Stability Dependence of Canopy Flows over a Flat Larch Forest

The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra. This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence. The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised at nearly half its value under stable conditions.     

Turbulence structure in a deciduous forest

Three-dimensional wind velocity components were measured at two levels above and at six levels within a fully-leafed deciduous forest. Greatest shear occurs in the upper 20% of the canopy, where over 70% of the foliage is concentrated. The turbulence structure inside the canopy is characterized as non-Gaussian, intermittant and highly turbulent. This feature is supported by large turbulence intensities, skewness and kurtosis values and by the large infrequent sweeps and ejections that dominate tangential momentum transfer. Considerable day/night differences were observed in the vertical profiles of the mean streamwise wind velocity and turbulence intensities since the stability of the nocturnal boundary layer dampens turbulence above and within the canopy.     

Influence of foliar density and thermal stability on profiles of Reynolds stress and turbulence intensity in a deciduous forest

Observations were made of turbulence in an extensive deciduous forest on level terrain using a vertical array of seven three-dimensional sonic anemometer/thermometers within and above the canopy. Data were collected through the period of leaf fall and over a range of thermal stabilities. A bulk canopy drag coefficient was nearly independent of the density of the forest but decreased greatly with the onset of nocturnal stability. The depth of penetration of momentum into the forest increased with leaf fall but, again, was greatly curtailed by stable conditions. Turbulent velocities decreased with increasing depth in the forest but relative turbulence intensities increased to mid-canopy levels. Leaf density influenced turbulence levels but not as strongly as did thermal stability. Thermal effects were adequately described by the single parameter h/L, where h is the canopy height and L is the Monin-Obukhov length. The longitudinal and vertical velocity correlation coefficient was larger in magnitude than expected in the upper layers of the forest but decreased to a small value in the lowest layers where the Reynolds stress was small. The ratio _w /u ^*, where u ^* is the local friction velocity, reflected changes in the uw correlation, becoming smaller than usual in the upper canopy layers. It is believed that these effects result from the intermittent, spatially coherent structures that are responsible for a large fraction of the momentum flux to the forest.     

The Effects of Canopy Leaf Area Index on Airflow Across Forest Edges: Large-eddy Simulation and Analytical Results

The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region close to the ground. It was demonstrated that the LES here predicted first-order and second-order mean velocity statistics within the canopy that agree with reported Reynolds-Averaged Navier–Stokes (RANS) model results, field and laboratory experiments. In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the mean velocity at the outflow edge into a superposition of ‘exit flow’ and BFS-like flow with their relative importance determined by LAI was explored.     

Large-eddy Simulation of Turbulent Flow across a Forest Edge. Part II: Momentum and Turbulent Kinetic Energy Budgets

Momentum and turbulent kinetic energy (TKE) budgets across a forest edge have been investigated using large-eddy simulation (LES). Edge effects are observed in the rapid variation of a number of budget terms across this vegetation transition. The enhanced drag force at the forest edge is largely balanced by the pressure gradient force and by streamwise advection of upstream momentum, while vertical turbulent diffusion is relatively insignificant. For variance and TKE budgets, the most important processes at the forest edge are production due to the convergence (or divergence) of the mean flow, streamwise advection, pressure diffusion and enhanced dissipation by canopy drag. Turbulent diffusion, pressure redistribution and vertical shear production, which are characteristic processes in homogeneous canopy flow, are less important at the forest transition. We demonstrate that, in the equilibrated canopy flow, a substantial amount of TKE produced in the streamwise direction by the vertical shear of the mean flow is redistributed in the vertical direction by pressure fluctuations. This redistribution process occurs in the upper canopy layers. Part of the TKE in the vertical velocity component is transferred by turbulent and pressure diffusion to the lower canopy levels, where pressure redistribution takes place again and feeds TKE back to the streamwise direction. In this TKE cycle, the primary source terms are vertical shear production for streamwise velocity variance and pressure redistribution for vertical velocity variance. The evolution of these primary source terms downwind of the forest edge largely controls the adjustment rates of velocity variances.     

