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1.
Heterogeneous Saltation: Theory, Observation and Comparison 总被引：7，自引：0，他引：7
In the theory of saltation, under development since the 1940s, it is often assumed that saltation is homogeneous, i.e., saltating particles are uniform in size and follow identical trajectories. This assumption is a limitation to the development of saltation theory. In this paper, as in some other studies made since the 1980s, we are concerned with the saltation of multi-sized particles in turbulent flows, a process that we refer to as heterogeneous saltation. The theory deals with several questions, including the variation of particle size distribution with height, the entrainment rates of particles in different size ranges, and the associated profiles of saltation flux, particle momentum flux and particle concentration. It is hypothesised that saltation is dynamically similar and ‘universal’ similarity functions can be established. The similarity function for saltation flux is presented. Field observations are carried out at the southern fringe of the Takla Makan Desert in April 2002. Measurements of streamwise saltation flux of 32 particle size groups are made using a sand particle counter, together with measurements of wind speed and other atmospheric variables. These data are used to validate the saltation theory, by examining whether the observed saltation flux and particle size distribution can be reproduced. It is shown that the theory is promising in predicting these quantities. 相似文献
2.
Boundary-Layer Meteorology - Surface shear stresses produced by wind and particle collision play a key role in aerodynamic entrainment and splash processes. The fluid shear stress at the surface... 相似文献
3.
Equilibrium Saltation: Mass Fluxes, Aerodynamic Entrainment, and Dependence on Grain Properties 总被引：3，自引：1，他引：3
An examination is given of the way in which the saltation layer isaffected by the characteristics of the particles. Special attentionis given to the potential importance of aerodynamic entrainmentduring steady state saltation, a topic for which the discussion is still unresolved. A new numerical model for saltation in steady stateis presented, which is focused on the computation of the horizontalmass flux. The numerical computations, combined with physical arguments, suggest that aerodynamic entrainment plays a more important role thangenerally assumed so far. A comparison of the model results is made with previous models, and with measurements of snow saltation that have been reported in the literature. 相似文献
4.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2006,119(1):57-79
Forced convection in a quasi-steady atmospheric boundary layer is investigated based on a large-eddy simulation (LES) model.
The performed simulations show that in the upper portion of the mixed layer the dimensionless (in terms of mixed layer scales)
vertical gradients of temperature, humidity, and wind velocity depend on the dimensionless height z/z
i
and the Reech number Rn. The peak values of variances and covariances at the top of the mixed layer, scaled in terms of the interfacial scales, are
functions of the interfacial Richardson number Ri. As a result expressions for the entrainment rates, in the case when the interfacial layer has a finite depth, and a condition
for the presence of moistening or drying regimes in the mixed layer, are derived. Profiles of dimensionless scalar moments
in the mixed layer are proposed to be expressed in terms of two empirical similarity functions F
m
and F
i
, dependent on dimensionless height z/z
i
, and the interfacial Richardson number Ri. The obtained similarity expressions adequately approximate the LES profiles of
scalar statistics, and properly represent the impact of stability, shear, and entrainment. They are also consistent with the
parameterization proposed for free convection in the first part of this paper. 相似文献
5.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(3):365-383
We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under
consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe
the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z
m
(defined by the maximum of the potential temperature gradient) and its standard deviation σ
m
, the mixed-layer height z
i
(defined by the minimum of the potential temperature flux), entrainment velocity W
e, and the entrainment flux H
i
. The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature
flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime,
and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured
values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer. 相似文献
6.
J. D. Wilson 《Boundary-Layer Meteorology》1982,22(4):399-420
By non-dimensionalizing a trajectory-simulation (TS) model of turbulent dispersion, it is shown that the dimensionless concentration z
0cu*/kQ (cu
*/kQ) due to a continuous line (area) source of strength Q in the atmospheric surface layer depends only on z/z
0, x/z
0, z
0/L and z
s/z0, where z
s is the source height. The TS model is used to tabulate concentration profiles due to ground-level line and area sources. Concentration profiles generated by the TS model for elevated sources are shown to be inconsistent with the Reciprocal Theorems of Smith (1957) and it is suggested that this is because the flux-mean gradient closure scheme inherent in the Reciprocal Theorem is invalid for an elevated source. 相似文献
7.
