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We present a new algorithm for the estimation of plant area density (PAD) profiles and plant area index (PAI) for forested areas based on data from airborne lidar. The new element in the algorithm is to scale and average returned lidar intensities for each lidar pulse, whereas other methods either do not use the intensity information at all, only use average intensity values or do not scale the intensity information, which can cause problems for heterogeneous vegetation. We compare the performance of the new and three previously published algorithms over two contrasting types of forest: a boreal coniferous forest with a relatively open structure and a dense beech forest. For the beech forest site, both summer (full leaf) and winter (bare trees) scans are analyzed, thereby testing the algorithm over a wide spectrum of PAIs. Whereas all tested algorithms give qualitatively similar results, absolute differences are large (up to 400 % for the average PAI at one site). A comparison with ground-based estimates shows that the new algorithm performs well for the tested sites, and further and more importantly – it never produces clearly dubious results. Specific weak points for estimation of PAD from airborne lidar data are addressed; the influence of ground reflections and the effect of small-scale heterogeneity, and we show how the effect of these points is minimized using the new algorithm. We further show that low-resolution gridding of PAD will lead to a negative bias in the resulting estimate according to Jensen’s inequality for concave functions, and that the severity of this bias is method-dependent. As a result, PAI magnitude as well as heterogeneity scales should be carefully considered when setting the resolution for PAD gridding of airborne lidar scans.

Parameterizing the effect of vertically-distributed vegetation through an effective roughness (z0,eff) – whereby momentum loss through a three-dimensional foliage volume is represented as momentum loss over an area at one vertical level – can facilitate the use of forest data in flow models, to any level of detail, and simultaneously reduce computational cost. Results of numerical experiments and comparison with observations show that a modelling approach based on z0,eff can estimate wind speed and turbulence levels over forested areas, at heights of interest for wind energy applications (∼60 m and higher), but only above flat terrain. Caution must be exercised in the application of such a model to zones of forest edges. Advanced flow models capable of incorporating local (distributed) drag forces are recommended for complex terrain covered by forest.

Solitary trees are natural land surface elements found in almost all climates, yet their influence on the surrounding airflow is poorly known. Here we use state-of-the-art, laser-based, remote sensing instruments to study the turbulent wind field in the near-wake region of a mature, open-grown oak tree. Our measurements provide for the first time a full picture of the mixing layer of high turbulence that surrounds the mean wind speed deficit. In this layer, we investigate the validity of a two-dimensional vectorial relation derived from the eddy-viscosity hypothesis, a hypothesis commonly used in modelling the turbulence transport of momentum and scalars in the atmosphere. We find that the momentum fluxes of the streamwise wind component can be adequately predicted by the transverse gradient of the mean flow. Using the mixing-length hypothesis we find that for this tree the corresponding turbulence length scale in the mixing layer can be approximated by one height-independent value. Further, the laser-based scanning technology used here was able to accurately reveal three-dimensional turbulent and spatially varying atmospheric flows over a large plane without seeding or intruding the atmospheric flow. This capability points to a new and more exact way of exploring the complex earth–atmosphere interactions.

Trees are important natural wind engineering elements in both the urban and the agricultural contexts, but their aerodynamic description has been limited by its focus on young, flexible trees. Here, we provide a reformulation of the classical drag equation, which is also suitable for mature and wind-adapted trees. The new formulation is based on results from a full-scale experiment with focus on a solitary oak tree, for which we determined all terms in the drag equation experimentally. We also present a new photographical method for the accurate quantification of the tree’s frontal area under highly variable outdoor lighting conditions. We used a database of images from a surveillance camera, from which a high-quality subset was automatically selected with machine learning algorithms. Compared to previous work on younger and smaller trees, the mature tree has a lower absolute value of the Vogel exponent, which indicates a relatively low degree of reconfiguration. The presented results underline the high efficiency of mature trees in reducing the momentum of the wind. The results can be used to quantify the effect of similar trees in wind simulations, and the new method for determining the frontal area can be applied in other tree measurement campaigns.