Verification of a One-Dimensional Model of $$\hbox {CO}_{2}$$ Atmospheric Transport Inside and Above a Forest Canopy Using Observations at the Norunda Research Station

A model of \(\hbox {CO}_{2}\) atmospheric transport in vegetated canopies is tested against measurements of the flow, as well as \(\hbox {CO}_{2}\) concentrations at the Norunda research station located inside a mixed pine–spruce forest. We present the results of simulations of wind-speed profiles and \(\hbox {CO}_{2}\) concentrations inside and above the forest canopy with a one-dimensional model of profiles of the turbulent diffusion coefficient above the canopy accounting for the influence of the roughness sub-layer on turbulent mixing according to Harman and Finnigan (Boundary-Layer Meteorol 129:323–351, 2008; hereafter HF08). Different modelling approaches are used to define the turbulent exchange coefficients for momentum and concentration inside the canopy: (1) the modified HF08 theory—numerical solution of the momentum and concentration equations with a non-constant distribution of leaf area per unit volume; (2) empirical parametrization of the turbulent diffusion coefficient using empirical data concerning the vertical profiles of the Lagrangian time scale and root-mean-square deviation of the vertical velocity component. For neutral, daytime conditions, the second-order turbulence model is also used. The flexibility of the empirical model enables the best fit of the simulated \(\hbox {CO}_{2}\) concentrations inside the canopy to the observations, with the results of simulations for daytime conditions inside the canopy layer only successful provided the respiration fluxes are properly considered. The application of the developed model for radiocarbon atmospheric transport released in the form of \(^{14}\hbox {CO}_{2}\) is presented and discussed.     

Turbulence Statistics Above And Within Two Amazon Rain Forest Canopies

The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth.     

Dawn‐to‐dusk evolution of air turbulence,temperature and sensible and latent heat fluxes above a forest canopy: Concepts,model and field comparisons

Abstract

Dawn‐to‐dusk evolution of air turbulence, sensible heat and latent heat above a forest during cloud‐free or near‐cloud‐free summer conditions is modelled by way of a system of differential equations. Temperatures in and above the canopy, near canopy‐top wind velocities, early morning leaf moisture (dew) and afternoon canopy ventilation (i.e. heat released from the canopy and from below the canopy) are included in the mathematical treatment. Computed results are compared with field data for atmospheric temperature and wind speed profiles up to 1200 m, within‐canopy temperature, and canopy‐level radiation, turbulent fluxes and wind speeds. Data were collected at a central New Brunswick mixed‐wood forest site dominated by spruce (Picea spp. ) and shade‐tolerant hardwoods for four representative summer days. It was found that the effective canopy temperature was not only affected by insolation, but also by the extent of canopy ventilation and the amount of dew on the foliage. The growth of the mixing layer was affected by canopy ventilation and by above‐canopy wind speeds. Model calculations closely simulated the meteorological observations.     

The Effects of Thermal Stratification on Clustering Properties of Canopy Turbulence

