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1.
Turbulence Structure of the Unstable Atmospheric Surface Layer and Transition to the Outer Layer 总被引:4,自引:4,他引:0
K.G. McNaughton 《Boundary-Layer Meteorology》2004,112(2):199-221
We present a new model of the structure of turbulence in the unstable atmospheric surface layer, and of the structural transition between this and the outer layer. The archetypal element of wall-bounded shear turbulence is the Theodorsen ejection amplifier (TEA) structure, in which an initial ejection of air from near the ground into an ideal laminar and logarithmic flow induces vortical motion about a hairpin-shaped core, which then creates a second ejection that is similar to, but larger than, the first. A series of TEA structures form a TEA cascade. In real turbulent flows TEA structures occur in distorted forms as TEA-like (TEAL) structures. Distortion terminates many TEAL cascades and only the best-formed TEAL structures initiate new cycles. In an extended log layer the resulting shear turbulence is a complex, self-organizing, dissipative system exhibiting self-similar behaviour under inner scaling. Spectral results show that this structure is insensitive to instability. This is contrary to the fundamental hypothesis of Monin--Obukhov similarity theory. All TEAL cascades terminate at the top of the surface layer where they encounter, and are severely distorted by, powerful eddies of similar size from the outer layer. These eddies are products of the breakdown of the large eddies produced by buoyancy in the outer layer. When the outer layer is much deeper than the surface layer the interacting eddies are from the inertial subrange of the outer Richardson cascade. The scale height of the surface layer, z
s, is then found by matching the powers delivered to the creation of emerging TEAL structures to the power passing down the Richardson cascade in the outer layer. It is z
s = u
*
3
/ks, where u
* is friction velocity, k is the von Kármán constant and s is the rate of dissipation of turbulence kinetic energy in the outer layer immediately above the surface layer. This height is comparable to the Obukhov length in the fully convective boundary layer. Aircraft and tower observations confirm a strong qualitative change in the structure of the turbulence at about that height. The tallest eddies within the surface layer have height z
s, so z
s is a new basis parameter for similarity models of the surface layer. 相似文献
2.
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
3.
The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch
forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two
heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that
of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly
stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the
RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra.
This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence.
The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind
spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant
eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised
at nearly half its value under stable conditions. 相似文献
4.
Yu Zhang Heping Liu Thomas Foken Quinton L. Williams Shuhua Liu Matthias Mauder Claudia Liebethal 《Boundary-Layer Meteorology》2010,136(2):235-251
During the Energy Balance EXperiment, the patch-by-patch, flood irrigation in a flat cotton field created an underlying surface
with heterogeneous soil moisture, leading to a dry (warm)-to-wet (cool) transition within the cotton field under northerly
winds. Moreover, the existence of an extremely dry, large bare soil area upstream beyond the cotton field created an even
larger step transition from the bare soil region to the cotton field. We investigated the turbulence spectra and cospectra
in the atmospheric surface layer (ASL) that was disturbed by large eddies generated over regions upstream and also influenced
by horizontal advection. In the morning, the ASL was unstable while in the afternoon a stable internal boundary layer was
observed at the site. Therefore, the turbulence data at 2.7 and 8.7 m are interpreted and compared in terms of interactions
between large eddies and locally generated turbulence under two atmospheric conditions: the unstable ASL beneath the convective
boundary layer (CBL) (hereafter the unstable condition) and the stable ASL beneath the CBL (hereafter the stable condition).
We identified the influences of multiple sizes of large eddies on ASL turbulence under both stratifications; these large eddies
with multiple sizes were produced over the dry patches and dry, large bare soil areas upstream. As a consequence of the disturbance
of large eddies, the broadening, erratic variability, and deviation of spectra and cospectra, relative to those described
by Monin–Obukhov similarity theory, are evident in the low- to mid-frequencies. Transfer of momentum, heat, and water vapour
by large eddies is distinctly observed from the turbulence cospectra and leads to significant run-to-run variations of residuals
of the surface energy balance closure. Our results indicate that these large eddies have greater influences on turbulence
at higher levels compared to lower levels, and in the unstable ASL compared to the stable ASL. 相似文献
5.