In this study, we investigate the relationship between the mean wind-induced load on an open-grown oak tree and the mean wind speed. The estimation of the wind load is based on bending moment observations on the stem, using strain gauges installed close to the base of the tree. The measurement campaign included periods where the crown was both with and without foliage. A new post-processing method to compensate for time- and temperature-dependent drifts of long-term strain gauge measurements is presented. The time-scale, at which the method is applied, is determined from the observed high coherence between the wind speed and the induced bending moments. The analysis shows that the wind load on the tree is proportional to the wind speed raised to an exponent of 1.60–1.68 and 1.90–2.01 in the case of a crown with and without leaves, respectively. The results indicate that the presence of foliage increases the wind-induced load by a factor of 2–3. The impact of the wind speed range on these results is discussed and compared to the measured wind deficit in the lee of the tree. The application of this method can potentially be extended to other tree mounted sensors.

This study reports simultaneous measurements of wind at single point positions up- and downstream of a tree and a numerical experiment with the aim of quantifying the interaction of a solitary tree and the wind field. Relative to the inflow velocity, the velocity deficit in the wake of the tree showed strong seasonal dependence, with wake velocities changing between 70% and 10% of the upstream value from no-leaf winter conditions to full-leaf summer conditions. Whereas for the winter tree the turbulence intensity in the wake is everywhere reduced relative to the upwind flow, for the summer tree the turbulent intensity is markedly reduced in the inner wake, but increased in the outer wake. For the numerical experiment, the combination of (i) a high-detail tree model, based on terrestrial lidar scanning, and (ii) observations of the total bending moment on the tree, taken from strain gauges mounted on the stem, provided the tree parameterization. By this approach, the drag coefficient is not calibrated to fit the observed wind speed in the wake, but the total observed bending moment. Mean wind speed observations in the wake of both the winter and summer tree were well reproduced by the model with mean absolute errors lower than or equal to 5% throughout the wake transect. Also the turbulence intensity in the wake were well reproduced for the summer tree, whereas it was overestimated for the winter tree. Effects of changing tree model and grid resolution are demonstrated and discussed. Based on the presented findings, we recommend to estimate the total bending moment (or drag force) on modelled trees to ensure transferability of results between different numerical setups.

Trees are known to reduce the wind momentum efficiently. Yet, firm quantitative estimates of their contribution to the land surface drag have remained elusive, partly because trees have complex shapes that consist of elastic multi-scale elements. This structural complexity makes trees inherently difficult to scale for wind tunnel studies. Here, we test a new method for quantifying the drag force on a solitary mature tree in its natural environment. The method is based on the application of mass and momentum conservation over a control volume that encloses the tree. For this control volume, the drag force is estimated through the momentum deficit in the wake. For the characterisation of the heterogeneous and high-gradient wind field in the wake, spatially distributed measurements of the wind vector were acquired using three synchronously scanning wind lidar instruments in a vertical plane encompassing the wake. The resulting drag force estimate is compared to a reference measurement from a tree-mounted sensor at the base of the stem. We find that the drag force in both methods shows a dependence on the wind speed raised to an exponent of 1.8 and that the drag force, based on the momentum deficit method, is consistently underestimated by 1 %–10 %. Potential reasons for this bias are discussed in light of the accuracy of both methods. The relatively close agreement between the two methods indicates that scanning Doppler lidar measurements can be used to determine the drag force on complex objects in their natural environment, such as trees.

Sonic anemometers provide point observations of the three-dimensional velocity field at high sampling rates and are crucial instruments for understanding and quantifying the fluxes of momentum, energy and scalars between the atmosphere and Earth’s surface. Since the beginning of sonic anemometry 50 years ago, the characterization of flow distortion, i.e. how the instrument structure alters the flow, has been an ongoing research topic. Multi-path sonic anemometry provides a new opportunity to research and understand flow distortion on the vertical velocity component, since several positions in the small measurement volume can be measured simultaneously. In this work, we use data from a flat terrain measurement campaign in 2020, in which several sonic anemometers were mounted on 4m towers placed 4m apart. The analysis is focused on the Multipath Class-A sonic anemometer (Metek GmbH, Germany), which provides vertical velocity observations from three vertical paths 120 degrees and 0.1m apart. Vertical velocities are also calculated from several combinations of the tilted paths. We investigate how the vertical velocity component is altered depending on wind direction relative to different parts of the instrument structure. We demonstrate that by an optimal combination of the different paths, the vertical velocity variance and fluxes can be significantly enhanced. We also show spectra, and especially look at the high frequency end of the spectrum, where the relative behaviour of the velocity components is known from fundamental turbulence theory. Further, the relative importance of transducer shadowing and pressure-induced blockage effects is discussed.

