Frontal Area Index Research Articles

Estimating zero-plane displacement height and aerodynamic roughness length using synthesis of LiDAR and SPOT-5 data

In this study, a combination of low and high density airborne LiDAR and satellite SPOT-5 HRG data were used in conjunction with ground measurements of forest structure to parameterize four models for zero-plane displacement height d(m) and aerodynamic roughness length z 0 m (m), over cool-temperate forests in Heihe River basin, an arid region of Northwest China. For the whole study area, forest structural parameters including tree height (Ht) (m), first branch height (FBH) (m), crown width (CW) (m) and stand density (SD)(trees ha − 1 ) were derived by stepwise multiple linear regressions of ground-based forest measurements and height quantiles and fractional canopy cover ( f c ) derived from the low density LiDAR data. The high density LiDAR data, which covered a much smaller area than the low density LiDAR data, were used to relate SPOT-5's reflectance to the effective plant area index (PAIe) of the forest. This was done by linear spectrum decomposition and Li–Strahler geometric–optical models. The result of the SPOT-5 spectrum decomposition was applied to the whole area to calculate PAIe (and leaf area index LAI). Then, four roughness models were applied to the study area with these vegetation data derived from the LiDAR and SPOT-5 as input. For validation, measurements at an eddy covariance site in the study area were used. Finally, the four models were compared by plotting histograms of the accumulative distribution of modeled d and z 0 m in the study area. The results showed that the model using by frontal area index (FAI) produced best d estimate, and the model using both LAI and FAI generated the best z 0 m . Furthermore, all models performed much better when the representative tree height was Lorey's mean height instead of using an arithmetic mean.

Aeolian sediment transport over vegetation canopies: A wind tunnel study with live plants

Abstract This wind tunnel study examines aeolian sediment transport in live plant canopies, whereas most previous studies have used model plants for this purpose. Experiments were performed with three canopy densities of Perennial Ryegrass ( Lolium perenne ) and with bare sand surfaces. The results suggest that both total sediment mass flux Q and PM 10 concentration in the air decreased exponentially with increasing canopy density. In the large-density canopy (frontal area index λ = 0.58), Q and PM 10 concentration were reduced to 0.01% and 0.4%, respectively, compared to the unplanted configuration. In the medium-density canopy ( λ = 0.16), Q and PM 10 concentration were reduced to 6.6% and 48.5%. In the small-density canopy ( λ = 0.03), however, Q and PM 10 concentration were increased to 117.5% and 145.6%. This is attributed to elevated shear stress on the sand bed caused by flow acceleration around the tussocks and vortical structures in their lee. Furthermore, the grasses were observed to trigger erosion by oscillating movements at the ground surface. It was also found that the vertical profiles of sediment mass flux in the medium- and large-density canopy strongly deviated from the exponential decay curve of the unplanted configuration, showing a local maximum at approximately twice the canopy height.

Aerodynamic Parameters of Urban Building Arrays with Random Geometries

It is difficult to describe the flow characteristics within and above urban canopies using only geometrical parameters such as plan area index (λ p ) and frontal area index (λ f ) because urban surfaces comprise buildings with random layouts, shapes, and heights. Furthermore, two types of ‘randomness’ are associated with the geometry of building arrays: the randomness of element heights (vertical) and that of the rotation angles of each block (horizontal). In this study, wind-tunnel experiments were conducted on seven types of urban building arrays with various roughness packing densities to measure the bulk drag coefficient (C d ) and mean wind profile; aerodynamic parameters such as roughness length (z o ) and displacement height (d) were also estimated. The results are compared with previous results from regular arrays having neither ‘vertical’ nor ‘horizontal’ randomness. In vertical random arrays, the plot of C d and z o versus λ f exhibited a monotonic increase, and z o increased by a factor of almost two for λ f = 48–70%. C d was strongly influenced by the standard deviation of the height of blocks (σ) when λ p ≥ 17%, whereas C d was independent of σ when λ p = 7%. In the case of horizontal random arrays, the plot of the estimated C d against λ f showed a peak. The effect of both vertical and horizontal randomness of the layout on aerodynamic parameters can be explained by the structure of the vortices around the blocks; the aspect ratio of the block is an appropriate index for the estimation of such features.