Based upon comparisons between published experimental data and simulated results on the vertical sand flux distribution in
the saltation layer, Shao’s similarity saltation model has been greatly improved by correcting the average vertical particle
lift-off velocity and using a more suitable universal roughness length. By the improved model, the vertical sand flux profile
over the bare, dry and loose uniform sandy surface, which is quite representative of real desert surfaces, can be reproduced
very well. Meanwhile, the surface transport rate and the characteristic and average saltation heights have been simulated
and analyzed in detail, disclosing their relationships with friction velocity, particle size and roughness length, and the
possible underlying mechanisms. Besides, the average particle lift-off velocity and the average mean vertical aerodynamic
action upon the ascending particle, which determine the saltation process, are explicitly expressed by parameters involved
in the similarity model, and their relationships with friction velocity, particle size and roughness length are also described
concisely. The corrected average particle lift-off velocity makes it possible to investigate the characteristic particle trajectory,
whose initial velocity equals the average lift-off velocity, so as to estimate the average particle against surface impacting
velocity and the average aerodynamic action upon the saltation process. 相似文献
8.
The Influences of Thermodynamic Characteristics on Aerodynamic Roughness Length over Land Surface 
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下载免费PDF全文 It has previously been shown that aerodynamic roughness length changes significantly along with nearsurface atmospheric thermodynamic state; however, at present, this phenomenon remains poorly understood, and very little research concerning this topic has been conducted. In this paper, by using the data of different underlying surfaces provided by the Experimental Co-observation and Integral Research in Semi-arid and Arid Regions over North China, aerodynamic roughness length (z0) values in stable, neutral, and unstable atmospheric stratifications are compared with one another, and the relationship between z0 and atmospheric thermodynamic stability (ζ) is analyzed. It is found that z0 shows great differences among the stable, neutral, and unstable atmospheric thermodynamic states, with the difference in z0 values between the fully thermodynamic stable condition and the neutral condition reaching 60% of the mean z0. Furthermore, for the wind speed range in which the wind data are less sensitive to z0, the surface z0 changes more significantly with ζ, and is highly correlated with both the Monin-Obukhov stability (ζ0) and the overall Richardson number (Rib), with both of their correlation coefficients greater than 0.71 and 0.47 in the stable and unstable atmospheric stratification, respectively. The empirical relation fitted with the experimental observations is quite consistent with the Zilitinkevich theoretical relation in the stable atmosphere, but the two are quite distinct and even show opposite variation tendencies in the unstable atmosphere. In application, however, verification of the empirical fitted relations by using the experimental data finds that the fitted relation is slightly more applicable than the Zilitinkevich theoretical relation in stable atmospheric stratification, but it is much more suitable than the Zilitinkevich relation in unstable atmospheric stratification. 相似文献
9.
Meteorological data obtained from sensors mounted on a 300-m antenna mast, including wind components and temperature fluctuations together with high resolution profile measurements, were used to calculate quantities which describe the most important processes during the development and decay of ground-based inversions. The vertical distribution of air temperature during the developing phase of radiation inversions has been measured continuously and compared with an existing simple prognostic model. Free and forced convection conditions have been investigated for the inversion lifting phase by measuring the heat fluxes at the surface (H
s) and at the inversion base (H
i), the heating rate of the mixed layer (/z)
m
, the friction velocity (u
*) and the entrainment rate (dh/dt). Comparisons between the directly measured entrainment rates and those calculated by models show good agreement. For nearly free convection conditions, the entrainment parameter (k = -H
i/Hs) has been determined to be about 0.25. 相似文献
10.