1. Interactions between wind and trees control energy exchanges between the atmosphere and forest canopies. This energy exchange can lead to the widespread damage of trees and wind is a key disturbance agent in many of the world’s forests. However, most research on this topic has focused on conifer plantations, where risk management is economically important, rather than broadleaf forests, which dominate the forest carbon cycle. This study brings together all available tree motion time-series data to systematically evaluate the factors influencing tree responses to wind loading, including data from both broadleaf and coniferous trees in forests and open environments. 2. We found that the two most descriptive features of tree motion were: (a) the fundamental frequency, which is a measure of the speed at which a tree sways and is strongly related to tree height, and (b) the slope of the power spectrum, which is related to the efficiency of energy transfer from wind to trees. Intriguingly, the slope of the power spectrum was found to remain constant from medium to high wind speeds for all trees in this study. This suggests that, contrary to some predictions, damping or amplification mechanisms do not change dramatically at high wind speeds and therefore wind damage risk is related, relatively simply, to wind speed. 3. Conifers from forests were distinct from broadleaves in terms of their response to wind loading. Specifically, the fundamental frequency of forest conifers was related to their size according to the cantilever beam model (i.e. vertically distributed mass), whereas broadleaves were better approximated by the simple pendulum model (i.e. dominated by the crown). Forest conifers also had a steeper slope of the power spectrum. We interpret these finding as being strongly related to tree architecture, i.e. conifers generally have a simple shape due to their apical dominance, whereas broadleaves exhibit a much wider range of architectures with more dominant crowns.

The study of wind–tree interaction represents a challenge in fluid dynamics partly because the three-dimensional tree crown and its branch structure are dynamically adapting to the mean and turbulent wind components. With the advent of infrared technologies these complex, three-dimensional, real time interactions between wind and the response of a tree can be assessed. The present work makes use of a Microsoft Kinect V2, which incorporates an infrared time-of-flight camera, to capture images containing point cloud data of a single garden tree in turbulent winds. The raw output from the sensor is used to translate three-dimensional information into mean crown positions and projected frontal canopy areas. This novel infrared time-of-flight sensor technique facilitates a better understanding of the behaviour of the tree crown against wind speed. Overall, less movement and larger crown areas are observed at lower wind speeds. As the wind speed increases, a general shrinkage in area, more swaying and a smoothing of the crown edges are noted. Correlations between the observed wind force exerted on the tree and the displacement of the crown centre as well as between the wind force and the projected frontal canopy area are determined. The displacement and the drag force are positively correlated for all wind speeds. The drag force and frontal canopy area are positively correlated at low wind speeds and negatively correlated at higher wind speeds with a bi-stable phase between. The infrared time-of-flight sensor has demonstrated its capacity to provide real-time three-dimensional scanning of trees in a wind testing facility providing new insights into the complex wind–tree interaction problem.

We present numerical predictions and experimental observations of the drag force on the major branches of a model tree as well as the wind field in its wake. The tree model was mounted on a force sensor in a wind tunnel. The numerical simulations solved both the Reynolds Averaged Navier Stokes and detached eddy simulations, in combination with an immersed boundary model representation of the tree, while the measurements were obtained in the WindEEE Dome. Contrary to previous studies on trees or fractal structures, we find that the simulated forces on the tree skeleton were strongly dependent on the roughness of its surface. Upon setting a proper surface roughness, the simulations were capable of predicting both forces and wake fields, which were in good agreement with the observations both in terms of mean wind speed, mean turbulence and spectra. Furthermore, both simulations and observations showed that turbulence and mean wind speed were reduced for up to a distance of two tree heights downwind of the tree skeleton, underlining the efficiency of trees to remove momentum from a flow.

The understanding of scalar fluxes observed in the lower atmosphere is a challenging task, when the underlying surface is non-uniform. In this paper, we apply a micro-scale flow model with a two-equation closure scheme to analyse the influence of the surface heterogeneity on a flux measurement in the area surrounding the 122-m tower at Riso (Denmark), which is a mosaic of water, agricultural areas and forests. These heterogeneities are clearly reflected in the tower-based observations of the turbulence statistics from a profile of six sonic anemometers and are also reproduced by the flow model. Using the two-dimensional mode of the model, in combination with the footprint estimator, we calculate the scalar flux footprints for the 122 m eddy-covariance location and compare these results to analytical footprint estimators, which are only valid for homogeneous terrain, but are commonly applied also for heterogeneous terrain. Whereas the results by the analytical footprint estimator indicate smooth source areas regardless of the surface heterogeneities, the footprint estimator based on the micro-scale model indicates source hotspots, which have a stronger weight in the footprint. The hotspots coincide with areas, where the mean vertical velocity is positive. The positive mean vertical velocity is, in turn, related to topography and forest edge effects on the flow. Relative to the surface roughness estimated from a sonic anemometer, a higher value of the surface roughness was needed for the analytical footprint estimator in order to coarsely match the flow model-based footprint result. Although neither footprint model can be directly verified, the difference in the results underlines that the analytical model should be used with caution in heterogeneous areas. We also estimate the effect of the surface flux source-strength on the observed CO2 flux. This step demonstrates a novel way of evaluating the CO2 exchange with the surface, which is useful for constraining models of the surface source or sink.

