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1.
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   

2.
Source/sink strengths and vertical fluxdistributions of carbon dioxide within and above arice canopy were modelled using measured meanconcentration profiles collected during aninternational rice experiment in Okayama, Japan (IREX96). The model utilizes an Eulerian higher-orderclosure approach that permits coupling of scalar andmomentum transport within vegetation to infer sourcesand sinks from mean scalar concentration profiles; theso-called `inverse problem'. To compute the requiredvelocity statistics, a Eulerian second-order closuremodel was considered. The model well reproducedmeasured first and second moment velocity statisticsinside the canopy. Using these modelled velocitystatistics, scalar fluxes within and above the canopywere computed and compared with CO2eddy-correlation measurements above the canopy. Goodagreement was obtained between model calculations offluxes at the top of the canopy and measurements. Close to the ground, the model predicted higherrespiratory fluxes when the paddy was drained comparedto when it was flooded. This is consistent with thefloodwater providing a barrier to diffusion ofCO2 from the soil to the atmosphere. TheEulerian sources and flux calculations were alsocompared to source and flux distributions estimatedindependently using a Lagrangian Localized Near Fieldtheory, the first study to make such a comparison.Some differences in source distributions werepredicted by these analyses. Despite this, thecalculated fluxes by the two approaches compared wellprovided a closure constant, accounting for theinfluence of `near-field' sources in the Eulerian fluxtransport term, was given a value of 1.5 instead ofthe value of 8 found in laboratory studies.  相似文献   

3.
Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are <5% atall heights. These results appear to be insensitive to theReynolds number (at high Reynolds numbers).  相似文献   

4.
In the first part of this study, results of a computational fluid dynamics simulation over an array of cubes have been validated against a set of wind-tunnel measurements. In Part II, such numerical results are used to investigate spatially-averaged properties of the flow and passive tracer dispersion that are of interest for high resolution urban mesoscale modelling (e.g. non resolved obstacle approaches). The results show that vertical profiles of mean horizontal wind are linear within the canopy and logarithmic above. The drag coefficient, derived from the numerical results using the classical formula for the drag force, is height dependent (it decreases with height). However, a modification of the formula is proposed (accounting for subgrid velocity scales) that makes the drag coefficient constant with height. Results also show that the dispersive fluxes are similar in magnitude to the turbulent fluxes, and that they play a very important role within the canopy. Vertical profiles of turbulent length scales (to be used in k–l closure schemes, where k is the turbulent kinetic energy and l a turbulent length scale) are also derived. Finally the distribution of the values around the mean over the reference volumes are analysed for wind and tracer concentrations.  相似文献   

5.
Eddy-covariance observations above the densely built-up Centre of Nanjing were made from December 2011 to August 2012. Separate eddy-covariance systems installed at two levels on a 36-m tower located on a rooftop were operated simultaneously, and observations grouped into two sectors (A, B) according to the prevalent wind directions. For sector A, where the nearby buildings are all below the lower measurement level, the sensible heat and momentum fluxes are generally greater at the upper level. For sector B, where several high-rise buildings are located upwind, the sensible heat and momentum fluxes at the upper level are close to those at the lower level. The analysis shows that the turbulent eddy characteristics differ between the two wind sectors, leading to a different behaviour of turbulent exchange between the two levels. A hypothesis is proposed that addresses the vertical variation of turbulent fluxes in the urban roughness sublayer (RSL). For sector A, the buildings block the flow, change the trajectory of scalars, and distort the footprint of scalar fluxes; this ‘blocking effect’ is believed to lead to a smaller sensible heat flux above the canopy layer. Such an effect should decrease with height in the RSL, explaining the increase of the observed turbulent heat flux with height. In addition, the presence of non-uniform building heights adversely affects turbulence organization around the canopy top, and likely elevates the inflection point of the mean flow to a higher elevation close to the upper measurement level, where larger shear results in a larger momentum flux. For sector B, wake effects from the nearby high-rise buildings strongly reduce turbulence organization at higher elevations, leading to similar sensible heat and momentum fluxes at both measurement levels.  相似文献   

