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Exposure correction is necessary for removing the distortion effects induced by nonstandard local exposure in raw near‐ground wind speed datasets. The accurate calculation of the exposure correction factor (ECF $$ \\mathrm{ECF} $$ ) for wind speeds requires reliable input of the local aerodynamic roughness length (z0 $$ {z}_0 $$ ). In this study, we evaluate the performance of an ECF $$ \\mathrm{ECF} $$formula suggested by the World Meteorological Organization and the estimation of z0 $$ {z}_0 $$based on gustiness model. The estimation of z0 $$ {z}_0 $$will be more reasonable if local zero‐plane displacement (zd $$ {z}_d $$ ) is considered under rough terrain conditions. An empirical linear relationship zd=C0z0 $$ {z}_d={C}_0{z}_0 $$is introduced, and the ratio C0=6 $$ {C}_0=6 $$is recommended for meteorological stations under rough terrain conditions in China coastline. The incorporation of zd $$ {z}_d $$into the ECF $$ \\mathrm{ECF} $$formula suggested by the World Meteorological Organization is further performed. Sensitivity analyses indicate that the z0 $$ {z}_0 $$estimates and ECF $$ \\mathrm{ECF} $$values are highly sensitive to factors such as mean wind duration, gust duration and anemometer height. Finally, we conducted case studies across 15 meteorological stations in China coastline, which revealed that our proposed method enhances the accuracy of both z0 $$ {z}_0 $$estimation and ECF $$ \\mathrm{ECF} $$calculation in comparison to the existing models. This paper presents a method that incorporates zero‐plane displacement (zd $$ {z}_d $$ ) into the estimation of land surface roughness length (z0 $$ {z}_0 $$ ) utilizing a gustiness model and then the calculation of exposure correction factor (ECF $$ \\mathrm{ECF} $$ ) for wind speeds. The method's application is illustrated through exposure corrections at 15 meteorological stations across China. Scatter plots are presented to compare the z0 $$ {z}_0 $$estimates (left panel) and ECFs $$ \\mathrm{ECFs} $$(right panel) derived from the proposed method (indicated by red squares) with those obtained from two previous methods. For common ranges of mean gust factors, the proposed method yields more accurate z0 $$ {z}_0 $$estimates and ECF $$ \\mathrm{ECF} $$calculations.

Nine methods to determine local-scale aerodynamic roughness length ( z 0 ) and zero-plane displacement ( z d ) are compared at three sites (within 60 m of each other) in London, UK. Methods include three anemometric (single-level high frequency observations), six morphometric (surface geometry) and one reference-based approach (look-up tables). A footprint model is used with the morphometric methods in an iterative procedure. The results are insensitive to the initial z d and z 0 estimates. Across the three sites, z d varies between 5 and 45 m depending upon the method used. Morphometric methods that incorporate roughness-element height variability agree better with anemometric methods, indicating z d is consistently greater than the local mean building height. Depending upon method and wind direction, z 0 varies between 0.1 and 5 m with morphometric z 0 consistently being 2–3 m larger than the anemometric z 0 . No morphometric method consistently resembles the anemometric methods. Wind-speed profiles observed with Doppler lidar provide additional data with which to assess the methods. Locally determined roughness parameters are used to extrapolate wind-speed profiles to a height roughly 200 m above the canopy. Wind-speed profiles extrapolated based on morphometric methods that account for roughness-element height variability are most similar to observations. The extent of the modelled source area for measurements varies by up to a factor of three, depending upon the morphometric method used to determine z d and z 0 .

Zero plane displacement height (d0) and momentum roughness length (z0m), describe the aerodynamic characteristics of a vegetated surface. Usually, d0 and z0m are assumed to be constant functions of the physical characteristics of the surface. Prior evidence collected from the literature and our examination of flux tower data show that d0 and z0m vary in time at sites with tree and shrub canopies, but not grasslands. The conventional explanations of these variations are based on linear functions of wind velocity and friction velocity, with little theoretical basis. This study explains the variation in aerodynamic parameters by matching four analytical canopy velocity models to a logarithmic above-canopy velocity profile at canopy height. d0 and z0m come out as functions of 2 non-dimensional terms, the canopy momentum absorption capacity (parameter) and a (measurable) Péclet number. To test the theories of variation, we analysed the velocity profiles from Ozflux and Ameriflux sites. None of the theories could recreate d0 and z0m at half-hourly intervals. However, the canopy velocity models were able better to recreate the distribution of the variations in d0 and z0m. Additionally, the estimates of canopy momentum absorption capacity varied consistently with phenological changes in the canopies, whereas, the fitting parameters of the linear regression of using wind speed and friction velocity did not exhibit physically interpretable variations. The canopy velocity models may offer better predictions with an accurate estimation of the canopy height, a horizontally homogeneous and rigid canopy, and incorporation of the roughness sublayer.