The intermittent structure of turbulence within the canopy sublayer (CSL) is sensitive to the presence of foliage and to the atmospheric stability regime. How much of this intermittency originates from amplitude variability or clustering properties remains a vexing research problem for CSL flows. Using a five-level set of measurements collected within a dense hardwood canopy, the clustering properties of CSL turbulence and their dependence on atmospheric stability are explored using the telegraphic approximation (TA). The binary structure of the TA removes any amplitude variability from turbulent excursions but retains their zero-crossing behaviour, and thereby isolating the role of clustering in intermittency. A relationship between the spectral exponents of the actual and the TA series is derived across a wide range of atmospheric stability regimes and for several flow variables. This relationship is shown to be consistent with a relationship derived for long-memory and monofractal processes such as fractional Brownian motion (fBm). Moreover, it is demonstrated that for the longitudinal and vertical velocity components, the vegetation does not appreciably alter fine-scale clustering but atmospheric stability does. Stable atmospheric stability conditions is characterized by more fine scale clustering when compared to other atmospheric stability regimes. For scalars, fine-scale clustering above the canopy is similar to its velocity counterpart but is significantly increased inside the canopy, especially under stable stratification. Using simplified scaling analysis, it is demonstrated that clustering is much more connected to space than to time within the CSL. When comparing intermittency for flow variables and their TA series, it is shown that for velocity, amplitude variations modulate intermittency for all stability regimes. However, amplitude variations play only a minor role in scalar intermittency. Within the crown region of the canopy, a ‘double regime’ emerges in the inter-pulse duration probability distributions not observed in classical turbulence studies away from boundaries. The double regime is characterized by a power-law distribution for shorter inter-pulse periods and a log-normal distribution for large inter-pulse periods. The co-existence of these two regimes is shown to be consistent with near-field/far-field scaling arguments. In the near-field, short inter-pulse periods are controlled by the source strength, while in the far-field long inter-pulse periods are less affected by the precise source strength details and more affected by the transport properties of the background turbulence.     

Estimating Heat Sources And Fluxes In Thermally Stratified Canopy Flows Using Higher-Order Closure Models

Over the past two decades, several inverse methods have been proposed to estimatescalar source and sink strengths from measured mean concentration profiles withinthe canopy volume (hereafter termed the `inverse' problem). These inverse methodscommonly assumed neutral atmospheric stability conditions for the entire canopyvolume. For non-neutral conditions, atmospheric stability corrections in inverseschemes were limited to adjusting the integral time scale or other flow statistics tomatch well-established surface-layer similarity relations above the canopy. Suchstability corrections do not explicitly consider the local stability effects within thecanopy volume. Currently, there is no satisfactory inverse scheme that explicitlyaccounts for local atmospheric stability for canopy turbulence. A Eulerian inversemethod that explicitly accounts for local atmospheric stability within the canopy isdeveloped using second-order closure principles. Field testing the method is conductedusing temperature measurements from two field experiments collected in an even-ageduniform loblolly pine forest. It is demonstrated that by accounting for local atmospheric stability in the inversion scheme, the agreement between modelled sensible heat flux calculations and measurements improve by 60% for stable conditions, 10% for near-neutral conditions and 20% for unstable conditions     

Simulation of Turbulent Flow in An Urban Forested Park Damaged by a Windstorm

The damage caused by windstorms to forest ecosystems is often very heterogeneous. In order to improve the stability of forested landscapes, it is of great importance to identify the factors responsible for this spatial variability. The structure of the landscape itself may play a role, through possible influences of canopy heterogeneities on the development of turbulence. For the purpose of investigating the role of landscape fragmentation on turbulence development, we used a numerical flow model with a k–ε turbulence scheme model, previously validated in simple cases with well-defined surface changes (roughness change and forest edge flow). A series of two- and three-dimensional simulations were performed over a heterogeneous urban forested park in Europe, which was severely damaged in various places by the Lothar windstorm in December 1999. The model shows the development of a region of strong turbulence, resulting from the generation of large wind shear at the top of the canopy. A sensitivity study shows how the location, extension and intensity of the region depend on canopy characteristics such as the leaf density, the nature of the edge or the presence of gaps and clearings. Simulations performed in conditions representative of the windstorm show that the location of the damaged areas corresponds very closely to the regions where the turbulent kinetic energy was above a certain threshold.     

Measurements and Langevin simulations of mean tracer concentration fields downwind from a circular line source inside an alfalfa canopy