Estimating Urban Canopy Parameters Using Synthetic Aperture Radar Data

Abstract This paper introduces a remote sensing–based approach to rapidly derive urban morphological characteristics using radar satellite data. The approach is based on the expectation that the magnitude of the synthetic aperture radar (SAR) backscatter can be related to urban canopy parameters (UCPs) describing the height, density, and roughness of buildings, trees, and other objects in cities. This hypothesis was tested with full-feature terrain elevation and SAR datasets for the Houston, Texas, metropolitan area. The backscatter magnitude was found to vary as expected across the city with higher backscatter values in the downtown tall building district relative to adjacent residential and commercial areas. To demonstrate the concept of using radar backscatter to estimate UCPs, relationships were derived between SAR backscatter and mean height, plan area fraction, and frontal area index of roughness elements (e.g., buildings and trees). In addition, SAR backscatter relationships were derived with roughness lengths computed using morphometric approaches. In all cases, the derived relationships were found to provide estimates of UCPs acceptable for use in meteorological models. Further testing using data from the Salt Lake City, Utah, metropolitan area validated the relationships and identified key areas for improvement for future research, including SAR instrument view angle differences and buildings split between SAR pixels.

Estimating aerodynamic resistance of rough surfaces using angular reflectance

Current wind erosion and dust emission models neglect the heterogeneous nature of surface roughness and its geometric anisotropic effect on aerodynamic resistance, and over-estimate the erodible area by assuming it is not covered by roughness elements. We address these shortfalls with a new model which estimates aerodynamic roughness length ( z 0) using angular reflectance of a rough surface. The new model is proportional to the frontal area index, directional, and represents the geometric anisotropy of z 0. The model explained most of the variation in two sets of wind tunnel measurements of aerodynamic roughness lengths ( z 0). Field estimates of z 0 for varying wind directions were similar to predictions made by the new model. The model was used to estimate the erodible area exposed to abrasion by saltating particles. Vertically integrated horizontal flux ( F h) was calculated using the area not covered by non-erodible hemispheres; the approach embodied in dust emission models. Under the same model conditions, F h estimated using the new model was up to 85% smaller than that using the conventional area not covered. These F h simulations imply that wind erosion and dust emission models without geometric anisotropic sheltering of the surface, may considerably over-estimate F h and hence the amount of dust emission. The new model provides a straightforward method to estimate aerodynamic resistance with the potential to improve the accuracy of wind erosion and dust emission models, a measure that can be retrieved using bi-directional reflectance models from angular satellite sensors, and an alternative to notoriously unreliable field estimates of z 0 and their extrapolations across landform scales.

A simple method for designation of urban ventilation corridors and its application to urban heat island analysis

This paper describes urban wind ventilation mapping, using the concept of “building frontal area index”, and uses the Kowloon peninsula of Hong Kong as an example of a dense, sub-tropical urban environment where ventilation is critical for human health. The frontal area index is calculated for uniform 100 m grid cells, based on three dimensional buildings in each cell, for eight different wind directions. The frontal area index is then correlated with a land use map, and the results indicate that high density commercial and industrial areas with large building footprints had higher values than other urban land use types. Using the map of frontal area index, the main ventilation pathways across the urban area are located using least cost path analysis in a raster GIS. Field measurements of urban winds confirmed the significance and functionality of these modelled ventilation paths. Comparison of the pathways with a map of the urban heat island suggests that ventilation is a key parameter in mitigating heat island formation in the study area. Planning and environmental authorities may use the derived frontal area index and ventilation maps as objective measures of environmental quality within a city, especially when temperatures in the inner city are a major concern.

Aerodynamic Parameters of Regular Arrays of Rectangular Blocks with Various Geometries

The aerodynamic effects of various configurations of an urban array were investigated in a wind-tunnel experiment. Three aerodynamic parameters characterising arrays—the drag coefficient (Cd), roughness length (zo) and displacement height (d)—are used for analysis. Cd is based on the direct measurement of the total surface shear using a floating element, and the other two parameters are estimated by logarithmic fitting of the measured wind profile and predetermined total drag force. The configurations of 63 arrays used for measurement were designed to estimate the effects of layout, wind direction and the height variability of the blocks on these parameters for various roughness packing densities. The results are summarised as follows: (1) The estimated Cd and zo of the staggered arrays peak against the plan area index (λp) and frontal area index (λf), in contrast with values for the square arrays, which are less sensitive to λp and λf. In addition, the square arrays with a wind direction of 45° have a considerably larger Cd, and the wind direction increases zo/H by up to a factor of 2. (2) The effect of the non-uniformity of roughness height on zo is more remarkable when λf exceeds 20%, and the discrepancy in zo is particularly remarkable and exceeds 200%. (3) The effect of the layout of tall blocks on Cd is stronger than that of short blocks. These results indicate that the effects of both wind direction and the non-uniformity of the heights of buildings on urban aerodynamic parameters vary greatly with λp and λf; hence, these effects should be taken into account by considering the roughness packing density.