There are many geometrical factors than can influence the aerodynamic parameters of urban surfaces and hence the vertical wind profiles found above. The knowledge of these parameters has applications in numerous fields, such as dispersion modelling, wind loading calculations, and estimating the wind energy resource at urban locations. Using quasi-empirical modelling, we estimate the dependence of the aerodynamic roughness length and zero-plane displacement for idealized urban surfaces, on the two most significant geometrical characteristics; surface area density and building height variability. A validation of the spatially-averaged, logarithmic wind profiles predicted by the model is carried out, via comparisons with available wind-tunnel and numerical data for arrays of square based blocks of uniform and heterogeneous heights. The model predicts two important properties of the aerodynamic parameters of surfaces of heterogeneous heights that have been suggested by experiments. Firstly, the zero-plane displacement of a heterogeneous array can exceed the surface mean building height significantly. Secondly, the characteristic peak in roughness length with respect to surface area density becomes much softer for heterogeneous arrays compared to uniform arrays, since a variation in building height can prevent a skimming flow regime from occurring. Overall the simple model performs well against available experimental data and may offer more accurate estimates of surface aerodynamic parameters for complex urban surfaces compared to models that do not include height variability.

Local aerodynamic roughness parameters (zero-plane displacement, zd, and aerodynamic roughness length, z0) are determined for an urban park and a suburban neighbourhood with a new morphometric parameterisation that includes vegetation. Inter-seasonal analysis at the urban park demonstrates zd determined with two anemometric methods is responsive to vegetation state and is 1–4 m greater during leaf-on periods. The seasonal change and directional variability in the magnitude of zd is reproduced by the morphometric methods, which also indicate z0 can be more than halved during leaf-on periods. In the suburban neighbourhood during leaf-on, the anemometric and morphometric methods have similar directional variability for both zd and z0. Wind speeds at approximately 3 times the average roughness-element height are estimated most accurately when using a morphometric method which considers roughness-element height variability. Inclusion of vegetation in the morphometric parameterisation improves wind-speed estimation in all cases. Results indicate that the influence of both vegetation and roughness-element height variability are important for accurate determination of local aerodynamic parameters and the associated wind-speed estimates.

We applied three approaches to estimate the zero-plane displacement d through the aerodynamic measurement height z (with z = z m - d and z m being the measurement height above the surface), and the aerodynamic roughness length z 0 , from single-level eddy covariance data. Two approaches (one iterative and one regression-based) were based on the universal function in the logarithmic wind profile and yielded an inherently simultaneous estimation of both d and z 0 . The third approach was based on flux–variance similarity, where estimation of d and consecutive estimation of z 0 are independent steps. Each approach was further divided into two methods differing either with respect to the solution technique (profile approaches) or with respect to the variable (variance of vertical wind and temperature, respectively). All methods were applied to measurements above a large, growing wheat field where a uniform canopy height and its frequent monitoring provided plausibility limits for the resulting estimates of time-variant d and z 0 . After applying, for each approach, a specific data filtering that accounted for the range of conditions (e.g. stability) for which it is valid, five of the six methods were able to describe the temporal changes of roughness parameters associated with crop growth and harvest, and four of them agreed on d to within 0.3 m most of the time. Application of the same methods to measurements with a more heterogeneous footprint consisting of fully-grown sugarbeet and a varying contribution of adjacent harvested fields exhibited a plausible dependence of the roughness parameters on the sugarbeet fraction. It also revealed that the methods producing the largest outliers can differ between site conditions and stability. We therefore conclude that when determining d for canopies with unknown properties from single-level measurements, as is increasingly done, it is important to compare the results of a number of methods rather than rely on a single one. An ensemble average or median of the results, possibly after elimination of methods that produce outliers, can help to yield more robust estimates. The estimates of z 0 were almost exclusively physically plausible, although d was considered unknown and estimated simultaneously with the methods and results described above.