Marieta Cristina Castillo Atsushi Inagaki Manabu Kanda 《Boundary-Layer Meteorology》2011,140(3):453-469
A large-eddy simulation of the atmospheric boundary layer, large enough to contain both an urban surface layer and a convective
mixed layer, was performed to investigate inner-layer and outer-layer scale motions. The objective was to determine the applicability
of Monin–Obukhov similarity theory to inner-layer motions, to investigate the influence of outer-layer motions on surface-layer
structure, as well as to assess the interaction of the two scales of motion. The urban surface roughness consisted of square-patterned
cubic buildings of dimension H (40 m). A spatial filter was used to decompose the two scales in the inertial sublayer. The horizontal square filter of size
10H was effective in separating the inner-layer (surface-layer height ≈ 2 H) and outer-layer scales (boundary-layer height δ ≈ 30H), where the Reynolds stress contribution of the inner layer dominates in the logarithmic layer (depth 2H). Similarity coefficients for velocity fluctuations were successfully determined for inner-layer motions in the surface layer,
proving the robustness of Monin–Obukhov similarity for surface-layer turbulence. The inner-layer structures exhibit streaky
structures that have similar streamwise length but narrower spanwise width relative to the streamwise velocity fluctuation
field, consistent with observations from an outdoor scale model. The outer-layer motions to some extent influence the location
of ejections and sweeps through updraft and downdraft motions, respectively, thus, disturbing the homogeneity and similarity
of inner-layer motions. Although the horizontal averages of the variances and covariance of motions reveal that the Reynolds
stresses are dominated by inner-layer structures, the localized influence of the interaction of outer-layer horizontal and
inner-layer vertical motions on the Reynolds stress is not insignificant. 相似文献
6.
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
7.
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
8.
Peter P. Sullivan James C. McWilliams Chin-Hoh Moeng 《Boundary-Layer Meteorology》1996,80(1-2):167-202
A method for performing nested grid calculations with a large-eddy simulation code is described. A common numerical method is used for all meshes, and the grid architecture consists of a single outer or coarse grid, and nested or fine grids, which overlap in some common region. Inter-grid communication matches the velocity, pressure and potential temperature fields in the overlap region. Resolved and sub-grid scale (SGS) turbulent fluxes and kinetic energy on the fine grid are averaged to the coarse grid using a conservation rule equivalent to Germano's identity used to develop dynamic SGS models.Simulations of a slightly convective, strong shear planetary boundary layer were carried out with varying surface-layer resolutions. Grid refinements in the (x, y, z) directions of up to (5, 5, 2) times were employed. Two-way interaction solutions on the coarse and fine meshes are successfully matched in the overlap region on an instantaneous basis, and the turbulent motions on the fine grid blend smoothly into the coarse grid across the grid interface. With surface-layer grid nesting, significant increases in resolved eddy fluxes and variances are found. The energy-scale content of the vertical velocity, and hence vertical turbulent fluxes, appear to be most influenced by increased grid resolution. Vertical velocity spectra show that the dominant scale shifts towards higher wavenumbers (smaller scales) and the magnitude of the peak energy is increased by more than a factor of 3 with finer resolution. Outside of the nested region the average heat and momentum fluxes and spectra are slightly influenced by the fine resolution in the surface layer. From these results we conclude that fine resolution is required to resolve the details of the turbulent motions in the surface layer. At the same time, however, increased resolution in the surface layer does not appreciably alter the ensemble statistics of the resolved and SGS motions outside of the nested region. 相似文献
9.
We present the power spectra of wind velocity and the cospectra of momentum and heat fluxes observed for different wind directions
over flat terrain and a large valley on the Loess Plateau. The power spectra of longitudinal (u) and lateral (v) wind speeds satisfy the −5/3 power law in the inertial subrange, but do not vary as observed in previous studies within
the low frequency range. The u spectrum measured at 32 m height for flow from the valley shows a power deficit at intermediate frequencies, while the v spectrum at 32 m downwind of the valley reaches another peak in the low frequency range at the same frequency as the u spectrum. The corresponding peak wavelength is consistent with the observed length scale of the convective outer layer at
the site. The v spectrum for flat terrain shows a spectral gap at mid frequencies while obeying inner layer scaling in its inertial subrange,
suggesting two sources of turbulence in the surface layer. All the spectra and cospectra from the valley direction show a
height dependency over the three levels. 相似文献
10.
