共查询到20条相似文献，搜索用时 31 毫秒
1.
G. G. Katul J. J. Finnigan D. Poggi R. Leuning S. E. Belcher 《Boundary-Layer Meteorology》2006,118(1):189-216
Topography influences many aspects of forest-atmosphere carbon exchange; yet only a small number of studies have considered
the role of topography on the structure of turbulence within and above vegetation and its effect on canopy photosynthesis
and the measurement of net ecosystem exchange of CO2 (Nee) using flux towers. Here, we focus on the interplay between radiative transfer, flow dynamics for neutral stratification,
and ecophysiological controls on CO2 sources and sinks within a canopy on a gentle cosine hill. We examine how topography alters the forest-atmosphere CO2 exchange rate when compared to uniform flat terrain using a newly developed first-order closure model that explicitly accounts
for the flow dynamics, radiative transfer, and nonlinear eco physiological processes within a plant canopy. We show that variation
in radiation and airflow due to topography causes only a minor departure in horizontally averaged and vertically integrated
photosynthesis from their flat terrain values. However, topography perturbs the airflow and concentration fields in and above
plant canopies, leading to significant horizontal and vertical advection of CO2. Advection terms in the conservation equation may be neglected in flow over homogeneous, flat terrain, and then Nee = Fc, the vertical turbulent flux of CO2. Model results suggest that vertical and horizontal advection terms are generally of opposite sign and of the same order
as the biological sources and sinks. We show that, close to the hilltop, Fc departs by a factor of three compared to its flat terrain counterpart and that the horizontally averaged Fc-at canopy top differs by more than 20% compared to the flat-terrain case. 相似文献
2.
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
3.
Turbulent Pressure and Velocity Perturbations Induced by Gentle Hills Covered with Sparse and Dense Canopies 总被引：1，自引：1，他引：0
How the spatial perturbations of the first and second moments of the velocity and pressure fields differ for flow over a train of gentle hills covered by either sparse or dense vegetation is explored using large-eddy simulation (LES). Two simulations are investigated where the canopy is composed of uniformly arrayed rods each with a height that is comparable to the hill height. In the first simulation, the rod density is chosen so that much of the momentum is absorbed within the canopy volume yet the canopy is not dense enough to induce separation on the lee side of the hill. In the second simulation, the rod density is large enough to induce recirculation inside the canopy on the lee side of the hill. For this separating flow case, zones of intense shear stress originating near the canopy-atmosphere interface persist all the way up to the middle layer, ‘contaminating’ much of the middle and outer layers with shear stress gradients. The implications of these persistent shear-stress gradients on rapid distortion theory and phase relationships between higher order velocity statistics and hill-induced mean velocity perturbations (Δu) are discussed. Within the inner layer, these intense shear zones improve predictions of the spatial perturbation by K-theory, especially for the phase relationships between the shear stress (~ ?Δu/?z) and the velocity variances, where z is the height. For the upper canopy layers, wake production increases with increasing leaf area density resulting in a vertical velocity variance more in phase with Δu than with ?Δu/?z. However, background turbulence and inactive eddies may have dampened this effect for the longitudinal velocity variance. The increase in leaf area density does not significantly affect the phase relationship between mean surface pressure and topography for the two simulations, though the LES results here confirm earlier findings that the minimum mean pressure shifts downstream from the hill crest. The increase in leaf area density and associated flow separation simply stretches this difference further downstream. This shift increases the pressure drag, the dominant term in the overall drag on the hill surface, by some 15%. With regards to the normalized pressure variance, increasing leaf area density increases ${\sigma_p/u_{*}^{2}}$ near the canopy top, where u * is the longitudinally averaged friction velocity at the canopy top and σ p is the standard deviation of the pressure fluctuations. This increase is shown to be consistent with a primitive scaling argument on the leading term describing the mean-flow turbulent interaction. This scaling argument also predicts the spatial variations in σ p above the canopy reasonably well for both simulations, but not inside the canopy. 相似文献
4.
Andrew N. Ross 《Boundary-Layer Meteorology》2011,141(2):179-199
Numerical simulations of scalar transport in neutral flow over forested ridges are performed using both a 1.5-order mixing-length
closure scheme and a large-eddy simulation. Such scalar transport (particularly of CO2) has been a significant motivation for dynamical studies of forest canopy–atmosphere interactions. Results from the 1.5-order
mixing-length simulations show that hills for which there is significant mean flow into and out of the canopy are more efficient
at transporting scalars from the canopy to the boundary layer above. For the case with a source in the canopy this leads to
lower mean concentrations of tracer within the canopy, although they can be very large horizontal variations over the hill.