A wind-tunnel experiment was designed and carried out to study the effect of a surface roughness transition on subfilter-scale
(SFS) physics in a turbulent boundary layer. Specifically, subfilter-scale stresses are evaluated that require parameterizations
and are key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. The surface transition
considered in this study consists of a sharp change from a rough, wire-mesh covered surface to a smooth surface. The resulting
magnitude jump in aerodynamic roughnesses, M = ln(z
01/z
02), where z
01 and z
02 are the upwind and downwind aerodynamic surface roughnesses respectively, is similar to that of past experimental studies
in the atmospheric boundary layer. The two-dimensional velocity fields used in this study are measured using particle image
velocimetry and are acquired at several positions downwind of the roughness transition as well as over a homogeneous smooth
surface. Results show that the SFS stress, resolved strain rate and SFS transfer rate of resolved kinetic energy are dependent
on the position within the boundary layer relative to the surface roughness transition. A mismatch is found in the downwind
trend of the SFS stress and resolved strain rate with distance from the transition. This difference of behaviour may not be
captured by some eddy-viscosity type models that parameterize the SFS stress tensor as proportional to the resolved strain
rate tensor. These results can be used as a benchmark to test the ability of existing and new SFS models to capture the spatial
variability SFS physics associated with surface roughness heterogeneities. 相似文献
11.
Velocity and Surface Shear Stress Distributions Behind a Rough-to-Smooth Surface Transition: A Simple New Model 总被引：1，自引：1，他引：0
A simple new model is proposed to predict the distribution of wind velocity and surface shear stress downwind of a rough-to-smooth
surface transition. The wind velocity is estimated as a weighted average between two limiting logarithmic profiles: the first
log law, which is recovered above the internal boundary-layer height, corresponds to the upwind velocity profile; the second
log law is adjusted to the downwind aerodynamic roughness and local surface shear stress, and it is recovered near the surface,
in the equilibrium sublayer. The proposed non-linear form of the weighting factor is equal to ln(z/z
01)/ln(δ
i
/z
01), where z, δ
i
and z
01 are the elevation of the prediction location, the internal boundary-layer height at that downwind distance, and the upwind
surface roughness, respectively. Unlike other simple analytical models, the new model does not rely on the assumption of a
constant or linear distribution for the turbulent shear stress within the internal boundary layer. The performance of the
new model is tested with wind-tunnel measurements and also with the field data of Bradley. Compared with other existing analytical
models, the proposed model shows improved predictions of both surface shear stress and velocity distributions at different
positions downwind of the transition. 相似文献
12.
We test a surface renewal model that is widely used over snow and ice surfaces to calculate the scalar roughness length (z s ), one of the key parameters in the bulk aerodynamic method. For the first time, the model is tested against observations that cover a wide range of aerodynamic roughness lengths (z 0). During the experiments, performed in the ablation areas of the Greenland ice sheet and the Vatnajökull ice cap in Iceland, the surface varied from smooth snow to very rough hummocky ice. Over relatively smooth snow and ice with z 0 below a threshold value of approximately 10?3 m, the model performs well and in accord with earlier studies. However, with growing hummock size, z 0 increases well above the threshold and the bulk aerodynamic flux becomes significantly smaller than the eddy-correlation flux (e.g. for z 0 = 0.01 m, the bulk aerodynamic flux is about 50% smaller). Apparently, the model severely underpredicts z s over hummocky ice. We argue that the surface renewal model does not account for the deep inhomogeneous roughness sublayer (RSL) that is generated by the hummocks. As a consequence, the homogeneous substrate ice grain cover becomes more efficiently ‘ventilated’. Calculations with an alternative model that includes the RSL and was adapted for use over hummocky ice, qualitatively confirms our observations. We suggest that, whenever exceedance of the threshold occurs (z 0 > 10?3 m, i.e., an ice surface covered with at least 0.3-m high hummocks), the following relation should be used to calculate scalar roughness lengths, ln (z s /z 0) = 1.5 ? 0.2 ln (Re *) ? 0.11(ln (Re *))2. 相似文献
13.