The aim of the present work is to obtain a better understanding of how to model the thermally stratified wind field over a forest during full diurnal cycles. The setup of the study assumes a horizontally homogeneous forest, with the objective of finding a simple and efficient way to model the canopy flow using time-dependent input data, obtained from measurements and mesoscale simulations. With this, new insights can be gained for future microscale modelling of complex forested terrains using mesoscale input data. In terrain without forest a diurnal cycle is commonly simulated by imposing time-dependent ground temperature. However, the presence of forests partially isolates the temperature at ground level from the flow above the canopy, making this common approach ineffective. This work proposes imposing the time-dependent net radiation at the forest canopy top to drive the thermal stratification changes along the diurnal cycle. To this end, several full days of simulation are driven by prescribing the net radiative heat flux balance measured on top of the canopy, together with a geostrophic pressure gradient. The advantage of the method is its simplicity and that the input data can be easily obtained from mesoscale modelling. When compared to the observations at the Swedish site Ryningsnas, the new method dramatically improves estimations of wind speed, wind direction and turbulent kinetic energy compared to simulations that only assume neutral stratification. Out of the variables studied, temperature and turbulent heat flux profiles were the ones that qualitatively followed the measurements the best, while wind speed and turbulent kinetic energy showed a larger disagreement.

We propose a method to assess the accuracy of atmospheric turbulence measurements performed by sonic anemometers and test it by analysis of measurements from two commonly used sonic anemometers, a Metek USA-1 and a Campbell CSAT3, at two locations in Denmark. The method relies on the estimation of the ratio of the vertical to the along-wind velocity power spectrum within the inertial subrange and does not require the use of another measurement as reference. When we correct the USA-1 to account for three-dimensional flow-distortion effects, as recommended by Metek GmbH, the ratio is very close to 4∕3 as expected from Kolmogorov's hypothesis, whereas non-corrected data show a ratio close to 1. For the CSAT3, non-corrected data show a ratio close to 1.1 for the two sites and for wind directions where the instrument is not directly affected by the mast. After applying a previously suggested flow-distortion correction, the ratio increases up to ≈1.2, implying that the effect of flow distortion in this instrument is still not properly accounted for.

Most of our knowledge on forest-edge flows comes from numerical and wind-tunnel experiments where canopies are horizontally homogeneous. To investigate the impact of tree-scale heterogeneities ( $${>}1$$ m) on the edge-flow dynamics, the flow in an inhomogeneous forest edge on Falster island in Denmark is investigated using large-eddy simulation. The three-dimensional forest structure is prescribed in the model using high resolution helicopter-based lidar scans. After evaluating the simulation against wind measurements upwind and downwind of the forest leading edge, the flow dynamics are compared between the scanned forest and an equivalent homogeneous forest. The simulations reveal that forest inhomogeneities facilitate flow penetration into the canopy from the edge, inducing important dispersive fluxes in the edge region as a consequence of the flow spatial variability. Further downstream from the edge, the forest inhomogeneities accentuate the canopy-top turbulence and the skewness of the wind-velocity components while the momentum flux remains unchanged. This leads to a lower efficiency in the turbulent transport of momentum within the canopy. Dispersive fluxes are only significant in the upper canopy. Above the canopy, the mean flow is less affected by the forest inhomogeneities. The inhomogeneities induce an increase in the mean wind speed that was found to be equivalent to a decrease in the aerodynamic height of the canopy. Overall, these results highlight the importance of forest inhomogeneities when looking at canopy–atmosphere exchanges in forest-edge regions.