These variations are closed linked to flow separation and recirculation in the canopy and can lead to maximum concentrations
near the separation point that exceed those over flat ground. Simple scaling arguments building on the analytical model of
Finnigan and Belcher (Q J Roy Meteorol Soc 130:1–29, 2004) successfully predict the variations in scalar concentration near the canopy top over a range of hills. Interestingly this
analysis suggests that variations in the components of the turbulent transport term, rather than advection, give rise to the
leading order variations in scalar concentration. The scaling arguments provide a quantitative measure of the role of advection,
and suggest that for smaller/steeper hills and deeper/sparser canopies advection will be more important. This agrees well
with results from the numerical simulations. A large-eddy simulation is used to support the results from the mixing-length
closure model and to allow more detailed investigation of the turbulent transport of scalars within and above the canopy.
Scalar concentration profiles are very similar in both models, despite the fact that there are significant differences in
the turbulent transport, highlighted by the strong variations in the turbulent Schmidt number both in the vertical and across
the hill in the large-eddy simulation that are not represented in the mixing-length model. 相似文献
5.
The deposition velocity (V
d) of nitric acid vapor over a fully leafed deciduous forest was estimated using flux/gradient theory. HNO3 deposition velocities ranged from 2.2 to 6.0cm/s with a mean V
don the order of 4.0cms-1. Estimates of V
dfrom a detailed canopy turbulence model gave deposition velocities of similar magnitude. The model was used to investigate the sensitivity of V
dto the leaf boundary-layer resistance and leaf area index (LAI). Although modeled deposition velocities were found to be sensitive to the parameterization of the leaf boundary-layer resistance, they were less sensitive to the LAI. Modeled V
d's were found to peak at LAI = 7. 相似文献
6.
Tirtha Banerjee Gabriel Katul Stefano Fontan Davide Poggi Mukesh Kumar 《Boundary-Layer Meteorology》2013,149(1):19-41
A streamfunction-vorticity formulation is used to explore the extent to which turbulent and turbulently inviscid solutions to the mean momentum balance explain the mean flow across forest edges and within cavities situated inside dense forested canopies. The turbulent solution is based on the mean momentum balance where first-order closure principles are used to model turbulent stresses. The turbulently inviscid solution retains all the key terms in the mean momentum balance but for the turbulent stress gradients. Both exit and entry versions of the forest edge problem are explored. The turbulent solution is found to describe sufficiently the bulk spatial patterns of the mean flow near the edge including signatures of different length scales reported in canopy transition studies. Next, the ‘clearing inside canopy’ or the so-called ‘cavity’ problem is solved for the inviscid and turbulent solutions and then compared against flume experiments. The inviscid solution describes the bulk flow dynamics in much of the zones within the cavity. In particular, the solution can capture the correct position of the bulk recirculation zone within the cavity, although with a weaker magnitude. The inviscid solution cannot capture the large vertical heterogeneity in the mean velocity above the canopy, as expected. These features are better captured via the first-order closure representation of the turbulent solution. Given the ability of this vorticity formulation to capture the mean pressure variations and the mean advective acceleration terms, it is ideal for exploring the distributions of scalars and roughness-induced flow adjustments on complex topography. 相似文献
7.
A one-dimensional analytical model that predicts foliage CO2 uptake rates, turbulent fluxes, and mean concentration throughout the roughness sub-layer (RSL), a layer that extends from the ground surface up to 5h, where h is canopy height, is proposed. The model combines the mean continuity equation for CO2 with first-order closure principles for turbulent fluxes and simplified physiological and radiative transfer schemes for foliage uptake. This combination results in a second-order ordinary differential equation in which soil respiration (R) and CO2 concentration well above the RSL are imposed as lower and upper boundary conditions, respectively. An inverse version of the model was tested against datasets from two contrasting ecosystems: a tropical forest (h = 40m) and a managed irrigated rice canopy (h = 0.7m), with good agreement noted between modelled and measured mean CO2 concentration profiles within the entire RSL. Sensitivity analysis on the model parameters revealed a plausible scaling regime between them and a dimensionless parameter defined by the ratio between external (R) and internal (stomatal conductance) characteristics controlling the CO2 exchange process. The model can be used to infer the thickness of the RSL for CO2 exchange, the inequality in zero-plane displacement between CO2 and momentum, and its consequences on modelled CO2 fluxes. A simplified version of the solution is well suited for being incorporated into large-scale climate models. Furthermore, the model framework here can be used to a priori estimate relative contributions from the soil surface and the atmosphere to canopy-air CO2 concentration, thereby making it synergetic to stable isotopes studies. 相似文献
8.