Characteristics of Boundary Layer over a Sequence of Small Localized Urban Canopies with Various Heights Obtained by Wind-Tunnel Experiment

Characteristics of a flow in the boundary layer over a sequence of small localized urban canopies with various heights were studied by wind-tunnel experiment under neutral stratification, where “small” means that the length (fetch) is insufficient for the flow to fully adapt to the surface geometry. The sequence was composed of two taller building groups, such as commercial buildings, and two roughness areas of lower obstacles, such as houses. In the sequence, the flow passed over boundaries from the building groups to the roughness areas, named downward boundaries, and boundaries from the roughness areas to the building groups, named upward boundaries. The flow was quite different depending on the boundary types. The focus was placed on equilibrium-like states of the flow and it was found that the flow behind the boundaries were in the states in short fetches.Relating to the downward boundary, flow over the roughness areas was obtained in the range L/H = 7.5 to 63, where H is the height of the upstream array and L is the distance from the end of the upstream array to the measurement position. It was found that the profiles of the Reynolds stress exhibited double-layered structure separated by kinks. In the range L/H > 19, the profiles had constant forms above the kinks and the flow was in some sort of equilibrium state. It was discussed schematically and found that the double-layered structure was explained well based on the three sources of turbulence: turbulence transported from the flow over the upstream building group, turbulence caused by the downward boundary, and turbulence caused by the surface obstacles.Relating to the upward boundary, flow over the building group (cubic array with a frontal area index of 0.25) was obtained. Two adaptation processes to equilibrium state were found: adaptation of transversal average of the streamwise velocity (U) and adaptation of transversal shear of U. The latter was in balanced state in shorter fetch than the former.

Parameterisation of aerodynamic roughness over boreal, cool- and warm-temperate forests

Roughness length and zero-plane displacement over boreal, cool- and warm-temperate forests were observed and parameterised using forest structure data. Previous models for roughness length and zero-plane displacement using leaf area index and frontal area index did not describe intersite differences, and the model for zero-plane displacement did not express seasonal variations with the change of leaf area that was smaller in dense forest than in sparse forest. The observed results show that intersite differences of normalised zero-plane displacement were related to stand density, and seasonal variations were related to leaf area index at each forest, with the degree depending on stand density. From these observations, a new concept is proposed for normalised zero-plane displacement: the basal part is primarily determined by the density of stems and branches (stand density), while the seasonal variation depends on the density of leaves (leaf area index), which is limited to the residual of the basal part. Based on this concept, a new model was developed and verified to express both intersite differences and seasonal variations in observed roughness length and zero-plane displacement.

An experimental investigation of the mean momentum budget inside dense canopies on narrow gentle hilly terrain

Recent theories and model calculations for flows inside canopies on gentle hilly terrain suggest that the impact of advection and pressure perturbations on the mean momentum budget remains problematic when the canopy adjustment length ( L c) is comparable to the hill half-length ( L) (referred to as narrow gentle hills). To progress on this problem, detailed laser Doppler anemometry (LDA) and water surface profile measurements were conducted in a large flume simulating a neutrally stratified boundary layer flow over a train of gentle hills covered by a dense canopy with L c/ L ≃ 1. The canopy was composed of an array of vertical cylinders with a frontal area index concentrated in the upper third to resemble a tall hardwood forest at maximum leaf area index. The data was presented in terms of component balance of the mean momentum equation decomposed into a background state and a perturbed state induced by topographic variation. We found that the measured and modelled pressure computed from the topographic shape function were not in phase, with the minimum pressure shifted downwind from the hill summit. We also showed that the recirculation region, predicted to occur on the lee side of the hill close to the ground, was sufficiently large to modify the mean streamlines both within the canopy sub-layer and just above the canopy. This adjustment in mean streamlines can be accounted for through an effective ground concept thereby retaining the usability of linear theory to model the mean pressure gradients. The LDA data suggested that the shear stress gradient remained significant at the bottom of the hill in the deeper layers of the canopy and was the leading term balancing the adverse pressure gradient in the recirculation region. The drag force was the leading contributor to the mean momentum balance near the canopy top and within the deeper layers of the canopy at the hill summit. However, we found that the drag force was not the primary term balancing the adverse pressure gradient within the recirculation zone. Advection was not only substantial above the canopy but remained significant in the deeper layers of the canopy near the hill summit as predicted by recent numerical simulations. In short, no one term in the mean momentum balance can be a priori neglected at all positions across a gentle narrow hill.

Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes

Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion.

