maintaining of turbulence by the velocity shear at any gradient Richardson number Ri, and distinguishes between the two principally different regimes: “strong turbulence” at ${Ri \ll 1}$ typical of boundary-layer flows and characterized by the practically constant turbulent Prandtl number Pr T; and “weak turbulence” at Ri > 1 typical of the free atmosphere or deep ocean, where Pr T asymptotically linearly increases with increasing Ri (which implies very strong suppression of the heat transfer compared to the momentum transfer). For use in different applications, the closure is formulated at different levels of complexity, from the local algebraic model relevant to the steady-state regime of turbulence to a hierarchy of non-local closures including simpler down-gradient models, presented in terms of the eddy viscosity and eddy conductivity, and a general non-gradient model based on prognostic equations for all the basic parameters of turbulence including turbulent fluxes. 相似文献
11.
Richard Bintanja 《Boundary-Layer Meteorology》2000,95(3):369-395
An atmospheric surface-layer model is used to investigate the interactionbetween suspended snow particles and the near-surface flow. Themodel incorporates the effects of upward diffusion, gravitational settling and sublimation of snow particles in 48 size classes, the effects of snowdrift sublimation on the heat and moisture budget of the surface layer, and the buoyancy destruction of turbulent kinetic energy (TKE) caused by the presence of suspended particles. A new term in the E- closure model representing the buoyancy destruction due to suspended particles is included in the prognostic equation for TKE. Generally, model results indicate that the presence of suspended particles causes significant decreases in TKE, the dissipation rate, turbulent length scales and eddy exchange coefficients (up to 40%). It is found that the reduction in the eddy exchangecoefficients is due mainly to reductions in turbulent length scales. Theassociated particle Richardson number peaks near the saltation-suspensioninterface, but at higher levels in the surface layer the particle-induced buoyancy can also significantly affect the flow. A detailed analysis of the various snowdrift quantities, the TKE budget and the particle buoyancy effects on the flow is presented. 相似文献
12.
13.
G. S. GOLITSYN 《大气科学进展》2009,26(3):585-598
The goal of this paper is to quantitatively formulate some necessary conditions for the
development of intense atmospheric vortices. Specifically, these criteria are discussed for tropical
cyclones (TC) and polar lows (PL) by using bulk formulas for fluxes of momentum, sensible heating,
and latent heating between the ocean and the atmosphere. The velocity scale is used in two forms: (1)
as expressed through the buoyancy flux b and the Coriolis parameter lc for rotating fluids convection,
and (2) as expressed with the cube of velocity times the drag coefficient through the formula for
total kinetic energy dissipation in the atmospheric boundary layer. In the quasistationary case the
dissipation equals the generation of the energy. In both cases the velocity scale can be expressed
through temperature and humidity differences between the ocean and the atmosphere in terms of the
reduced gravity, and both forms produce quite comparable velocity scales. Using parameters b and lc,
we can form scales of the area and, by adding the mass of a unit air column, a scale of the total
kinetic energy as well. These scales nicely explain the much smaller size of a PL, as compared to
a TC, and the total kinetic energy of a TC is of the order 1018-1019 J. It will be shown
that wind of 33 m s-1 is produced when the total enthalpy fluxes between the ocean and the
atmosphere are about 700 W m-2 for a TC and 1700 W m-2 for a PL, in association with
the much larger role of the latent heat in the first case and the stricter geostrophic constraints
and larger static stability in the second case. This replaces the mystical role of 26oC as
a criterion for TC origin.
The buoyancy flux, a product of the reduced gravity and the wind speed, together with the atmospheric
static stability, determines the rate of the penetrating convection. It is known from the observations
that the formation time for a PL reaching an altitude of 5--6 km can be only a few hours, and a day, or
even half a day, for a TC reaching 15--18 km. These two facts allow us to construct curves on the plane
of Ts and ΔT=Ts-Ta to determine possibilities for forming an intense vortex. Here, Ta is the atmospheric temperature at the height z=10 m. A PL should have ΔT>20oC in accordance
with the observations and numerical simulations. The conditions for a TC are not so straightforward but
our diagram shows that the temperature difference of a few degrees, or possibly even a fraction of a
degree, might be sufficient for TC development for a range of static stabilities and development times. 相似文献
14.