Qi Zhang Ming Chang Shengzhen Zhou Weihua Chen Xuemei Wang Wenhui Liao Jianing Dai ZhiYong Wu 《Asia-Pacific Journal of Atmospheric Sciences》2017,53(4):519-536
There has been a rapid growth of reactive nitrogen (Nr) deposition over the world in the past decades. The Pearl River Delta region is one of the areas with high loading of nitrogen deposition. But there are still large uncertainties in the study of dry deposition because of its complex processes of physical chemistry and vegetation physiology. At present, the forest canopy parameterization scheme used in WRF-Chem model is a single-layer “big leaf” model, and the simulation of radiation transmission and energy balance in forest canopy is not detailed and accurate. Noah-MP land surface model (Noah-MP) is based on the Noah land surface model (Noah LSM) and has multiple parametric options to simulate the energy, momentum, and material interactions of the vegetation-soil-atmosphere system. Therefore, to investigate the improvement of the simulation results of WRF-Chem on the nitrogen deposition in forest area after coupled with Noah-MP model and to reduce the influence of meteorological simulation biases on the dry deposition velocity simulation, a dry deposition single-point model coupled by Noah- MP and the WRF-Chem dry deposition module (WDDM) was used to simulate the deposition velocity (Vd). The model was driven by the micro-meteorological observation of the Dinghushan Forest Ecosystem Location Station. And a series of numerical experiments were carried out to identify the key processes influencing the calculation of dry deposition velocity, and the effects of various surface physical and plant physiological processes on dry deposition were discussed. The model captured the observed Vd well, but still underestimated the Vd. The self-defect of Wesely scheme applied by WDDM, and the inaccuracy of built-in parameters in WDDM and input data for Noah-MP (e.g. LAI) were the key factors that cause the underestimation of Vd. Therefore, future work is needed to improve model mechanisms and parameterization. 相似文献
9.
Numerical simulations of flow over hills that are partially covered with a forest canopy are performed. This represents a much more realistic situation than previous studies that have generally concentrated on hills that are fully-forested. The results show that the flow over the hill is sensitive to where on the hill the forest is positioned. In particular, for low slopes flow separation is predominantly located within the forest on the lee slope. This has implications for the transport of scalars in the forest canopy. For large hills the results show more variability in scalar concentrations within the canopy compared to either a fully-forested hill or a patch of forest over flat terrain. These results are likely to have implications for a range of applications including the siting and interpretation of flux measurements over forests in complex terrain, predicting wind damage to trees and wind-farm developments. Calculation of the hill-induced pressure drag and canopy-plus-surface stress shows a strong sensitivity to the position of the forest relative to the hill. Depending on the position of the forest the individual drag terms may be strongly enhanced or reduced and may even change sign. The net impact is generally to reduce the total drag compared to an equivalent fully-forested hill, but the amount of the reduction depends strongly on the position of the forest canopy on the hill. In many cases with large, wide hills there is a clear separation of scales between the adjustment of the canopy to a forest edge (of order 6 ? 8L c, where L c is the canopy adjustment length scale) and the width of the hill. This separation means that the hill-induced pressure and flow fields and the forest-edge induced pressure and flow fields can in some sense be considered as acting separately. This provides a means of explaining the combined effects of partial forestation and terrain. It also offers a simple method for modelling the changes in drag over a hill due to partial forest cover by considering the impact of the hill and the partial canopy separately. Scaling arguments based on this idea successfully collapse the modelled drag over a range of different hill widths and heights and for different canopy parameters. This offers scope for a relatively simple parametrization of the effects of partial forest cover on the drag over a hill. 相似文献
10.
11.