6.
Scalar Fluxes from Urban Street Canyons Part II: Model   总被引：1，自引：1，他引：0  
A practical model is developed for the vertical flux of a scalar, such as heat, from an urban street canyon that accounts for variations of the flow and turbulence with canyon geometry. The model gives the magnitude and geometric dependence of the flux from each facet of the urban street canyon, and is shown to agree well with wind-tunnel measurements described in Part I. The geometric dependence of the flux from an urban street canyon is shown to be determined by two physical processes. Firstly, as the height-to-width ratio of the street canyon increases, so does the roughness length and displacement height of the surface. This increase leads to a reduction in the wind speed in the inertial sublayer above the street canyons. Since the speed of the circulations in the street are proportional to this inertial sublayer wind speed, the flux then reduces with the inertial sublayer wind speed. This process is dominant at low height-to-width ratios. Secondly, the character of the circulations within the street canyon also varies as the height-to-width ratio increases. The flow in the street is partitioned into a recirculation region and a ventilated region. When the street canyon has high height-to-width ratios the recirculation region occupies the whole street canyon and the wind speeds within the street are low. This tendency decreases the flux at high height-to-width ratios. These processes tend to reduce the flux density from the individual facets of the street canyon, when compared to the flux density from a horizontal surface of the same material. But the street canyon has an increased total surface area, which means that the total flux from the street canyon is larger than from a horizontal surface. The variations in scalar flux from an urban street canyon with geometry is over a factor of two, which means that the physical mechanisms responsible should be incorporated into energy balance models for urban areas.  相似文献   

7.
We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer.  相似文献   

8.
Scalar fluxes from urban street canyons. Part I: Laboratory simulation   总被引：3，自引：3，他引：0  
Flow over urban surfaces depends on surface morphology and interaction with the boundary layer above. However, the effect of the flow on scalar fluxes is hard to quantify. The naphthalene sublimation technique was used to quantify scalar vertical fluxes out of a street canyon under neutral conditions. For an array of eight canyons with aspect ratio H/W=0.75 (here, H is building height and W is the street width), increased flux was observed in the first two or three canyons for moderate and low roughness upstream. This is consistent with predictions of the length scale for initial adjustment of flow to an urban canopy. The flux was constant after the initial adjustment region and thus dependent only on local geometry. For a street canyon in the equilibrium part of the array, each facet of the street canyon was coated with naphthalene to simulate scalar release from street, walls and roof, to evaluate the effect of street canyon geometry on fluxes for H/W=0.25, 0.6, 1 and 2. Fluxes from the roof and downstream wall were considerably larger than fluxes from the street and upstream wall, and only the flux from the downstream wall exhibited a simple decrease with H/W. For each H/W there was a monotonic decrease between downstream wall, street and upstream wall transfer. This suggests that flow decelerates around the recirculation region in the lee of the upstream building, i.e. a recirculating jet rather than a symmetrical vortex. The addition of a second source within the street canyon resulted in reduced fluxes from each facet for H/W>0.25, due to increased concentration of naphthalene in the canyon air.  相似文献   

9.
A Wind Tunnel Model for Quantifying Fluxes in the Urban Boundary Layer   总被引：9，自引：6，他引：3  
Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity wT relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient wT/U represents the ventilation efficiency of the canyon (here, wT is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. wT/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in wT/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes.  相似文献   