Song-Lak Kang 《Boundary-Layer Meteorology》2009,132(1):59-81
A theoretical approach suggests that the surface heterogeneity on a scale of tens of kilometres can generate mesoscale motions
that are not in a quasi-stationary state. The starting point of the theoretical approach is the equations of horizontal velocity
and potential temperature that are low-pass filtered with a mesoscale cut-off wavelength. The transition of the generated
mesoscale motions from a quasi-stationary state to a non-stationary state occurs when horizontal advection is strong enough
to level out the potential temperature gradient on the surface heterogeneity scale. Large-eddy simulations (LES) suggest that
the convective boundary layer (CBL) changes to a non-stationary state when forced by a surface heat-flux variation of amplitude
of 100W m−2 or higher and a wavelength of the order of 10 km. Spectral analysis of the LES reveals that when the mesoscale motions are
in a quasi-stationary state, the energy provided by the surface heat-flux variation remains in organized mesoscale motions
on the scale of the surface variation itself. However, in a non-stationary state, the energy cascades to smaller scales, with
the cascade extending down into the turbulence scale when the wavelength of the surface heat-flux variation is on a scale
smaller than 100 times the CBL height. The energy transfer from the generated mesoscale motions to the CBL turbulence results
in the absence of a spectral gap between the two scales. The absence of an obvious spectral gap between the generated mesoscale
motions and the turbulence raises questions about the applicability of mesoscale models for studies on the effect of high-amplitude
surface heterogeneity on a scale of tens of kilometres.
The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
11.
黄土高原复杂地形受中尺度运动影响的稳定边界层湍流特征 总被引:1,自引:0,他引:1
利用兰州大学半干旱气候与环境观测站(Semi-Arid Climate and Environment Observatory of Lanzhou University,简称SACOL)2008年12月观测资料,研究了稳定边界层湍流特征.使用涡动相关资料研究湍流通量时,定义湍流的平均时间τ内的中尺度运动是造成湍流统计量变化范围大的主要原因,稳定情形? τ取几十秒至几分钟.对梯度理查森数大于0.3的强稳定情形的湍流尺度分解(MRD)谱分析表明,感热通量在112.4~449.9 s存在谱隙,尺度大于谱隙的中尺度运动造成了通量观测资料离散性大,甚至有支配性影响.动量通量的谱隙在112.4~224.9 s之间.弱风时,中尺度运动的影响更大,垂直风速标准差以0.1的比率随中尺度风速变化;垂直风速标准差同广义风速表现出很好的相关性,并随着广义风速消失而消失.三维风速标准差与摩擦速度呈很好的线性关系,垂直、水平、横风风速的无量纲标准差分别为1.35、2.54、2.21.对湍流动能的研究发现,在梯度理查森数大于0.3的条件下,仍然存在连续的湍流.以湍动能为依据,分析了湍流的平稳时间长度,其长度随稳定度变化而变化,2008年12月7~11日从133.5 s变化到856.2 s,湍流平稳时间长度反映了中尺度运动的发生频率. 相似文献
12.
雷暴大风过程中对流层中低层动量通量和动能通量输送特征研究 总被引:2,自引:0,他引:2
2014年5月31日北京发生一次雷暴大风过程。以雷达资料同化结果为初始场,对此次过程进行高分辨率数值模拟。采用非静力平衡和非地转平衡的经向动量方程和质量权重动能方程,利用模拟资料,对雷暴大风过程中经向动量和质量权重动能进行收支分析,以此来研究雷暴过程中对流层中低层动量通量和动能通量输送特征,讨论地面大风的可能成因。分析结果表明,在对流层中低层,经向动量通量散度是影响经向动量局地变化的主要强迫项。雷暴系统后部的入流把中低层的经向动量倾斜向下输送,系统前部对流云区中低层的下沉气流也向下输送经向动量。这两支下传动量通量先后与近地面经向动量的水平通量汇合,向系统前沿输送经向动量。在北京西北部地形阻挡作用下,经向动量通量在系统前端近地面辐合,促进那里的经向动量局地增长,有利于增强那里的南风。质量动能收支的特征与经向动量收支类似,在近地面层质量动能的局地变化主要是由质量动能通量散度引起的。系统后部入流把中层质量动能向下传输到近地面层,然后与近地面质量动能的水平通量汇合,向系统前沿输送质量动能。相对来说,近地面层经向动量和质量动能的水平通量比下传通量更重要,这主要与低层较强的东南急流有关。 相似文献
13.
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
14.
A. G. Goulart B. E. J. Bodmann M. T. M. B. de Vilhena P. M. M. Soares D. M. Moreira 《Boundary-Layer Meteorology》2011,138(1):61-75
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
15.