Flow Over Hills Covered by a Plant Canopy: Extension to Generalised Two-Dimensional Topography 总被引：1，自引：1，他引：0
An understanding of how topography, including that covered by a plant canopy, influences the flow and turbulence is important in many meteorological and micrometeorological applications. We revisit the framework of Finnigan and Belcher (Q J Roy Meteorol Soc 130:1–29, 2004) for neutral flow over sinusoidal two-dimensional topography covered by a canopy and present a minor modification that enables a formal match between the forms for the flow within and above the canopy, including within the deep canopy. The modification then allows the easy extension of the framework to generalised two-dimensional topography. The revised framework affirms the conclusions of Finnigan and Belcher (op.cit.); above the canopy the flow perturbations are maximised near to where the perturbation pressure is a minimum, whereas deep within the canopy the flow perturbations are maximised when the streamwise gradient of the perturbation pressure is largest. However, the extension to general topography strongly illustrates that it is the pressure perturbation, not the hill shape directly, that controls the magnitude and location of the topographic impacts on the flow. For isolated ridges with a plateau, the flow perturbations above the canopy maximise, not at the ridge crest, but where the hill curvature is largest. Multiple recirculation regions can also exist within the canopy, as determined by the streamwise gradient of the pressure perturbation. These results indicate that knowledge of the terrain over a larger area than hither-to-thought is required to assess whether the use of micrometeorological techniques is appropriate at individual sites. 相似文献
12.
Lei Xiao''en 《Acta Meteorologica Sinica》1996,10(1):118-128
A new dry deposition velocity pattern (NDDVP) for the study of region-scale dry deposition processes is developed. The mean ratio between NDDVP and 1022 experimental data of dry deposition velocity Vd is 1.06±0.82. The result shows that NDDVP is well consistent with experimental data. Practical cases are forecasted by the high resolution regional acid deposition model (EM3) with both NDDVP and old Vd pattern. The maximum ratio between the central concentrations for SO4- can reach 2.4 only due to different Vd patterns. 3-D distributions of species concentrations and dry depositions forecasted by NDDVP are better than those by the old Vd pattern. 相似文献
13.
M. R. Raupach 《Boundary-Layer Meteorology》1979,16(3):467-486
Simultaneous profile and eddy correlation flux data gathered over Thetford Forest, U.K., have been analysed to find values of the vertical turbulent diffusivities K M, K H and K E (for momentum, heat and water vapour transfer, respectively) at a reference height z R, nine roughness lengths above the zero-plane displacement d. The results show: (i), that values of K M over the forest are not significantly different from these predicted by semiempirical diabatic influence functions appropriate to much smoother surfaces such as short grass; and (ii), that K H and K E exceed their values predicted from the semiempirical functions by an average factor of 2 or more in unstable, near neutral and slightly stable conditions. These conclusions are strongly dependent on the assumed behaviour of d, here taken as 0.76 tree heights, independent of both property and stability. Consideration is given to an alternative analysis procedure, in which values of the zero-plane displacements d H and d E for heat and water vapour respectively, are obtained from the data by assuming K H and K E to be given by semiempirical diabatic influence functions; this procedure is shown to be unacceptable on both practical and physical grounds. To account for the anomalies in K H and K E, a mechanism is proposed in which the horizontally inhomogeneous temperature structure of the canopy causes free convection to be maintained by discrete; localized heat sources and/or sinks, effectively enhancing turbulent transport processes even in near neutral conditions. 相似文献
14.
Christopher Garrett 《Dynamics of Atmospheres and Oceans》1983,7(4):265-277
If a spot of tracer is released into a turbulent flow, the peak concentration at some subsequent time will initially be much greater than that implied by a solution for the ensemble average concentration at fixed points. For two-dimensional turbulence three areas may be defined: (1) an area Ad related to the ensemble average concentration field; (2) an area Ap defined in terms of the relative dispersion of particles seeded into the patch after a short initial diffusion time; and (3) the area At occupied by tracer. It is argued that Ad grows linearly with time, whereas Ap and At grow exponentially; Ap faster than At. Thus, the concentration field is significantly streaky, even within the particle domain, until At becomes comparable with Ad. The time taken for this to occur is estimated; after this time, fluctuations about the ensemble average concentration field should not be greater than those given by a simple mixing length argument. In three-dimensional turbulence the volume Vt of the tracer domain grows much more rapidly than the volume Vp of the particle domain if the merging of streaks is ignored. However, Vt cannot be greater than Vp so streaks must merge and Vp can be used to provide a rough estimate of peak concentration, or concentration variance. 相似文献
15.