Aerodynamic Parameters of Regular Arrays of Rectangular Blocks with Various Geometries 总被引：8，自引：8，他引：0
The aerodynamic effects of various configurations of an urban array were investigated in a wind-tunnel experiment. Three aerodynamic
parameters characterising arrays—the drag coefficient (C
d
), roughness length (z
o) and displacement height (d)—are used for analysis. C
d
is based on the direct measurement of the total surface shear using a floating element, and the other two parameters are
estimated by logarithmic fitting of the measured wind profile and predetermined total drag force. The configurations of 63
arrays used for measurement were designed to estimate the effects of layout, wind direction and the height variability of
the blocks on these parameters for various roughness packing densities. The results are summarised as follows: (1) The estimated
C
d
and z
o of the staggered arrays peak against the plan area index (λ
p
) and frontal area index (λ
f
), in contrast with values for the square arrays, which are less sensitive to λ
p
and λ
f
. In addition, the square arrays with a wind direction of 45° have a considerably larger C
d
, and the wind direction increases z
o/H by up to a factor of 2. (2) The effect of the non-uniformity of roughness height on z
o is more remarkable when λ
f
exceeds 20%, and the discrepancy in z
o is particularly remarkable and exceeds 200%. (3) The effect of the layout of tall blocks on C
d
is stronger than that of short blocks. These results indicate that the effects of both wind direction and the non-uniformity
of the heights of buildings on urban aerodynamic parameters vary greatly with λ
p
and λ
f
; hence, these effects should be taken into account by considering the roughness packing density. 相似文献
14.
Yaping Shao 《Boundary-Layer Meteorology》2005,115(2):319-338
A similarity theory for saltation is presented. To derive the theory, a saltation model is developed and applied to simulating particle motion in turbulent flows. The numerical results are then fitted to simple universal expressions, or similarity functions. This approach allows the determination of saltation mass flux and other quantities. While the theory can be general, we focus on studying the saltation mass flux profile q(z). It is shown that q(z) is determined by friction velocity, aerodynamic roughness length and soil particle size distribution. There are two limiting situations, i.e., the saltation of large particles in weak turbulence and that of small particles in strong turbulence, for these two cases, q(z) is respectively exponential and Gaussian. Modified saltation has an intermediate saltation mass flux profile. For multi-sized particles, q(z) is a weighted superposition of many different profiles. The theory is compared with wind-tunnel observations, and uncertainties of the theory are discussed. 相似文献
15.
For flow over natural surfaces, there exists a roughness sublayer within the atmospheric surface layer near the boundary. In this sublayer (typically 50z
0 deep in unstable conditions), the Monin-Obukhov (M-O) flux profile relations for homogeneous surfaces cannot be applied. We have incorporated a modified form of the M-O stability functions (Garratt, 1978, 1980, 1983) in a mesoscale model to take account of this roughness sublayer and examined the diurnal variation of the boundary-layer wind and temperature profiles with and without these modifications. We have also investigated the effect of the modified M-O functions on the aerodynamic and laminar-sublayer resistances associated with the transfer of trace gases to vegetation. Our results show that when an observation height or the lowest level in a model is within the roughness sublayer, neglect of the flux-profile modifications leads to an underestimate of resistances by 7% at the most. 相似文献
16.
Edgar L. Andreas 《Boundary-Layer Meteorology》2011,141(3):333-347
A common parametrization over snow-covered surfaces that are undergoing saltation is that the aerodynamic roughness length
for wind speed (z
0) scales as au*2/g{\alpha u_\ast^2/g}, where u
* is the friction velocity, g is the acceleration of gravity, and α is an empirical constant. Data analyses seem to support this scaling: many published plots of z
0 measured over snow demonstrate proportionality to u*2{u_\ast^2 }. In fact, I show similar plots here that are based on two large eddy-covariance datasets: one collected over snow-covered
Arctic sea ice; another collected over snow-covered Antarctic sea ice. But in these and in most such plots from the literature,
the independent variable, u
*, was used to compute z
0 in the first place; the plots thus suffer from fictitious correlation that causes z
0 to unavoidably increase with u
* without any intervening physics. For these two datasets, when I plot z
0 against u
* derived from a bulk flux algorithm—and thus minimize the fictitious correlation—z
0 is independent of u
* in the drifting snow region, u
* ≥ 0.30 ms−1. I conclude that the relation z0 = au*2/g{z_0 = \alpha u_\ast^2/g} when snow is drifting is a fallacy fostered by analyses that suffer from fictitious correlation. 相似文献
17.