Manabu Kanda 《Boundary-Layer Meteorology》2006,118(1):151-168
Turbulent organized structures (TOS) above building arrays were investigated using a large-eddy simulation (LES) model for
a city (LES-CITY). Square and staggered building arrays produced contrasting behaviour in terms of turbulence that roughly
corresponded to the conventional classification of ‘D-type’ and ‘K-type’ roughness, respectively: (1) The drag coefficients
(referred to the building height) for staggered arrays were sensitive to building area density, but those for square arrays
were not. (2) The relative contributions of ejections to sweeps (S2/S4) at the building height for square arrays were sensitive to building area density and nearly equalled or exceeded 1.0 (ejection
dominant), but those for staggered arrays were insensitive to building area density and were mostly below 1.0 (sweep dominant).
(3) Streaky patterns of longitudinal low speed regions (i.e., low speed streaks) existed in all flows regardless of array
type. Height variations of the buildings in the square array drastically increased the drag coefficient and modified the turbulent
flow structures. The mechanism of D-type and K-type urban-like roughness flows and the difference from vegetation flows are
discussed. Although urban-like roughness flows exhibited mixed properties of mixing layers and flat-wall boundary layers as
far as S2/S4 was concerned, the turbulent organized structures of urban-like roughness flows resembled those of flat-wall boundary layers. 相似文献
15.
A study of the neutrally-stratified flow within and over an array of three-dimensional buildings (cubes) was undertaken using
simple Reynolds-averaged Navier—Stokes (RANS) flow models. These models consist of a general solution of the ensemble-averaged,
steady-state, three-dimensional Navier—Stokes equations, where the k-ε turbulence model (k is turbulence kinetic energy and ε is viscous dissipation rate) has been used to close the system of equations. Two turbulence
closure models were tested, namely, the standard and Kato—Launder k-ε models. The latter model is a modified k-ε model designed specifically to overcome the stagnation point anomaly in flows past a bluff body where the standard k-ε model overpredicts the production of turbulence kinetic energy near the stagnation point. Results of a detailed comparison
between a wind-tunnel experiment and the RANS flow model predictions are presented. More specifically, vertical profiles of
the predicted mean streamwise velocity, mean vertical velocity, and turbulence kinetic energy at a number of streamwise locations
that extend from the impingement zone upstream of the array, through the array interior, to the exit region downstream of
the array are presented and compared to those measured in the wind-tunnel experiment. Generally, the numerical predictions
show good agreement for the mean flow velocities. The turbulence kinetic energy was underestimated by the two different closure
models. After validation, the results of the high-resolution RANS flow model predictions were used to diagnose the dispersive
stress, within and above the building array. The importance of dispersive stresses, which arise from point-to-point variations
in the mean flow field, relative to the spatially-averaged Reynolds stresses are assessed for the building array. 相似文献
16.
Pierre Aumond Valery Masson Christine Lac Benoit Gauvreau Sylvain Dupont Michel Berengier 《Boundary-Layer Meteorology》2013,146(1):65-80
We use the mesoscale meteorological model Meso-NH, taking the drag force of trees into account under stable, unstable and neutral conditions in a real case study. Large-eddy simulations (LES) are carried out for real orography, using a regional forcing model and including the energy and water fluxes between the surface (mostly grass with some hedges of trees) and the atmosphere calculated using a state-of-the-art soil-vegetation-atmosphere-transfer model. The formulation of the drag approach consists of adding drag terms to the momentum equation and subgrid turbulent kinetic energy dissipation, as a function of the foliage density. Its implementation in Meso-NH is validated using Advanced Regional Prediction System simulation results and measurements from Shaw and Schumann (Boundary-Layer Meteorol, 61(1):47?C64, 1992). The simulation shows that the Meso-NH model successfully reproduces the flow within and above homogeneous covers. Then, real case studies are used in order to investigate the three different boundary layers in a LES configuration (resolution down to 2 m) over the ??Lannemezan 2005?? experimental campaign. Thus, we show that the model is able to reproduce realistic flows in these particular cases and confirm that the drag force approach is more efficient than the classical roughness approach in describing the flow in the presence of vegetation at these resolutions. 相似文献
17.
Computational Fluid Dynamical Modelling of Concentration Fluctuations in an Idealized Urban Area 总被引：2，自引：2，他引：0
Turbulent mixing induces variability in concentration that is important in many applications, such as reactive plumes, risk
assessments or odour impact analyses (when the effects can have time scales on the order of a second). In urban canopies,
the variability may be modified by the presence of buildings. Our purpose is to study concentration fluctuation variance in
built-up areas using an Eulerian approach. We performed numerical simulations with the computational fluid dynamics model
Mercure_Saturne, which is a three-dimensional model adapted to atmospheric flow and pollutant dispersion. We use a k − ϵ turbulence closure and predict the concentration variance with a transport equation model. The model performance is evaluated
with the near-full scale experiment MUST (Mock Urban Setting Test), a field experiment conducted in Utah’s West Desert Test
Center. The modelled root-mean-square of the concentration fluctuations is compared to measurements for 20 of the MUST trials.