Leaf area index(LAI) of forest is the sum of vertical projection area of plant leaves per unit area. Stand density refers to the number of trees per unit area. These two parameters are important indexes to characterize the wind resistance effect of forest underlying surface. This paper presents a theoretical model of the flow field above a forest canopy layer and the variation of forest aerodynamic roughness (Z0), zero-plane displacement (d), and friction speed (U *) with stand density and leaf area index (LAI) were investigated. The results show that stand density does not affect the variation of Z0, d, or u* with LAI. The aerodynamic roughness first increases and then decreases with increasing LAI, and the ratio of roughness to forest height Z0/h is below 0.16. The value of d increases to a maximum with increasing of LAI and then remains stable. Moreover, the maximum value increases with increasing stand density. The maximum d/h ratios corresponding to stand densities of 400 ha−1, 1000 ha−1 and 1600 ha−1 were 0.68, 0.88, and 0.93, respectively. Friction speed decreased with increasing LAI, then tends to become stable. The minimum friction speed decreases with increasing stand density and the minimum values of u* corresponding to stand densities of 400 ha−1, 1000 ha−1, and 1600 ha−1 were 0.65, 0.48, and 0.44, respectively. At the minimum friction velocity, the corresponding LAI was found to increase with increasing stand density.

This paper presents a framework to estimate aerodynamic roughness over specific height (zo/H) and zero plane displacement (d/H) over various landscapes in Kelantan State using airborne LiDAR data. The study begins with the filtering of airborne LiDAR, which produced ground and non-ground points. The ground points were used to generate digital terrain model (DTM) while the non-ground points were used for digital surface model (DSM) generation. Canopy height model (CHM) was generated by subtracting DTM from DSM. Individual trees in the study area were delineated by applying the Inverse Watershed segmentation method on the CHM. Forest structural parameters including tree height, height to crown base (HCB) and diameter at breast height (DBH) were estimated using existing allometric equations. The airborne LiDAR data was divided into smaller areas, which correspond to the size of the zo/H and d/H maps i.e. 50 m and 100 m. For each area individual tree were reconstructed based on the tree properties, which accounts overlapping between crowns and trunks. The individual tree models were used to estimate individual tree frontal area and the total frontal area over a specific ground surface. Finally, three roughness models were used to estimate zo/H and d/H for different wind directions, which were assumed from North/South and East/West directions. The results were shows good agreements with previous studies that based on the wind tunnel experiments.

A series of experiments have been made in a wind tunnel to investigate the ventilation of snow by shear. We argue that the zero-plane displacement can be used as a convenient indicator of ventilation, and that this can be obtained from measurements of mean velocity profiles in conditions of zero pressure gradient. Measurements made over a natural snow surface show a zero-plane displacement depth of less than 5 mm, but practical considerations preclude extensive use of snow for these measurements. Instead, the influence of permeability is investigated using reticulated foams in place of snow. We demonstrate that the foam and snow have similar structure and flow-relevant properties. Although the surface of the foam is flat, the roughness lengths increase by two orders of magnitude as the permeability increases from 6 x 10-⁹ to 160 x 10-⁹ m². The zero-plane displacement for the least permeable foams is effectively zero, but more than 15 mm for the most permeable foams. Our data compare well to the few studies available in the literature. By analogy to conditions over snow surfaces, we suggest that shear-driven ventilation of snow is therefore limited to the upper few millimetres of snow surfaces.

Field experiments were conducted on green gram during spring-summer seasons of 2011 and 2012 with four dates of sowing and five varieties arranged in a strip-plot design to study the aerodynamic characteristics of green gram at BCKV, Kalyani, West Bengal. Micro-cup anemometers were placed at 0.5, 1.0, 1.5, 2.0 and 2.5 m above the crop canopy on a wooden mast. The zero-plane displacement (d), roughness length (Z0) and drag coefficient (Cd) were found to increase with crop age but the effect of dates of sowing was not prominent except at the final stage of growth. For a LAI value of 1.5, “d” values was 0.254 m and the “Cd” value was 0.015. Both the “Cd” and “d” increased with plant height and LAI. The “d” and “Z0” had significant positive correlation with the total dry matter and crop growth rate irrespective of dates of sowing.