Applying erroneous roughness lengths can have a large impact on the estimated performance of wind turbines, particularly in forested areas. In this study, a new method called the Objective Roughness Approach (ORA), which converts tree height maps created using airborne lidar scans to roughness maps suitable for wind modeling, is evaluated via cross-predictions between different anemometers at a complex forested site with seven tall meteorological masts using the Wind Atlas Analysis and Application program (WAsP). The cross-predictions were made using ORA maps created at four spatial resolutions and from four freely available roughness maps based on land-use classifications. The validation showed that the use of ORA maps resulted in a closer agreement with observational data for all investigated resolutions compared to the land-use maps. Further, when using the ORA maps, the risk of making large errors (25 %) in predicted power density was reduced by 40–50 % compared to satellite based products with the same resolution. The results could be further improved for high-resolution ORA maps by adding the displacement height. The improvement when using the ORA maps came down to two factors, first they had a higher roughness length for forests, which was confirmed to by increasing the forest roughness value of the land-use based maps to the value of the ORA map, and second, due to the higher resolution of the ORA data, since the ORA maps with the highest resolution had the largest reduction in mean absolute errors.

Global oceans are an important sink of atmospheric carbon dioxide (CO2). Therefore, understanding the air–sea flux of CO2 is a vital part in describing the global carbon balance. Eddy covariance (EC) measurements are often used to study CO2 fluxes from both land and ocean. Values of CO2 are usually measured with infrared absorption sensors, which at the same time measure water vapor. Studies have shown that the presence of water vapor fluctuations in the sampling air potentially results in erroneous CO2 flux measurements resulting from the cross sensitivity of the sensor. Here measured CO2 fluxes from both enclosed-path Li-Cor 7200 sensors and open-path Li-Cor 7500 instruments from an inland measurement site are compared with a marine site. Also, new quality control criteria based on a relative signal strength indicator (RSSI) are introduced. The sampling gas in one of the Li-Cor 7200 instruments was dried by means of a multitube diffusion dryer so that the water vapor fluxes were close to zero. With this setup the effect that cross sensitivity of the CO2 signal to water vapor can have on the CO2 fluxes was investigated. The dryer had no significant effect on the CO2 fluxes. The study tested the hypothesis that the cross-sensitivity effect is caused by hygroscopic particles such as sea salt by spraying a saline solution on the windows of the Li-Cor 7200 instruments during the inland field test. The results confirm earlier findings that sea salt contamination can affect CO2 fluxes significantly and that drying the sampling air for the gas analyzer is an effective method for reducing this signal contamination.

The main purpose of this study is to evaluate the near-zero wind velocity measurement performance of two separate 1.5 μm all-fiber coherent Doppler lidars (CDL). The performance characterization is performed through the presentation of the results from two separate atmospheric field campaigns. In one campaign, a recently developed continuous wave (CW) CDL benefiting from an image-reject front-end was deployed. The other campaign utilized a different CW CDL, benefiting from a heterodyne receiver with intermediate frequency (IF) sampling. In both field campaigns the results are compared against a sonic anemometer, as the reference instrument. The measurements clearly show that the image-reject architecture results in more accurate measurements of radial wind velocities close to zero. Close-to-zero velocities are usually associated with the vertical component of the wind and are important to characterize.

An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensu ...

The New European Wind Atlas project will create a freely accessible wind atlas covering Europe and Turkey, develop the model chain to create the atlas and perform a series of experiments on flow in many different kinds of complex terrain to validate the models. This paper describes the experiments of which some are nearly completed while others are in the planning stage. All experiments focus on the flow properties that are relevant for wind turbines, so the main focus is the mean flow and the turbulence at heights between 40 and 300 m. Also extreme winds, wind shear and veer, and diurnal and seasonal variations of the wind are of interest. Common to all the experiments is the use of Doppler lidar systems to supplement and in some cases replace completely meteorological towers. Many of the lidars will be equipped with scan heads that will allow for arbitrary scan patterns by several synchronized systems. Two pilot experiments, one in Portugal and one in Germany, show the value of using multiple synchronized, scanning lidar, both in terms of the accuracy of the measurements and the atmospheric physical processes that can be studied. The experimental data will be used for validation of atmospheric flow models and will by the end of the project be freely available. This article is part of the themed issue ‘Wind energy in complex terrains’.