A scheme for drag partition over rough surfaces

We present a drag partition scheme for surfaces of various roughness densities. The total drag is partitioned into three components: a pressure drag, a skin drag due to momentum transfer to the surfaces of roughness elements and a surface drag due to momentum transfer to the ground surface. We introduce an effective frontal area index to characterize the mutual sheltering effect of roughness elements on drag partition and propose a method for the calculation of the drag components. The drag partition scheme is then applied to the calculation of bulk quantities such as roughness length and zero-plane displacement height. The results are compared with wind-tunnel measurements. Roughness length is found to increase with frontal area index λ to a maximum at about λ = 0.2 , and then decreases with λ . This finding is in agreement with observations. The drag partition scheme has applications to studying scalar exchange in boundary layers over rough surfaces, such as urban boundary layers.

WAsP in the Forest

AbstractThis article compares mean wind estimates from a WAsP analysis for three forest sites and one site near a forest with measurements taken at the sites. By standard WAsP settings for forest, the mean wind speed at the sites was overestimated. Agreement between the estimates and the measurements improved significantly if displacement height and roughness length as calculated from the forest mast data were used or if a simple model estimate of roughness length and displacement height based on stand density (frontal area index) was used. The two estimates of displacement height and roughness length (mast data and simple model) did not agree well with each other. One reason for this may be that all evaluated sites are windy and that both d and z0 depend on the wind speed. All analysed forest sites are dense, in which case the influence from the roughness sublayer is limited and the effect on mean wind speeds from this layer could not be evaluated. Copyright © 2005 John Wiley & Sons, Ltd.

An approach to deriving roughness length and zero-plane displacement height from satellite data, prototyped with BOREAS data

Global climate models typically use relatively crude estimates of roughness length, often based on constant values for land cover category. However, large variations may exist within any of the vegetation categories. Global large-scale estimates of roughness length, such as required by climate models, cannot rely on field experiments for practical reasons and must, therefore, be derived from satellite observations. This paper develops a satellite retrieval method for the roughness parameters, momentum roughness length, z o, and displacement height, d. The formulas, based on the observational data of Raupach [Raupach, M.R., 1992. Bound.-Layer Meteor. 60, 375–395; Raupach, M.R., 1994. Bound.-Layer Meteor. 71, 211–216] and Lindroth [Lindroth, A., 1993. Bound.-Layer Meteor. 66, 265–272], are applicable to most types of vegetation. Raupach relates the momentum roughness length and zero-plane displacement height to the frontal area index, λ, and to the height of the vegetation, h. The frontal area index is related to the shape of an average tree or plant crown and to the average density of the canopy elements. The crown shape factor includes the aspect (height-to-width) ratio of the crown and the geometric shape of the crown. Lindroth relates the momentum roughness length to the leaf area index (LAI) and to the height of the trees. The functional form of z o/ h is peaked in both frontal area index, with the maximum occurring at λ=0.152, and average plant LAI ( L p), with the maximum occurring at L p=0.878. As λ and L p increase, d/ h also increases and approaches an upper limit asymptotically at high values of λ and L p. As the vegetation height is difficult to obtain from present satellite data, retrievals are restricted to fractional values (i.e. z o/ h and d/ h) for global application. This retrieval then requires data for the crown aspect ratio, the canopy density (fractional overstory canopy cover) and the LAI. Data from BOREAS ( BOReal Ecosystem Atmosphere Study) are used to illustrate this satellite retrieval method.

Observed Wind Profiles and Turbulence Fluxes over an ice Surface with Changing Surface Roughness

Wind profile and eddy-correlation data obtained at two sites on a melting glacier surface in Iceland during the summer of 1996 are presented. Throughout the experiment the surface roughness increased rapidly from smooth to very rough, with the largest roughness element height obtained being about 1.7 m. In a layer close to the rough surface we find that the wind speed profiles were disturbed showing horizontal inhomogeneities as in a roughness sublayer. Its height was approximately two times the height of the main roughness elements (h) at both sites throughout the experiment. From the wind profiles and eddy-correlation data we calculated corrections for the displaced zero plane as a function of time and compared these with results obtained from a drag partitioning model. In general, the agreement was reasonable considering the ranges of uncertainty but the results indicate that the increasing horizontal anisotropy of the surface probably limits the use of the model. The values obtained for the roughness lengths are in good agreement with those calculated from a simple linear model, i.e., z0/h = 0.5λ with λ the frontal area index. Above the roughness sublayer the wind profiles, normalised standard deviations of wind speed, and the balance of the turbulence kinetic energy budget behaved as over an ideal homogeneous surface thereby confirming similarity of the flow.