Two Langevin simulations of trajectories of marked fluid elements in inhomogenous turbulence, where the Lagrangian length and vertical velocity scales are height dependent, were compared with field data. A CO₂ tracer was released from a circular line source and the concentration profiles were measured for diffusion distances of 50 and 100 cm inside and above an alfalfa canopy.One of the simulations, suggested by Wilson et al. (1983), biases the vertical velocities by adding a mean upward drift. The second simulation proposed here by-passes this difficulty by reflecting marked particles according to a probability calculated from the gradient in vertical velocity variance between the beginning and the end of each step. This simulation also makes use of a constant time-scale within the canopy, following preliminary results from a turbulence experiment within a forest (Leclerc, 1987).Comparing the results of these simulations with the field data shows that the simulation proposed by Wilson et al. (1983) does not correctly reproduce the difusion for the larger fetch in systems exhibiting strong gradients in vertical velocity variance. Instead, the modelled plumes exhibit a bulge at the source height whereas the field data show smooth profiles. In addition, the modelled plumes overestimate the vertical spread of the plumes, which is possibly due to the inadequacy of the approach in severely inhomogeneous systems. In contrast, the results from the tracer experiments indicate that the diffusion can be better reproduced with the use of a reflection probability calculated at each step. The discrepancies between the experimental results and the simulation using a reflection probability are attributed to stability effects.     

The Turbulence Structure of the Stable Atmospheric Boundary Layer Around a Coastal Headland: Aircraft Observations and Modelling Results

The turbulence structure of a stable marine atmospheric boundary layer in the vicinity of a coastal headland is examined using aircraft observations and numerical simulations. Measurements are drawn from a flight by the NCAR C-130 around Cape Mendocino on the coast of northern California on June 7 1996 during the Coastal Waves 96 field program. Local similarity scaling of the velocity variances is found to apply successfully within the continuously turbulent layer; the empirical scaling function is similar to that found by several previous studies. Excellent agreement is found between the modelled and observed scaling results. No significant change in scaling behaviour is observed for the region within the expansion fan that forms downstream of the Cape, suggesting that the scaling can be applied to horizontally heterogeneous conditions; however, the precise form of the function relating scaled velocities and stability is observed to change close to the surface. This result, differences between the scaling functions found here and in other studies, and the departure of these functions from the constant value predicted by the original theory, leads us to question the nature of the similarity functions observed. We hypothesize that the form of the functions is controlled by non-local contributions to the velocity variance budgets, and that differences in the non-local terms between studies explain the differences in the observed scaling functions.     

A Flume Experiment on the Adjustment of the Mean and Turbulent Statistics to a Transition from Short to Tall Sparse Canopies

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.     

Dispersive Stresses at the Canopy Upstream Edge

The derivation of flow and mass transfer models in canopy and porous media environments involves the spatial-averaging of the flow properties and their subscale equations. The averaging of the momentum equation generates the dispersive stress terms that represent the subscale spatial variations of the unresolved velocity field. While previous studies ignored the dispersive stresses in their flow models, recent evidence indicates that the dispersive stresses may be important. Here we focus our attention on the magnitude of the normal dispersive stresses in the entry region of a ‘forest patch’, where the in-canopy velocities are large and the longitudinal derivatives do not cancel out. Highly detailed particle image velocimetry measurements, at a temporal and spatial resolution of 5 Hz and 1.4 mm, are obtained inside and around a 1-m long model canopy which consists of transparent vertical cylinders 6 mm in diameter and 74.3 mm high (h). The cylinders are randomly distributed to form a relatively sparse forest patch with a leaf area density of 7.56 m⁻¹ and a fluid volume fraction (porosity) of 0.965. We present results of the double averaged flow properties at three different regions of the forest patch; the upstream edge (x ≈ 0), the fully-developed interior region (x ≈ 10h) and the downstream edge (x ≈ 13h). We find that the normal dispersive stresses around the entry region of the forest patch are significantly larger than the normal Reynolds stresses. An order of magnitude analysis of the relevant terms in the momentum equation indicates that the longitudinal derivatives of the dispersive stresses are of the same order of magnitude as that of the drag force and similar to that of the horizontal convection term. The longitudinal derivatives of the Reynolds stresses are smaller, though cannot be ignored. Comparing these results with the characteristic profiles measured in the fully-developed region indicates that the dispersive stresses, which are generated at the forest patch entrance, decrease along an adjustment region while maintaining their profile shape. We find that the dispersive stresses influence the rate at which momentum penetrates into the canopy. These observations suggest that under certain flow conditions, dispersive stresses may dominate the momentum balance and therefore must be considered in future canopy and porous media flow models.     

