共查询到20条相似文献，搜索用时 31 毫秒
1.
Qin Wenhan 《Acta Meteorologica Sinica》1995,9(1):78-86
Using data on leaf area density and wind profiles above and within canopies of wheat,rice,soybean and corn,thecenter-of-pressure method (CPM),originally proposed by Thom (1971),is first validated in the field.Aphysically-based model for directly calculating zero-displacement height (d) is derived.The comparison between thefriction velocity (u_*) estimated with CPM and that with eddy correlation technique shows that CPM not only works wellin the field,but also produces more steady and accurate estimates of aerodynamic parameters (which are hardly affectedby atmospheric thermal stability),than those with the widely-used log-profile fitting method in diabatic atmosphere.The results presented in this paper also demonstrate that the ratios of d,z_0 to crop height h usually vary with canopy ar-chitecture,atmospheric stratification and turbulent exchange intensity,and are not just constants as commonly assumedor used.d/h raises with an increase of relative height of the maximum foliage layer and wind extinction coefficientwithin the canopy.Only for crops with short stem and moderate foliage density,can the relations d= 0.64h,z_0=0.08hkeep stable.In addition,for long stem crops or sparse canopies,the fairly large shear stress at the soil surface and the va-riation of the exponent in the relationship between drag coefficient and wind speed undoubtedly influence the accuracyof CPM to a certain extent. 相似文献
2.
An existing second-order closure model is modified to include the effects on mean and turbulent motions of form and viscous drag in vegetative canopies. The additional physical mechanisms represented by the closure are viscous and pressure drag on canopy elements, their role in momentum absorption, in the creation of fine scale turbulent eddies and in enhancing the total viscous dissipation in the canopy airspace. Viscous dissipation is split into a standard 'isotropic contribution associated with the spectral eddy cascade and a foliage contribution associated with work against pressure and viscous drag on the foliage. Changes in the turbulent time scale that result from these mechanisms are included in the standard parameterisations of third moments and of the eddy cascade contribution to dissipation. The model is tested against a wind- tunnel 'wheat canopy, a corn canopy and a eucalypt canopy, a height range from 50 mm to 12.6 m. Model results show that the parameterisations of foliage interaction used in the closure are sufficiently robust to reproduce second-moment profiles within and above vegetative canopies to a high degree of accuracy without resorting to 'tuning of the model constants. The model also shows the natural emergence of two length scales, one associated with the familiar eddy cascade isotropic contribution to total dissipation and the other associated with the length scales of the canopy elements. 相似文献
3.
Momentum Transfer By A Mountain Meadow Canopy: A Simulation Analysis Based On Massman's (1997) Model
Using a mountain meadow as a case study it is the objective of the present paper todevelop a simple parameterisation for the within-canopy variation of the phytoelementdrag (Cd) and sheltering (Pm) coefficients required for Massman's model of momentum transfer by vegetation. A constant ratio between Cd and Pm is found to overestimate wind speed in the upper canopy and underestimate it in the lower canopy.Two simple parameterisations of Cd/Pm as a function of the plant area density and the cumulative plant area index are developed, using values optimised by least-squares regression between measured and predicted within-canopy wind speeds. A validation with independently measured data indicates that both parameterisations work reliably for simulating wind speed in the investigated meadow. Model predictions of the normalised zero-plane displacement height and the momentum roughness length fall only partly within the range of values given in literature, which may be explained by the accumulation of plantmatter close to the soil surface specific for the investigated canopies. The seasonal course of the normalised zero-plane displacement height and the momentum roughness length are discussed in terms of the seasonal variation of the amount and density of plant matter. 相似文献
4.