10.
Large-eddy simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cube arrays. The results from our model, hereafter LES-CITY, are shown to agree with laboratory experiments. We investigated the systematic influence of cube density on turbulent flow characteristics by performing numerical experiments for cube areal densities from 0 to 44%. The following results were obtained: (1) The dispersive momentum flux was quite large within the canopy layer due to a mean stream re-circulation, whereas it was smaller above the canopy. The spatial variation of temporally averaged momentum in the roughness sub-layer was 20% or less of the total kinematic surface drag. (2) The temporally and spatially-averaged flow structure confirmed the existence of conventionally described canyon flow regimes; isolated, interfacial, and wake. However, the intermittency of the canyon flow for all cube densities was quite large and the stream patterns were never persistent. (3) Turbulent organized structures (TOS) similar to those observed in turbulent surface-layer flows were simulated, which are characterized by longitudinally-elongated low speed streaks and the corresponding shorter streamwise vortices. The streaks in sparse and dense canopy flows were likely to be aligned to the street line and to the roof lines, respectively. Such heterogeneity of TOS partially accounts for the large spatial variation of momentum flux. (4) In contrast to the mixing layer analogy of vegetation flows, the TOS and the resulting turbulent statistics of urban flow above the canopy resembled those in surface layers. The recirculation within the canopy significantly influenced the turbulent statistical properties.  相似文献   

11.
Sensible heat, latent heat, and other scalar fluxes cannot be measuredwithin short dense canopies, e.g., straw mulches, with standard approachessuch as eddy correlation, Bowen ratio-energy balance, aerodynamic, andvariance methods. However, recently developed surface renewal models, thatare based on the fact that most of the turbulent transfer within and abovecanopies is associated with large-scale coherent eddies, which are evidentas ramp patterns in scalar time series, offer a feasible solution. Wepresent a new air renewal model that calculates sensible heat flux atdifferent heights within and above a canopy from the average cubictemperature structure function, sampled at a moderate rate, and measuredaverage friction velocity. The model is calibrated and tested with datameasured above and within a Douglas-fir forest and above a straw mulch andbare soil. We show that the model describes half-hour variations ofsensible heat flux very well, both within the canopy and roughnesssublayers and in the inertial sublayer, for stable and unstable atmosphericconditions. The combined empirical coefficient that appears in the modelhas an apparently universal value of about 0.4 for all surfaces andheights, which makes application of the model particularly simple. Themodel is used to predict daytime and nighttime sensible heat flux profileswithin the straw mulch and within a small bare opening in the mulch.  相似文献   

12.
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   

13.
The two-scalar covariance budget is significant within the canopy sublayer (CSL) given its role in modelling scalar flux budgets using higher-order closure principles and in estimating the segregation ratio for chemically reactive species. Despite its importance, an explicit expression describing how the two-scalar covariance is modified by inhomogeneity in the flow statistics and in the vertical variation in scalar emission or uptake rates within the canopy volume remains elusive even for passive scalars. To progress on a narrower version of this problem, an analytical solution to the two-scalar covariance budget in the CSL is proposed for the most idealized flow conditions: a stationary and planar homogeneous flow inside a uniform and dense canopy with a constant leaf area density distribution. The foliage emission (or uptake) source strengths are assumed to vary exponentially with depth while the forest floor emission is represented as a scalar flux. The analytical solution is a superposition of a homogeneous part that describes how the two-scalar covariance at the canopy top is transported and dissipated within the canopy volume, and an inhomogeneous part governed by local production mechanisms of the two-scalar covariance. The homogeneous part is primarily described by the canopy adjustment length scale, and the attenuation coefficients of the turbulent kinetic energy and the mean velocity. Conditions for which the vertical variation of the two-scalar covariance is controlled by the rapid attenuation in the mean velocity and turbulent kinetic energy profiles, vis-à-vis the vertical variation of the scalar source strength, are explicitly established. This model also demonstrates how dissimilarity in the emissions from the ground, even for the extreme binary case with one scalar turned ‘on’ and the other scalar turned ‘off’, modifies the vertical variation of the two-scalar covariance within the CSL. To assess its applicability to field conditions, the analytical model predictions were compared with observations made at two different forest types—a sparse pine forest at the Hyytiälä SMEAR II-station (in Finland) and a dense alpine hardwood forest at Lavarone (in Italy). While the model assumptions do not represent the precise canopy morphology, attenuation properties of the turbulent kinetic energy and the mean velocity, observed mixing length, and scalar source attenuation properties for these two forest types, good agreement was found between measured and modelled two scalar covariances for multiple scalars and for the triple moments at the Hyytiälä site.  相似文献   