Large-Eddy Simulation of Coherent Turbulence Structures Associated with Scalar Ramps Over Plant Canopies

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.     

Organised Motion and Radiative Perturbations in the Nocturnal Canopy Sublayer above an Even-Aged Pine Forest

Using time series measurements of velocity, carbon dioxideand water vapour concentration, and temperature collected justabove a 15 m tall even-aged pine forest, we quantify the roleof organized motion on scalar and momentum transport withinthe nocturnal canopy sublayer (CSL). We propose a frameworkin which the nocturnal CSL has two end-members, bothdominated by organised motion. These end-members representfully developed turbulent flows at near-neutral or slightly stablestratification and no turbulence for very stable stratification.Our analysis suggests that ramps dominate scalar transport fornear-neutral and slightly stable conditions, while linear canopywaves dominate the flow dynamics for very stable conditions.For intermediate stability, the turbulence is highly damped andoften dominated by fine scale motions. Co-spectral analysissuggests that ramps are the most efficient net scalar mass-transportingagent while linear canopy waves contribute little to net scalartransport between the canopy and atmosphere for averagingintervals that include complete wave cycles. However, canopywaves significantly contribute to the spectral properties of thescalar time series. Ramps are the most frequently occurringorganised motion in the nocturnal CSL for this site.Numerous night-time runs, however, resided between thesetwo end-members. Our analysis suggests that whenradiative perturbations are sufficient large (>20 W m^-2 innet radiation), the flow can switch from being highly dampedfine-scale turbulence to being organized with ramp-like properties. We also found that when ramps are already the dominant eddymotion in the nocturnal CSL, radiative perturbations have aminor impact on scalar transport. Finally, in agreement withprevious studies, we found that ramps and canopy waves havecomparable length scales of about 30–60 metres. Consequencesto night-time flux averaging are also discussed.     

HNO3 deposition to a deciduous forest

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.     

Turbulence Statistics Inside and Over Forest: Influence on Footprint Prediction

Observations of wind statistics within and above a Scots pine forest are comparedwith those predicted from an analytical second-order closure model. The roughnesssublayer (RSL) effects, and the influence of stability on similarity functions, arestudied using observations. The commonly accepted forms of similarity functionsdescribe the influence of diabatic effects above the RSL well. According to earlierstudies they are expected also to apply within the RSL. As an exception, the averagewind speed normalised with friction velocity was found to be invariant with stabilityclose to the canopy top under unstable conditions. Lagrangian stochastic trajectorysimulations were used to evaluate the influence of canopy turbulence profiles onfootprint prediction. The main uncertainty was found to arise from parameterisationof the random forcing term in the Lagrangian velocity equation. The influence ofdiabatic conditions was studied, and it was found that thermal stability affectssignificantly the footprint function above the forest canopy, but significantuncertainty exists because of uncertainties in the formulation of stability functions.     

Footprint Analysis For Measurements Over A Heterogeneous Forest

The air flow and vertical distribution of sources/sinks inside aforest canopy have been taken into accountin the analysis of the contribution of sources/sinks to measured fluxes and concentrations above a forest. Thestochastic estimators for concentrations and fluxes are described and their evaluation is performed by simulationof an ensemble of fluid parcel trajectories. The influence of the forest canopy on the footprint is important forobservation levels up to a few times the forest height. The influence of along-wind turbulent diffusion, whichanalytical atmospheric surface layer (ASL) footprint models do not account for, is significant even at higherlevels. The footprint analysis has been performed to deduce the Douglas fir canopy carbon dioxide uptake from eddycovariance flux measurements above a mixed Douglas fir–beech forest during the pre-leaf periods of the beech.The scatter in the results indicates that such an analysis is limited, presumably due to horizontal inhomogenetiesin flow statistics, which were not included in trajectory simulation. The analysis, however, is useful for theestimation of the qualitative effect of the forest canopy on the footprint function.