M. R. Raupach 《Boundary-Layer Meteorology》1979,16(4):467-486
Simultaneous profile and eddy correlation flux data gathered over Thetford Forest, U.K., have been analysed to find values of the vertical turbulent diffusivitiesK M,K H andK E (for momentum, heat and water vapour transfer, respectively) at a reference heightz R, nine roughness lengths above the zero-plane displacementd. The results show: (i), that values ofK M over the forest are not significantly different from these predicted by semiempirical diabatic influence functions appropriate to much smoother surfaces such as short grass; and (ii), thatK H andK E exceed their values predicted from the semiempirical functions by an average factor of 2 or more in unstable, near neutral and slightly stable conditions. These conclusions are strongly dependent on the assumed behaviour ofd, here taken as 0.76 tree heights, independent of both property and stability. Consideration is given to an alternative analysis procedure, in which values of the zero-plane displacementsd H andd E for heat and water vapour respectively, are obtained from the data by assumingK H andK E to be given by semiempirical diabatic influence functions; this procedure is shown to be unacceptable on both practical and physical grounds. To account for the anomalies inK H andK E, a mechanism is proposed in which the horizontally inhomogeneous temperature structure of the canopy causes free convection to be maintained by discrete; localized heat sources and/or sinks, effectively enhancing turbulent transport processes even in nearneutral conditions. 相似文献
16.
Turbulent Intensities and Velocity Spectra for Bare and Forested Gentle Hills: Flume Experiments 总被引：1，自引：1，他引：0
To investigate how velocity variances and spectra are modified by the simultaneous action of topography and canopy, two flume
experiments were carried out on a train of gentle cosine hills differing in surface cover. The first experiment was conducted
above a bare surface while the second experiment was conducted within and above a densely arrayed rod canopy. The velocity
variances and spectra from these two experiments were compared in the middle, inner, and near-surface layers. In the middle
layer, and for the canopy surface, longitudinal and vertical velocity variances () were in phase with the hill-induced spatial mean velocity perturbation (Δu) around the so-called background state (taken here as the longitudinal mean at a given height) as predicted by rapid distortion
theory (RDT). However, for the bare surface case, and remained out of phase with Δu by about L/2, where L is the hill half-length. In the canopy layer, wake production was a significant source of turbulent energy for , and its action was to re-align velocity variances with Δu in those layers, a mechanism completely absent for the bare surface case. Such a lower ‘boundary condition’ resulted in longitudinal
variations of to be nearly in phase with Δu above the canopy surface. In the inner and middle layers, the spectral distortions by the hill remained significant for the
background state of the bare surface case but not for the canopy surface case. In particular, in the inner and middle layers
of the bare surface case, the effective exponents derived from the locally measured power spectra diverged from their expected
− 5/3 value for inertial subrange scales. These departures spatially correlated with the hill surface. However, for the canopy
surface case, the spectral exponents were near − 5/3 above the canopy though the minor differences from − 5/3 were also
correlated with the hill surface. Inside the canopy, wake production and energy short-circuiting resulted in significant departures
from − 5/3. These departures from − 5/3 also appeared correlated with the hill surface through the wake production contribution
and its alignment with Δu. Moreover, scales commensurate with Von Karman street vorticies well described wake production scales inside the canopy,
confirming the important role of the mean flow in producing wakes. The spectra inside the canopy on the lee side of the hill,
where a negative mean flow delineated a recirculation zone, suggested that the wake production scales there were ‘broader’
when compared to their counterpart outside the recirculation zone. Inside the recirculation zone, there was significantly
more energy at higher frequencies when compared to regions outside the recirculation zone. 相似文献
17.
Pablo E. S. Oliveira Ot��vio C. Acevedo Osvaldo L. L. Moraes Hans R. Zimermann Claudio Teichrieb 《Boundary-Layer Meteorology》2013,146(1):45-64
A 1-year set of measurements of CO2 and energy turbulent fluxes above and within a 25-m pine forest in southern Brazil is analyzed. The study focuses on the coupling state between two levels and its impact on flux determination by the eddy-covariance method. The turbulent series are split in their typical temporal scales using the multiresolution decomposition, a method that allows proper identification of the time scales of the turbulent events. Initially, four case studies are presented: a continually turbulent, a continually calm, a calm then turbulent, and an intermittent night. During transitions from calm to turbulent, large scalar fluxes of opposing signs occur at both levels, suggesting the transference of air accumulated in the canopy during the stagnant period both upwards and downwards. Average fluxes are shown for the entire period as a function of turbulence intensity and a canopy Richardson number, used as an indicator of the canopy coupling state. Above the canopy, CO2 and sensible heat fluxes decrease in magnitude both at the neutral and at the very stable limit, while below the canopy they increase monotonically with the canopy Richardson number. Latent heat fluxes decrease at both levels as the canopy air becomes more stable. The average temporal scales of the turbulent fluxes at both levels approach each other in neutral conditions, indicating that the levels are coupled in that case. Average CO2 fluxes during turbulent periods that succeed very calm ones are appreciably larger than the overall average above the canopy and smaller than the average or negative within the canopy, indicating that the transfer of air accumulated during calm portions at later turbulent intervals affects the flux average. The implications of this process for mean flux determination are discussed. 相似文献
18.