The roughness length for momentum (z0m), zero-plane displacementheight (d), and roughness length for heat (z0h) are importantparameters used to estimate land-atmosphere energy exchange. Although many different approaches have been developed to parameterizemomentum and heat transfer, existing parameterizations generally utilizehighly simplified representations of vegetation structure. Further, a mismatch exists between the treatments used for momentum and heat exchange and those used for radiative energy exchanges. In this paper, parameterizations are developed to estimate z0m, d, and z0h for forested regimes using information related to tree crown density and structure. The parameterizations provide realistic representationfor the vertical distribution of foliage within canopies, and include explicit treatment for the effects of the canopy roughness sublayer and leaf drag on momentum exchange. The proposed parameterizationsare able to realistically account for site-to-site differences in roughness lengths that arise from canopy structural properties.Comparisons between model predictions and field measurements show good agreement, suggesting that the proposed parameterizations capture the most important factors influencing turbulent exchange of momentumand heat over forests. 相似文献
5.
An analytical one-dimensional second-order closure model is developed to describe the within canopy velocity variances, turbulent intensities, dissipation rates, Lagrangian time scale and Lagrangian far field diffusivities for vegetation canopies of arbitrary structure and density. The model incorporates and extends the model of momentum transfer developed by Massman (1997) and the model of within canopy velocity variances developed by Weil (unpublished) from the second-order closure model of Wilson and Shaw (1977). Model predictions of within and above canopy velocity variances, turbulent intensities, dissipation rates and the Lagrangian time scale are in reasonable agreement with previously measured or estimated values for these parameters. The present model suggests that the Lagrangian time scale and the far field diffusivity could be strongly dependent upon foliage structure and density through the foliage effects on the velocity variances. A simple formulation for the Lagrangian time scale at canopy height is derived from model results. Taken as a whole, the present model may provide a relatively simple way to incorporate turbulence parameters into models of soil/canopy/atmosphere mass transfer. 相似文献
6.
A numerical model was developed to simulate neutrally stratified air flow over and through a forest edge. The spatially averaged equations for turbulent flow in vegetation canopies are derived as the governing equations. A first-order closure scheme with the capability of accounting for the bulk momentum transport process in vegetation canopies is employed. The averaged equations are solved numerically by a fractional time-step method and successive relaxation. The asymptotic solution in time is regarded as the steady-state solution. Comparisons of model output to the field measurements of Raynor (1971) indicate that the model provides a realistic mean flow.Momentum balance computations show that the pressure gradient induced by the wind blowing against the forest edge is significant and has the same order of magnitude as the drag force in the edge region. The edge effect involves the generation of drag forces, the appearance of a large pressure gradient, the upward deflection of mean flow and the transport of momentum into the edge of the canopy. 相似文献
7.
Dennis D. Baldocchi Shashi B. Verma Norman J. Rosenberg 《Boundary-Layer Meteorology》1983,25(1):43-54
Air flow was observed above and within canopies of a number of kinds of soybeans. The Clark cultivar and two isolines of the Harosoy cultivar were studied in 1979 and 1980, respectively. Wind speed above the canopy was measured with cup anemometers. Heated thermistor anemometers were used to measure air flow within the canopy. Above-canopy air flow was characterized in terms of the zero-plane displacement (d), roughness parameter (z o) and drag coefficient (C d). d and z o were dependent on canopy height but were independent of friction velocity in the range 0.55 to 0.75 m s?1 · C d for the various canopies ranged from 0.027 to 0.035. Greater C d values were measured over an erectophile canopy than over a planophile canopy. C d was not measurably affected by differences in leaf pubescence. Within-canopy wind profiles were measured at two locations: within and between rows. The wind profile was characterized by a region of great wind shear in the upper canopy and by a region of relatively weak wind shear in the middle canopy. Considerable spatial variability in wind speed was evident, however. This result has significant implications for canopy flow modeling efforts aimed at evaluating transport in the canopy. In the lower canopy, wind speed within a row increased with depth whereas wind speed between two rows decreased with depth. The wind speeds at the two locations tended to converge to a common value at a height near 0.10 m. The attenuation of within-canopy air flow was stronger in canopies with greater foliage density. Canopy flow attenuation seemed to decrease with increasing wind speed, suggesting that high winds distorted the shape of the canopy in such a manner that the penetration of wind into the canopy increased. 相似文献
8.
9.