14.
A stochastic trajectory model was used to estimate scalar fluxfootprints in neutral stabilityfor canopies of varying leaf area distributions andleaf area indices. An analytical second-order closure model wasused to predict mean wind speed, second moments and the dissipationrate of turbulent kinetic energy within a forest canopy.The influence of source vertical profile on the flux footprint wasexamined. The fetch is longer for surface sourcesthan for sources at higher levels in the canopy. In order tomeasure all the flux components, and thus the total flux, with adesired accuracy, sources were located at the forest floor in thefootprint function estimation. The footprint functions werecalculated for five observation levels above the canopy top. Itwas found that at low observation heights both canopy density andcanopy structure affect the fetch. The higher abovethe canopy top the flux is measured, the more pronounced is the effectof the canopy structure. The forest fetch for flux measurements isstrongly dependent on the required accuracy: The 90% flux fetchis greater by a factor of two or more compared to the 75% fetch. Theupwind distance contributing 75% of flux is as large as 45 timesthe difference between canopy height and the observation heightabove the canopy top, being even larger for low observationlevels.  相似文献   

15.
Accurate simulation of air quality at neighbourhood scales (on order of 1-km horizontal grid spacing) requires detailed meteorological fields inside the roughness sub-layer (RSL). Since the assumptions of the roughness approach, used by most of the mesoscale models, are unsatisfactory at this scale, a detailed urban and rural canopy parameterisation, called DA-SM2-U, is developed inside the Penn State/NCAR Mesoscale Model (MM5) to simulate the meteorological fields within and above the urban and rural canopies. DA-SM2-U uses the drag-force approach to represent the dynamic and turbulent effects of the buildings and vegetation, and a modified version of the soil model SM2-U, called SM2-U(3D), to represent the thermodynamic effects of the canopy elements. The turbulence length scale is also modified inside the canopies. SM2-U(3D) assesses the sensible and latent heat fluxes from rural and urban surfaces in each of the computational layers inside the canopies by considering the shadowing effect, the radiative trapping by the street canyons, and the storage heat flux by the artificial surfaces. DA-SM2-U is tested during one simulated day above the city of Philadelphia, U.S.A. It is shown that DA-SM2-U is capable of simulating the important features observed in the urban and rural RSL, as seen in the vertical profiles of the shear stress, turbulent kinetic energy budget components, eddy diffusivity, potential air temperature, and specific humidity. Within the canopies, DA-SM2-U simulates the decrease of the wind speed inside the dense canopies, the skirting of the flow around the canopy blocks, warmer air inside the vegetation canopy than above open areas during the night and conversely during the day, and constantly warmer air inside the urban canopy. The comparison with measurements shows that the surface air temperature above rural and urban areas is better simulated by DA-SM2-U than by the `standard version' of MM5.  相似文献   