The model shows good agreement with the measurements, with the fraction of predictions within a factor of two of observations
of 60.1%, with better results for horizontal lines of detectors than for the detectors on vertical masts (with fractions of
predictions within a factor of two of observations of respectively 66.4% and 52.6%). The influence of different parameters
on the fluctuation variance is also studied and we show the importance of taking into account the stability of the stratification
when modelling the turbulent kinetic energy. 相似文献
18.
Daniel F. Nadeau Eric R. Pardyjak Chad W. Higgins Harinda Joseph S. Fernando Marc B. Parlange 《Boundary-Layer Meteorology》2011,141(2):301-324
A simple model to study the decay of turbulent kinetic energy (TKE) in the convective surface layer is presented. In this
model, the TKE is dependent upon two terms, the turbulent dissipation rate and the surface buoyancy fluctuations. The time
evolution of the surface sensible heat flux is modelled based on fitting functions of actual measurements from the LITFASS-2003
field campaign. These fitting functions carry an amplitude and a time scale. With this approach, the sensible heat flux can
be estimated without having to solve the entire surface energy balance. The period of interest covers two characteristic transition
sub-periods involved in the decay of convective boundary-layer turbulence. The first sub-period is the afternoon transition,
when the sensible heat flux starts to decrease in response to the reduction in solar radiation. It is typically associated
with a decay rate of TKE of approximately t
−2 (t is time following the start of the decay) after several convective eddy turnover times. The early evening transition is the
second sub-period, typically just before sunset when the surface sensible heat flux becomes negative. This sub-period is characterized
by an abrupt decay in TKE associated with the rapid collapse of turbulence. Overall, the results presented show a significant
improvement of the modelled TKE decay when compared to the often applied assumption of a sensible heat flux decreasing instantaneously
or with a very short forcing time scale. In addition, for atmospheric modelling studies, it is suggested that the afternoon
and early evening decay of sensible heat flux be modelled as a complementary error function. 相似文献
19.
Impact of Sea-Spray on the Atmospheric Surface Layer 总被引：1，自引：0，他引：1
The feedback effects of sea-spray on the heat and momentum fluxes under equilibrium conditions associated with winds of tropical
cyclones are investigated using a one-dimensional coupled sea-spray and atmospheric surface-layer (ASL) model. This model
is capable of simulating the microphysical aspects of the evaporation of saline water droplets of various sizes and their
dynamic and thermal interaction with the turbulence mixing that is simulated by the Mellor–Yamada 1.5-order closure scheme.
Sea-spray droplet generation is described by a state-of-the-art parametrization that predicts the size spectrum of sea-spray
droplets for a given surface forcing. The results from a series of simulations indicate the way in which evaporating droplets
of various sizes modify the turbulence mixing near the surface, which in turn affects further droplet evaporation. All these
results are direct consequences of the effects of sea-spray on the balance of turbulent kinetic energy in the spray-filled
surface layer. In particular, the overall impact of sea-spray droplets on the mean wind depends on the wind speed at the level
of sea-spray generation. When the wind speed is below 40 m s−1, the droplets are small in size and tend to evaporate substantially and thus cool the spray-filled layer, while for wind
speeds above 50 m s−1, the size of the droplets is so large that they do not have enough time to evaporate much before falling back into the sea.
The sensible heat carried by the droplets is released to the ambient air, increasing the buoyancy of the surface layer and
enhancing the turbulent mixing. The suspension of sea-spray droplets reduces the buoyancy and makes the surface layer more
stable, decreasing the friction velocity and the downward turbulent mixing of momentum. The results from the numerical experiments
also suggest that, in order not to violate the constant flux assumption critical to the Monin–Obukhov similarity theory, a
displacement equal to the mean wave height should be included in the logarithmic profiles of the wind and thermal fields. 相似文献
20.
S. S. Zilitinkevich I. Esau N. Kleeorin I. Rogachevskii R. D. Kouznetsov 《Boundary-Layer Meteorology》2010,135(3):505-511
We give a new derivation of the familiar linear relation for the dimensionless velocity gradient in the stably stratified surface layer and provide physical and empirical grounds for its universal applicability in stationary homogeneous turbulence over the whole range of static stabilities from Ri = 0 to very large Ri. Combining this relation with the budget equation for the turbulent kinetic energy we obtain the “equilibrium formulation” of the turbulent dissipation length scale, and recommend it for use in turbulence closure models. 相似文献