When validating the performance of a flow model in forested areas, it is important that the model accurately represents the forest effects. This presentation concerns the use of remote-sensing technology for describing forest effects, and more specifically, how positioning lidar data can be transferred into a parametrization of forests in wind models. The presentation covers three scales: the single tree, the forest edges and clearings, and the large-scale forested landscape in which the forest effects are parameterized with a roughness length. Flow modeling results and validation against observations are presented along with the different forest presentations for each of the cases. In a new research project called InnoWind,the use of satellite-based alternatives to airborne lidar campaigns are investigated, and examples of satellite products in wind power modeling are discussed.

The suitability of Copernicus Global Land Service products for wind assessment is investigated using two approaches. In the first approach the CORINE land cover database and the pan-European highresolution products were considered as input to atmospheric flow models. The CORINE data were used as input for modelling the wind conditions over a Danish near-coastal region. The flow model results were compared to alternative use of USGS land cover. Significant variations in the wind speed were found between the two atmospheric flow model results. Furthermore the wind speed from the flow model was compared to meteorological observations taken in a tall mast and from ground based remote-sensing wind profiling lidars. It is shown that simulations using CORINE provide better wind flow results close to the surface as compared to those using USGS on the investigated site. The next step towards improvement of flow model inputs is to investigate in further detail applied use of satellite maps in forested areas. 75% of new land-based wind farms are planned in or near forests in Europe. In forested areas the near surface atmospheric flow is more challenging to calculate than in regions with low vegetation because the tall vegetation to a high degree influences the atmospheric flow. Also in many forests the variation in forest plant structure is high. The forest structure depends on the tree height, the tree density, the existence of clearings, the types of leafs and branches and their structure. So the method of assigning one typical roughness length for land cover type ‘forest’ is at many sites not sufficient. This method assumes that all land cover classes can be represented with one value each. In our second approach, we look at a forested area in Northern Denmark, where an aerial lidar data observing terrain height, tree height and derived plant parameters provided a novel input for atmospheric flow modelling in forested areas. The flow model results were compared to horizontally scanning wind lidar observations and the results are very promising. Since, aerial lidar data are not available everywhere, we discuss the possibility of using similar Copernicus Global Land Service products as input to the flow model.

A velocity spectral tensor model was evaluated from the single-point measurements of wind speed. The model contains three parameters representing the dissipation rate of specific turbulent kinetic energy, a turbulence length scale and the turbulence anisotropy. Sonic anemometer measurements taken over a forested and an agricultural landscape were used to calculate the model parameters for neutral, slightly stable and slightly unstable atmospheric conditions for a selected wind speed interval. The dissipation rate above the forest was nine times that at the agricultural site. No significant differences were observed in the turbulence length scales between the forested and agricultural areas. Only a small difference was observed in the turbulence anisotropy at the two sites, except near the surface, where the forest turbulence was more isotropic. The turbulence anisotropy remained more or less constant with height at the forest site, whereas the turbulence became more isotropic with height for the agricultural site. Using the three parameters as inputs, we quantified the performance of the model in coherence predictions for vertical separations. The model coherence of all the three velocity components was overestimated for the analyzed stability classes at both sites. As expected from the model approximations, the model performed better at both sites for neutral stability than slightly stable and unstable conditions. The model prediction of coherence of the along-wind and vertical components was better than that of the cross-wind component. No significant difference was found between the performance of the model at the forested and the agricultural areas. © 2014 The Authors. Wind Energy published by John Wiley & Sons, Ltd.

For a single wind turbine, the efficiency of extracting energy from the wind depends on the ability to align the wind turbine with the dominating wind direction. Considering average power production, yaw misalignment is relevant when the wind turbine operates with maximum power coefficient. On the other hand, the power production is less sensitive to yaw misalignment in high wind speeds, where the available energy in the wind field is higher than the maximum wind turbine capacity. In a wind farm, the interaction between nearby wind turbines alters the flow, and the power production is reduced. The present study investigates how yaw misalignment affects the power production in these wake situations compared to yaw misalignment effects for a wind turbine in the free-stream. Two generic cases are presented in this paper, offshore and forest, where the atmospheric conditions alter the morphology of the wake and, therefore, the power output of a yawed wind turbine operating in wake conditions. The results show that, for a conventional downstream spacing further than 3 rotor diameters, yaw misalignment results in larger power loss in wake situations than in free-stream. In wake situations, the presented results also show that the spatial distribution of the deficit influences the relative power loss when the wind turbine is operating in yawed conditions.