16.
Large-eddy simulations were performed of a neutrally-stratified turbulent flow within and above an ideal, horizontally- and vertically-homogeneous plant canopy. Three simulations were performed for shear-driven flows in small and large computational domains, and a pressure-driven flow in a small domain, to enable the nature of canopy turbulence unaffected by external conditions to be captured. The simulations reproduced quite realistic canopy turbulence characteristics, including typical ramp structures appearing in time traces of the scalar concentration near the canopy top. Then, the spatial structure of the organised turbulence that caused the scalar ramps was examined using conditional sampling of three-dimensional instantaneous fields, triggered by the occurrence of ramp structures. A wavelet transform was used for the detection of ramp structures in the time traces. The ensemble-averaged results illustrate that the scalar ramps are associated with the microfrontal structure in the scalar, the ejection-sweep structure in the streamwise and vertical velocities, a laterally divergent flow just around the ramp-detection point, and a positive, vertically-coherent pressure perturbation. These vertical structures were consistent with previous measurements made in fields or wind tunnels. However, the most striking feature is that the horizontal slice of the same structure revealed a streamwise-elongated region of high-speed streamwise velocity impacting on another elongated region of low-speed velocity. These elongated structures resemble the so-called streak structures that are commonly observed in near-wall shear layers. Since elongated structures of essentially similar spatial scales were observed in all of the runs, these streak structures appear to be inherent in near-canopy turbulence. Presumably, strong wind shear formed just above the canopy is involved in their formation. By synthesis of the ensemble-averaged and instantaneous results, the following processes were inferred for the development of scalar microfronts and their associated flow structures: (1) a distinct scalar microfront develops where a coherent downdraft associated with a high-speed streak penetrates into the region of a low-speed streak; (2) a stagnation in flow between two streaks of different velocities builds up a vertically-coherent high-pressure region there; (3) the pressure gradients around the high-pressure region work to reduce the longitudinal variations in streamwise velocity and to enhance the laterally-divergent flow and lifted updrafts downstream of the microfront; (4) as the coherent mother downdraft impinges on the canopy, canopy-scale eddies are formed near the canopy top in a similar manner as observed in conventional mixing-layer turbulence.  相似文献   

17.
Measurements and Computations of Flow in an Urban Street System   总被引：1，自引：1，他引：0  
We present results from laboratory and computational experiments on the turbulent flow over an array of rectangular blocks modelling a typical, asymmetric urban canopy at various orientations to the approach flow. The work forms part of a larger study on dispersion within such arrays (project DIPLOS) and concentrates on the nature of the mean flow and turbulence fields within the canopy region, recognising that unless the flow field is adequately represented in computational models there is no reason to expect realistic simulations of the nature of the dispersion of pollutants emitted within the canopy. Comparisons between the experimental data and those obtained from both large-eddy simulation (LES) and direct numerical simulation (DNS) are shown and it is concluded that careful use of LES can produce generally excellent agreement with laboratory and DNS results, lending further confidence in the use of LES for such situations. Various crucial issues are discussed and advice offered to both experimentalists and those seeking to compute canopy flows with turbulence resolving models.  相似文献   

18.
An integrated canopy micrometeorological model is described for calculating CO2, water vapor and sensible heat exchange rates and scalar concentration profiles over and within a crop canopy. The integrated model employs a Lagrangian random walk algorithm to calculate turbulent diffusion. The integrated model extends previous Lagrangian modelling efforts by employing biochemical, physiological and micrometeorological principles to evaluate vegetative sources and sinks. Model simulations of water vapor, CO2 and sensible heat flux densities are tested against measurements made over a soybean canopy, while calculations of scalar profiles are tested against measurements made above and within the canopy. The model simulates energy and mass fluxes and scalar profiles above the canopy successfully. On the other hand, model calculations of scalar profiles inside the canopy do not match measurements.The tested Lagrangian model is also used to evaluate simpler modelling schemes, as needed for regional and global applications. Simple, half-order closure modelling schemes (which assume a constant scalar profile in the canopy) do not yield large errors in the computation of latent heat (LE) and CO2 (F c ) flux densities. Small errors occur because the source-sink formulation of LE andF c are relatively insensitive to changes in scalar concentrations and the scalar gradients are small. On the other hand, complicated modelling frames may be needed to calculate sensible heat flux densities; the source-sink formulation of sensible heat is closely coupled to the within-canopy air temperature profile.  相似文献   

19.
Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet’s impact on nocturnal turbulent exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear sheltering, mixing is reduced. In the absence of the latter, ‘upside-down’ eddies are primarily responsible for the enhanced velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height. The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes.  相似文献   

20.
Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough to a rougher surface, the thickness of the internal boundary layer scales as x 4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure than vortex shedding.  相似文献   

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