In this study, a detailed model of an urban landscape has been re-constructed inthe wind tunnel and the flow structure inside and above the urban canopy has beeninvestigated. Vertical profiles of all three velocity components have been measuredwith a Laser-Doppler velocimeter, and an extensive analysis of the measured meanflow and turbulence profiles carried out. With respect to the flow structure inside thecanopy, two types of velocity profiles can be distinguished. Within street canyons,the mean wind velocities are almost zero or negative below roof level, while closeto intersections or open squares, significantly higher mean velocities are observed.In the latter case, the turbulent velocities inside the canopy also tend to be higherthan at street-canyon locations. For both types, turbulence kinetic energy and shearstress profiles show pronounced maxima in the flow region immediately above rooflevel.Based on the experimental data, a shear-stress parameterization is proposed, inwhich the velocity scale, us, and length scale, zs, are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, ds, is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature.The length scales derived from the shear-stress parameterization also allowdetermination of appropriate scales for the mean wind profile. The roughnesslength, z0, and displacement height, d0, can both be described as fractions of the distance, zs - ds, between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth zs - ds as the roughness layer. With respect to the lower part of the canopy (z < ds) the flow behaves as a skimming flow. Correlations between the length scales zs and ds and morphometric parameters are discussed.The mean wind profiles above the urban structure follow a logarithmic windlaw. A combination of morphometric estimation methods for d0 and z0 with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u*, appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughnesssublayer, a local scaling approach results in good agreement between measuredand predicted mean wind profiles. 相似文献
18.
Edgar L Andreas Rachel E. Jordan Aleksandar P. Makshtas 《Boundary-Layer Meteorology》2005,114(2):439-460
A 4-month deployment on Ice Station Weddell (ISW) in the western Weddell Sea yielded over 2000 h of nearly continuous surface-level meteorological data, including eddy-covariance measurements of the turbulent surface fluxes of momentum, and sensible and latent heat. Those data lead to a new parameterization for the roughness length for wind speed, z0, for snow-covered sea ice that combines three regimes: an aerodynamically smooth regime, a high-wind saltation regime, and an intermediate regime between these two extremes where the macroscale or `permanent' roughness of the snow and ice determines z0. Roughness lengths for temperature, zT, computed from this data set corroborate the theoretical model that Andreas published in 1987. Roughness lengths for humidity,zQ, do not support this model as conclusively but are all, on average, within an order of magnitude of its predictions. Only rarely arezTand zQ equal to z0. These parameterizations have implications for models that treat the atmosphere-ice-ocean system. 相似文献
19.
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
20.
An Improvement of the Louis Scheme for the Surface Layer in an Atmospheric Modelling System 总被引：1，自引：1，他引：0
Yu Song 《Boundary-Layer Meteorology》1998,88(2):239-254
The Louis scheme for calculating the vertical eddy fluxes within the atmospheric surface layer is improved by broadening the original assumptions. In our approach, the momentum and heat transfer roughness lengths (z0 and zT respectively) can be different, and z0 need not be negligibly small compared with the lowest height (z) in modelling. For these conditions, we choose more consistent wind and potential temperature profile forms, then derive new algorithms for calculating fluxes. Improvement is demonstrated for a wide range of z/L (L is the Obukhov length), z/z0 and z0 zT, by comparing these fluxes with those derived from a theoretical surface-layer model. The improved algorithms can be used in atmospheric modelling systems for more varied surfaces and a wide range of atmospheric stability. 相似文献