On the basis of improving the algorithm of the mixing length in and above forest canopies, a PBL numerical model including the multi-layer, heterogeneous vegetation is developed. Simulations indicate that different treatments of mixing length can make a great difference in the wind field especially for dense forest, and results from the improved mixing length scheme are in better agreement with observations than those from the original scheme. It may be expected that the improved mixing length scheme can lead to more ra-tional turbulent transfer than the original one. From the sensitivity experiments, we obtain the characteris-tics of both wind and temperature profiles in and above plant canopies, e.g., during the daytime, a stable thermal stratification exists near the surface in the canopies, but a neutral or slightly unstable condition ap-pears above plant canopies, while at night the reverse situations occur; the increase of the temperature of the dense-forest case is less than that of the sparse-forest case; the windspeed is reduced within the canopy lay-er and the large wind shear occurs near the treetop, etc. 相似文献
10.
1.IntroductionItiswellknownthattheecosystemcangreatlyinfluencebothlocalclimateandgeneralcirculation.Onthenumericalstudyoftheturbulenceinandaboveforestcanopies,alotofsignificantstudieshavebeendone.Inallthesestudies,modelsaregenerallydividedintotwotypes:oneis'K--theory'type(Waggoner,1975;Gross,1987;Gross,1988,Jietal.,1989;Schilling,I991;Dickinsonetal.,1993;Wang,1996),theotherappliesthehigher--orderclosuremethod(Wilsonetal.,1977,Yamada,1982;Yinetal.,1989)ortheLagrangianmethod(Rampach,1987;R… 相似文献
11.
12.
Characteristics of Momentum and Heat Transfer over Semiarid Grasslands with Different Grazing Intensities in Inner Mongolia, China 
下载免费PDF全文
下载免费PDF全文 The drag coefficient (C d) and heat transfer coefficient (C h) with the bulk transfer scheme are usually used to calculate the momentum and heat fluxes in meteorological models.The aerodynamic roughness length (z 0m) and thermal roughness length (z 0h) are two crucial parameters for bulk transfer equations.To improve the meteorological models,the seasonal and interannual variations of z 0m,z 0h,coefficient kB 1,C d,and C h were investigated based on eddy covariance data over different grazed semiarid grasslands of Inner Mongolia during the growing seasons (May to September) from 2005 to 2008.For an ungrazed Leymus chinensis grassland (ungrazed since 1979),z 0m and z 0h had significant seasonal and interannual variations.z 0m was affected by the amount and distribution of rainfall.kB 1 exhibited a relatively negative variation compared with z 0h,which indicates that the seasonal variation of z 0h cannot be described by kB 1.To parameterize z 0m and z 0h,the linear regressions between ln(z 0m),ln(z 0h),and the leaf area index (LAI) were performed with R 2 =0.71 and 0.83.The monthly average kB 1 was found to decrease linearly with LAI.The four-year averaged values of C d and C h were 4.5×10 3 and 3.9×10 3,respectively.The monthly average C d only varied by 8% while the variation of C h was 18%,which reflects the different impacts of dead vegetation on momentum and heat transfer at this natural grassland.Moreover,with the removal of vegetation cover,grazing intensities reduced z 0m,z 0h,C d,and C h. 相似文献
13.
多层城市冠层模式的建立及数值试验研究 总被引：4，自引：1，他引：3
为在城市气象数值模拟中更好地体现由城市发展引起的下垫面土地利用改变及人为活动对大气过程的影响,建立了基于建筑物三维分布的多层城市冠层模式,冠层内动力方程组考虑了建筑物冠层拖曳力的作用及雷诺应力的影响,通过引入建筑物宽度、间距以及垂直分布密度指数等建筑物形态特征参数,以更好地体现城市复杂地表对大气温度、湿度及动量方程的影响.同时,该模式分屋顶、4个侧壤及地面分别考虑辐射及能量平衡求解表面温度,计算各表面与大气的通量交换,并考虑辐射阴影效应、冠层内部各个面之间的可视因子、以及与冠层内建筑物密度指数、可视因子等相关的多重反射辐射导致的辐射截陷作用.模式的离线检验结果表明:(1)冠层模式计算风廓线与风洞实验测量数据吻合良好;(2)离线冠层模式能够模拟实际小区的风速、温度垂直廓线,并能够较好地体现小区内气温日变化.冠层模式与区域边界层模式耦合检验结果表明:(1)耦合模拟的近地面(2 m处)气温及地表温度的结果明显优于传统的水泥平板方案,尤其是在夜间,水泥平板方案与实测气温最大偏差4 K左右,耦合模拟方案为1-2 K;(2)耦合模拟方案考虑了建筑物对冠层之上的拖曳力影响以及建筑物形态结构对雷诺应力的影响,风速(10 m处)计算结果与观测值相差约在1 m/s,水泥平板方案偏差3 m/s左右. 相似文献
14.