A method is presented for development of satellite green vegetation fraction (GVF) time series for use in the Weather Research and Forecasting (WRF) model. The GVF data is in the WRF model used to describe the temporal evolution of many land surface parameters, in addition to the evolution of vegetation. Several high-resolution GVF products, derived from high-quality satellite retrievals from Moderate Resolution Imaging Spectroradiometer images, were produced and their performance was evaluated in long-term WRF simulations. The atmospheric conditions during the 2006 heat wave year over Europe were simulated since significant interannual variability in vegetation seasonality was found. Such interannual variability is expected to increase in the coming decades due to climatic changes. The simulation using a quadratic normalized difference vegetation index to GVF relationship resulted in consistent improvements of modeled temperatures. The model mean temperature cold bias was reduced by 10 % for the whole domain and by 20–45 % in areas affected by the heat wave. The study shows that WRF simulations during heat waves and droughts, when vegetation conditions deviate from the climatology, require concurrent land surface properties in order to produce accurate results.

The flow near tall forest edges is complex, yet poorly described. A field experiment using two meteorological masts equipped with sonic anemometers and a horizontally staring lidar was performed upwind and downwind of the interface between an open flat farmland and a tall (hc = 24 m) beech forest. Data obtained during near-neutral conditions are presented for the wind direction towards the forest. Results from a high leaf area index period are compared with those from a low leaf area index period. For both periods, the wind speed increased above the forest and decreased within the forest, relative to the measurements upwind of the edge. The lidar data taken at several positions between the masts at 1.25hc show that the minimum wind speed occurred just upwind of the edge. At the 1.25hc level, at the forest mast, the momentum flux () increased strongly over the forest and positive values were recorded during the high leaf area index period. A spectral analysis revealed that approximately half of this change was caused by low-frequency, positively correlated eddies along the streamline. The remaining increase can qualitatively be explained with the concept of eddy-blocking by the canopy top, which could also explain the observed increase in lateral variance and the decrease in the vertical variance. Despite the short distance to the edge of approximately 1.5hc, the beginning of a new internal boundary layer was visible at 1.04hc as a decrease in the vertical momentum flux. At this level, as well as within the forest, the results depended on the wind speed. The presented findings enhance the understanding of the forest edge flow and are useful for model verification and development.

In-canopy mixing ratio gradients and above-canopy fluxes of several volatile organic compounds (VOCs) were measured using a commercial proton transfer reaction mass spectrometer (PTR-MS) in a European beech (Fagus sylvatica) forest in Denmark. Fluxes of methanol were bidirectional: Emission occurred during both day and night with highest fluxes (0.2 mg C m−2 h−1) during a warm period; deposition occurred dominantly at daytime. Confirming previous branch-level measurements on beech, the forest’s monoterpene emissions (0–0.5 mg C m−2 h−1), and in-canopy mixing ratios showed a diurnal cycle consistent with light-dependent emissions; a result contrasting temperature-only driven emissions of most conifer species. Also emitted was acetone, but only at ambient temperatures exceeding 20°C. Slow deposition dominated at lower temperatures. Our in-canopy gradient measurements contrast with earlier results from tropical and pine forest ecosystems in that they did not show this beech ecosystem to be a strong sink for oxygenated VOCs (OVOCs). Instead, their gradients were flat and only small deposition velocities (

An analysis of flow tilt angles from a fetch-limited beech forest site with clearings is presented in the context of vertical advection of carbon dioxide. Flow angles and vertical velocities from two sonic anemometers by different manufacturers were analyzed. Instead of using rotations, where zero-flow angles were assumed for neutral flow, the data was interpreted in relation to upstream and downstream forest edges. Uncertainties caused by flow distortion, vertical misalignment and limited sampling time (statistical uncertainty) were evaluated and found to be highly significant. Since the attack angle distribution of the wind on the sonic anemometer is a function of atmospheric stratification, an instrumental error caused by imperfect flow distortion correction is also a function of the atmospheric stratification. In addition, it is discussed that the sonic anemometers have temperature dependent off-sets. These features of the investigated sonic anemometers make them unsuitable for measuring vertical velocities over highly turbulent forested terrain. By comparing the sonic anemometer results to that of a conically scanning Doppler lidar (Dellwik et al., 2010b), sonic anemometer accuracy for measuring mean flow tilt angles was estimated to between 2° and 3°. Use of planar fit algorithms, where the mean vertical velocity is calculated as the difference between the neutral and non-neutral flow, does not solve this problem of low accuracy and is not recommended. Because of the large uncertainties caused by flow distortion and vertical alignment, it was only possible to a limited extent to relate sonic anemometer flow tilt angles to upwind forest edges, but the results by the lidar indicated that an internal boundary layer affect flow tilt angles at 21m above the forest. This is in accordance with earlier studies at the site. Since the mean flow tilt angles do not follow the terrain, an estimate of the vertical advection term for near-neutral conditions was calculated using profile measurements of carbon dioxide. The estimated advection term is large, but it is not recommended to include it in the surface carbon balance, unless all terms in the carbon dioxide conservation equation can be precisely estimated.