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
19.
Investigating a Hierarchy of Eulerian Closure Models for Scalar Transfer Inside Forested Canopies 总被引：3，自引：3，他引：0
Jehn-Yih Juang Gabriel G. Katul Mario B. Siqueira Paul C. Stoy Heather R. McCarthy 《Boundary-Layer Meteorology》2008,128(1):1-32
Modelling the transfer of heat, water vapour, and CO2 between the biosphere and the atmosphere is made difficult by the complex two-way interaction between leaves and their immediate microclimate. When simulating scalar sources and sinks inside canopies on seasonal, inter-annual, or forest development time scales, the so-called well-mixed assumption (WMA) of mean concentration (i.e. vertically constant inside the canopy but dynamically evolving in time) is often employed. The WMA eliminates the need to model how vegetation alters its immediate microclimate, which necessitates formulations that utilize turbulent transport theories. Here, two inter-related questions pertinent to the WMA for modelling scalar sources, sinks, and fluxes at seasonal to inter-annual time scales are explored: (1) if the WMA is to be replaced so as to resolve this two-way interaction, how detailed must the turbulent transport model be? And (2) what are the added predictive skills gained by resolving the two-way interaction vis-à-vis other uncertainties such as seasonal variations in physiological parameters. These two questions are addressed by simulating multi-year mean scalar concentration and eddy-covariance scalar flux measurements collected in a Loblolly pine (P. taeda L.) plantation near Durham, North Carolina, U.S.A. using turbulent transport models ranging from K-theory (or first-order closure) to third-order closure schemes. The multi-layer model calculations with these closure schemes were contrasted with model calculations employing the WMA. These comparisons suggested that (i) among the three scalars, sensible heat flux predictions are most biased with respect to eddy-covariance measurements when using the WMA, (ii) first-order closure schemes are sufficient to reproduce the seasonal to inter-annual variations in scalar fluxes provided the canonical length scale of turbulence is properly specified, (iii) second-order closure models best agree with measured mean scalar concentration (and temperature) profiles inside the canopy as well as scalar fluxes above the canopy, (iv) there are no clear gains in predictive skills when using third-order closure schemes over their second-order closure counterparts. At inter-annual time scales, biases in modelled scalar fluxes incurred by using the WMA exceed those incurred when correcting for the seasonal amplitude in the maximum carboxylation capacity (V cmax, 25) provided its mean value is unbiased. The role of local thermal stratification inside the canopy and possible computational simplifications in decoupling scalar transfer from the generation of the flow statistics are also discussed.
相似文献
“The tree, tilting its leaves to capture bullets of light; inhaling, exhaling; its many thousand stomata breathing, creating the air”. Ruth Stone, 2002, In the Next Galaxy
20.
Wilfried Brutsaert 《Boundary-Layer Meteorology》1979,16(3):365-388
A differential equation is obtained to describe the concentration of passive admixtures (water vapor, sensible heat, pollutants, CO2, etc.) of turbulent flow inside a dense and uniform vegetational canopy. The profiles of eddy diffusivity, wind speed and shear stress are assumed to be exponential decay functions of depth below the top of the canopy. This equation is solved for the case of a vegetation with constant concentration of the admixture at the foliage surfaces. The solution is used to formulate bulk mass or heat transfer coefficients, which can be applied to practical problems involving surfaces covered with a vegetation or with similar porous or fibrous roughness elements. The results are shown to be consistent with experimental data presented by Chamberlain (1966), Garratt and Hicks (1973) and Garratt (1978). Calculations with the model illustrate that, as compared to its behavior over surfaces with bluff roughness elements, ln(z 0/ z oc ) (where z 0 is the momentum roughness and Z oc the scalar roughness) for permeable roughness elements is relatively insensitive to u * and practically independent of z 0. 相似文献