Turbulence above and within canopies has characteristics distinct from that over rough surfaces. The vertical transport of
momentum and scalars is dominated by coherent structures whose origin is now thought to be the result of the unstable inflexion
in the profile of the mean wind speed established by the application of canopy drag. This distinctive property leads to the
failure of the standard Monin–Obukhov flux–profile relationships over homogeneous canopies, relationships that are assumed
in many surface exchange schemes within numerical weather prediction and general circulation models. A modification of the
flux–profile relationships is presented that incorporates the effects of the canopy turbulence. The subsequent impacts on
the evolution of the surface energy balance and boundary-layer state are investigated within a simple numerical model for
the evolution of the boundary layer and canopy state. By comparing cases with and without the modification it is shown that
canopy-generated turbulence can lead, not only to the alteration of the flux–profile relationships above the canopy, but also
to a different evolution of the surface energy balance and differences in near-surface conditions that would be significant
in numerical weather prediction. More fundamentally, the modifications to the flux–profile relationships imply that parameters
such as the roughness length and displacement height for canopies should not be considered as invariant properties, but rather
as properties that depend on the flow and hence vary systematically with the diabatic stability of the boundary layer. 相似文献
15.
综合考虑梯度输送和阵性穿透作用，并对植被上方的过渡层作适当处理，提出了植被微气象的一阶闭合模型。它对温、湿、风等物理量垂直分布的模拟精度良好。引用系统优化理论和方法，对模型中参数作多维非线性搜索，结果客观可靠。由于植物本身的调节适应能力和反馈作用，大气与植被间的动量与水热交换机制和过程不尽相同。在动量输送过程中，梯度产生项和阵性穿透皆不可忽视，尤其在群体下部，更为重要；而在热量和水分输送过程中，阵性穿透作用在低矮密集植被中的贡献甚微，梯度产生项起主导作用。 相似文献
16.
A wind-profile index for canopy flow 总被引：5，自引：0，他引：5
Ronald M. Cionco 《Boundary-Layer Meteorology》1972,3(2):255-263
Canopy wind profiles can often be represented by an exponential function. The associated attenuation index,a, is found to be proportional to [(Flexibility)(Leaf Area)(Density)]1/3. Leastsquare values of the index have been calculated for wind profiles in about a dozen natural and artificial canopies which included oats, wheat, corn, rice, sunflowers, larch trees, citrus trees, Xmas trees, plastic strips, wooden pegs and bushel baskets. It is found that canopy flow is a function of canopy density, element flexibility, and height and that the behaviour of artificial canopy elements is compatible with that of natural vegetation. The same calculations also show that the attenuation coefficient: (a) is not a universal constant, (b) is however, rather limited in range (-0.3 to 3.0), (c) varies with stage of growth, and (d) increases as density and flexibility increase. A compilation ofa-values for several canopies reveals that lowa-values correspond to sparsely arrayed rigid elements while higha-values correspond to densely arrayed and flexible elements. Finally, lowa-values appear to be relatively independent of wind speed, while higha-values tend to increase as wind speeds increase. 相似文献
17.