Estimation of vegetation aerodynamic roughness of natural regions using frontal area density determined from satellite imagery

Parameterizations of the frontal area index and canopy area index of natural or randomly distributed plants are developed and applied to the estimation of local aerodynamic roughness using satellite imagery. The formulas are expressed in terms of the subpixel fractional vegetation cover and one non-dimensional geometric parameter that characterizes the plant's shape. Geometrically similar plants and Poisson distributed plant centers are assumed. An appropriate averaging technique to extend satellite pixel-scale estimates to larger scales is provided. The parameterization is applied to the estimation of aerodynamic roughness using satellite imagery for a 2.3 km 2 coniferous portion of the Landes Forest near Lubbon, France, during the 1986 HAPEX-Mobilhy Experiment. The canopy area index is estimated first for each pixel in the scene based on previous estimates of fractional cover obtained using Landsat Thematic Mapper imagery. Next, the results are incorporated into Raupach's [Raupach, M.R., 1994. Boundary-Layer Meteorol. 71, 211–216] analytical formulas for momentum roughness and zero-plane displacement height. The estimates compare reasonably well to reference values determined from measurements taken during the experiment and to published literature values. The approach offers the potential for estimating regionally variable vegetation aerodynamic roughness lengths over natural regions using satellite imagery when there exists only limited knowledge of the vegetated surface.

A parameterization of momentum roughness length and displacement height for a wide range of canopy densities

Abstract. Values of the momentum roughness length, z0, and displacement height, d, derived from wind profiles and momentum flux measurements, are selected from the literature for a variety of sparse canopies. These include savannah, tiger-bush and several row crops. A quality assessment of these data, conducted using criteria such as available fetch, height of wind speed measurement and homogeneity of the experimental site, reduced the initial total of fourteen sites to eight. These datapoints, combined with values carried forward from earlier studies on the parameterization of z0 and d, led to a maximum number of 16 and 24 datapoints available for d and z0, respectively. The data are compared with estimates of roughness length and displacement height as predicted from a detailed drag partition model, R92 (Raupach, 1992), and a simplified version of this model, R94 (Raupach, 1994). A key parameter in these models is the roughness density or frontal area index, λ. Both the comprehensive and the simplified model give accurate predictions of measured z0 and d values, but the optimal model coefficients are significantly different from the ones originally proposed in R92 and R94. The original model coefficients are based predominantly on measured aerodynamic parameters of relatively closed canopies and they were fitted `by eye'. In this paper, best-fit coefficients are found from a least squares minimization using the z0 and d values of selected good-quality data for sparse canopies and for the added, mainly closed canopies. According to a statistical analysis, based on the coefficient of determination (r2), the number of observations and the number of fitted model coefficients, the simplified model, R94, is deemed to be the most appropriate for future z0 and d predictions. A CR value of 0.35 and a cd1 value of about 20 are found to be appropriate for a large range of canopies varying in density from closed to very sparse. In this case, 99% of the total variance occurring in the d-data across 16 selected canopies can be explained, whereas the analogous value for the z0-data (24 datapoints available) is 81%. This makes the R94 model, with only two coefficients and its relatively simple equations, a useful universal tool for predicting z0 and d values for all kinds of canopies. For comparison, a similar fitting exercise is made using simple linear equations based on obstacle height only (e.g. Brutsaert, 1982) and another formula involving canopy height as well as roughness density (Lettau, 1969). The fitted Brutsaert equations explain 98% and 62% of the variance in the d and z0-data, respectively. Lettau's equation for prediction of z0 performs unsatisfactorily (r2 values <0, even after fitting of the coefficient) and so it is concluded that the drag partition model is definitely the most effective for prediction of the momentum roughness lengths for a wide rang of canopy densities.

The effect of roughness elements on wind erosion threshold

A theory is developed to describe the dependence upon roughness density of the threshold friction velocity ratio Rt, the ratio of the threshold friction velocity of an erodible surface without roughness to that of the surface with nonerodible roughness present. The roughness density is quantified by the frontal area index λ. The prediction is Rt = (1 − mσλ)−½(1 + mβλ)−½, where β is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; σ is the basal‐to‐frontal area ratio of the roughness elements; and m (< 1) is a parameter accounting for differences between the average substrate surface stress and the maximum stress on the surface at any one point. The prediction is well verified by four independent data sets.

Experiments on scalar dispersion within a model plant canopy part I: The turbulence structure

This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that: (i) The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy. (ii) In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss. (iii) In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller. (iv) Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence. (v) Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.