Applied model for the growth of the daytime mixed layer 总被引:5,自引:2,他引:5
A slab model is proposed for developing the height of the mixed layer capped by stable air aloft. The model equations are closed by relating the consumption of energy (potential and kinetic) at the top of the mixed layer to the production of convective and mechanical turbulent kinetic energy within the mixed layer. By assuming that the temperature difference at the top of the mixed layer instantaneously adjusts to the actual meteorological conditions without regard to the initial temperature difference that prevailed, the model is reduced to a single differential equation which easily can be solved numerically. When the mixed layer is shallow or the atmosphere nearly neutrally stratified, the growth is controlled mainly by mechanical turbulence. When the layer is deep, its growth is controlled mainly by convective turbulence. The model is applied on a data set of the evolution of the height of the mixed layer in the morning hours, when both mechanical and convective turbulence contribute to the growth process. Realistic mixed-layer developments are obtained. 相似文献
16.
Mitsuaki Horiguchi Taiichi Hayashi Ahoro Adachi Shigeru Onogi 《Boundary-Layer Meteorology》2012,144(2):179-198
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels. 相似文献
17.
Gary K. Greenhut 《Boundary-Layer Meteorology》1981,20(1):75-100
The momentum flux data obtained by the gust probe aboard the NOAA DC-6 aircraft during GATE are analyzed. Vertical profiles are obtained for Phases I and III and correlated with vertical wind velocity profiles using the geostrophic departure method. Reasonable agreement is obtained using the horizontal equations of motion with negligible advective acceleration. The vertical profiles of momentum flux and wind speed variance compare well with the numerical model results of Deardorff (1972) and Wyngaard et al. (1974). Vertical distributions of power spectra for vertical eddy motion and cospectra corresponding to the momentum flux components are obtained along with the height variation of the dominant length scales of vertical eddy motion and the dissipation rate of turbulence kinetic energy. When normalized by mixed-layer similarity, these results agree well with previous determinations in the boundary layer over tropical oceans and over land. 相似文献
18.
An atmospheric surface-layer (ASL) experiment conducted at a meteorological site in the Oostelijk-Flevoland polder of the Netherlands is described. Turbulent fluctuations of wind velocity, air temperature and static pressure were measured, using three 10 m towers.Simultaneous turbulent signals at several heights on the towers were used to investigate the properties of the turbulent structures which contribute most significantly to the turbulent vertical transports in the unstable ASL. These turbulent structures produce between 30 and 50% of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50% of the mean turbulent vertical heat transport; in both cases this occurs during 15 to 20% of the total observation time.The translation speed of the turbulent structures equals the wind speed averaged over the depth of the ASL, which scales on the surface friction velocity. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is significantly smaller than that of the internal shear layer, which is associated with the temperature interface. The turbulent structures in the unstable ASL are determined by a large-scale temperature field: Convective motions, which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The curvature of the vertical profile of mean horizontal alongwind velocity forces the alignment of the convective cells in the flow direction (Kuettner, 1971), which have an average length of several hundreds of metres and an average width of a few tens of metres. This mechanism leads to the formation of turbulent structures, which extend throughout the depth of the ASL. 相似文献
19.
We quantify the role of the convective buoyant structures and the remainder turbulence, here called background turbulence, in the convective atmospheric boundary layer in horizontally homogeneous, dry and barotropic conditions. Three filtering methods to separate the structures and the background turbulence are first evaluated. These are: short-time averaging, Fourier filtering and proper orthogonal decomposition. The Fourier method turns out to be the most appropriate for the present purpose. The decomposition is applied to two cases: one with no mean flow and another with moderate mean wind speed. It is shown that roughly 85 % of the vertical flux of the potential temperature and about 72 % of the kinetic energy is carried by the structures in the mixed layer in both cases. The corresponding percentage for the potential temperature variance is 81 % in the zero mean-wind case and 76 % in the moderate mean-wind case. The structures are responsible for as much as 94 % of the momentum flux in the mixed layer of the moderate mean-wind case. In the surface layer the background turbulence is generally more important than the structure contribution in both cases. The budget of the potential temperature flux is analyzed in detail and it is shown that its turbulent transport term is mostly built up by the structures but also the interaction between the structures and the background turbulence plays a significant role. The other important budget terms are shown to be dominated by the structures except for the pressure–temperature gradient covariance. 相似文献
20.
Large-Eddy Simulation of the Daytime Boundary Layer in an Idealized Valley Using the Weather Research and Forecasting Numerical Model 总被引:1,自引:1,他引:0
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