An Analytical Model for Mean Wind Profiles in Sparse Canopies 总被引：2，自引：2，他引：0
Weiguo Wang 《Boundary-Layer Meteorology》2012,142(3):383-399
Existing analytical models for mean wind profiles within canopies are applicable only in dense canopy scenarios, where all
momentum is absorbed by canopy elements and, hence, the effect of the ground on turbulent mixing is not important. Here, we
propose a new analytical model that can simulate mean wind profiles within sparse canopies under neutral conditions. The model
adopts a linearized canopy-drag parametrization and a first-order turbulence closure scheme taking into account the effects
of both the ground and canopy elements on turbulent mixing. The resulting wind profile within a sparser canopy appears to
be more like a logarithmic form, with the no-slip condition at the ground being satisfied. The analytical solution converges
exactly to the standard surface-layer logarithmic wind profile in the case of zero canopy density (i.e., no-canopy scenario)
and tends to be an exponential wind profile for a dense canopy; this feature is unique compared with existing analytical models
for canopy wind profiles. Results from the new model are in good agreement with those from laboratory experiments and numerical
simulations. 相似文献
18.
The turbulent flow in and above plant canopies is of fundamental importance to the understanding oftransport processes of momentum,heat and mass between plant canopies and atmosphere,and to microme-teorology.The Reynolds stress equation model(RSM)has been applied to calculate the turbulence in cano-pies in this paper.The calculated mean wind velocity profiles,Reynolds stress,turbulent kinetic energy andviscous dissipation rate in a corn canopy and a spruce forest are compared with field observed data and withWilson's and Shaw's model.The velocity profiles and Rynolds stress calculated by both models are in goodagreement,and the length scale of turbulence appears to be similar. 相似文献
19.
Synchronous sonic anemometric measurements at five heightswithin a mixed coniferous forest were used to test two different parameterisations ofcanopy architecture in the application of a second-order turbulence closure model. Inthe computation of the leaf drag area, the aerodynamic sheltering was replaced with anarchitectural sheltering, assumed to be analogous to the clumping index defined in radiativetransfer theory. Consequently, the ratio of leaf area density and sheltering factor was approximatedby the effective leaf area or the mean contact number, both obtained from the inversion of non-destructive optical measurements. The first parameter represents the equivalentrandomly dispersed leaf area in terms of shading, the second is the average number of leavesthat a straight line intercepts penetrating the canopy with a certain zenith angle. Theselection of this direction was determined by the analysis of the mean angle of the wind vectorduring sweep events. The drag coefficient values obtained from the inversion of themomentum flux equation, using the two proposed parameterisations, are in good agreement withvalues found in the literature. The predicted profiles of turbulence statistics reasonablymatch actual measurements, especially in the case of the mean contact numberparameterisation. The vertical profile of leaf drag area, obtained by forcing the turbulence modelto match the observed standard deviation of vertical velocity (w), is intermediatebetween the two empirical ones. Finally, the proposed canopy parameterisations were appliedto a Lagrangian transport model to predict vertical profiles of air temperature, H2O andCO2 concentration. 相似文献
20.
Ali Chahine Sylvain Dupont Carole Sinfort Yves Brunet 《Boundary-Layer Meteorology》2014,151(3):557-577
Wind-flow dynamics has been extensively studied over horizontally uniform canopies, but agricultural plantations structured in rows such as vineyards have received less attention. Here, the wind flow over a vineyard is studied in neutral stratification from both large-eddy simulation (LES) and in situ measurements. The impact of row structure on the wind dynamics is investigated over a range of wind directions from cross-row to down-row, and a typical range of row aspect ratio (row separation/height ratio). It is shown that the mean flow over a vineyard is similar to that observed in uniform canopies, especially for wind directions from cross-row to diagonal. For down-row winds, the mean flow exhibits noticeable spatial variability across each elementary row-gap pattern, as the wind is channeled in the inter-row. This spatial variability increases with the aspect ratio. With down-row winds the turbulent structures are also more intermittent and generate larger turbulent kinetic energy and momentum flux. The displacement height and roughness length of the vineyard vary with the aspect ratio in a way similar to their variation with canopy density in uniform canopies. Both parameters take smaller values in down-row wind flow, for which the canopy appears more open. The analysis of velocity spectra and autocorrelation functions shows that vineyard canopies share similar features to uniform canopies in terms of turbulent coherent structures, with only minor changes with wind direction. 相似文献