Turbulent water vapour fluxes measured with closed-path eddy correlation (EC) systems are unintentionally low-pass filtered by the system in a manner that varies with environmental conditions. Why and how is described here. So is the practical method that systematically corrects long-term flux datasets for this substantial measurement error. In contrast to earlier studies, a large number of spectra and raw data have been used in the analysis to define the low-pass filtering characteristic of the EC system. This revealed that the cut-off frequency of the closed-path EC system for water vapour concentration measurements decreases exponentially with increasing relative humidity. After correction for this unintended filtering, the fluxes are consistent with CO2 and H2O fluxes that were measured with an open-path sensor at the same time. The correction of water vapour flux measurements over a Beech forest in Soro, Zealand, Denmark, amounted on average to 42% of the measured flux, while it was only 4% for the CO2 flux, which was measured with the same EC system. We recommend using the described method to correct water vapour fluxes measured in any closed-path EC system for unintended low-pass filtering effects. Other than for CO2 is the magnitude of the correction for water vapour flux measurements unsatisfactorily high, i.e. the EC system needs to be technically improved. Our results suggest that such high correction can be avoided by keeping relative humidity in the entire gas transport system of the EC system lower than 30%, e.g. by heating intake filters and tubes.

The difficulty of obtaining accurate information about the canopy structure is a current limitation towards higher accuracy in numerical predictions of the wind field in forested terrain. The canopy structure in computational fluid dynamics is specified through the frontal area density and this information is required for each grid point in the three-dimensional computational domain. By using raw data from aerial LiDAR scans together with the Beer-Lambert law, we propose and test a method to calculate and grid highly variable and realistic frontal area density input. An extensive comparison with ground-based measurements of the vertically summed frontal area density (or plant area index) and tree height was used to optimize the method, both in terms of plant area index magnitude and spatial variability. The resolution of the scans was in general low (

The influence of an internal boundary layer and a roughness sublayer on flux–profile relationships for momentum and sensible heat have been investigated for a closed beech forest canopy with limited fetch conditions. The influence was quantified by derivation of local scaling functions for sensible heat flux and momentum (Φ h and Φ m) and analysed as a function of atmospheric stability and fetch. For heat, the influences of the roughness sublayer and the internal boundary layer were in agreement with previous studies. For momentum, the strong vertical gradient of the flow just above the canopy top for some wind sectors led to an increase in Φ m, a feature that has not previously been observed. For a fetch of 500 m over the beech forest during neutral atmospheric conditions, there is no height range at the site where profiles can be expected to be logarithmic with respect to the local surface. The different influence of the roughness sublayer on Φ h and Φ m is reflected in the aerodynamic resistance for the site. The aerodynamic resistance for sensible heat is considerably smaller than the corresponding value for momentum.

A wind resource estimation study based on a series of 62 satellite wind field maps is presented. The maps were retrieved from imaging synthetic aperture radar (SAR) data. The wind field maps were used as input to the software RWT, which calculates the offshore wind resource based on spatial averaging (footprint modelling) of the wind statistic in each satellite image. The calculated statistics can then be input to the program WAsP and used in lieu of in-situ observations by meteorological instruments. A regional wind climate map based on satellite SAR images delineates significant spatial wind speed variations. The site of investigation was Horns Rev in the North Sea, where a meteorological time series is used for comparison. The advantages and limitations of these new techniques, which seem particularly useful for mapping of the regional wind climate, are discussed. Copyright © 2005 John Wiley & Sons, Ltd.

Wind maps are retrieved from ERS-2 Synthetic Aperture Radar (SAR) scenes by the CMOD-IFR2 and CMOD4 algorithms for 61 cases at the Horns Rev site in the North Sea and compared to meteorological in situ observations from a mast located 14 km offshore. The in situ data are corrected for flow distortion and sea-level changes prior to validating the SAR wind maps. The SAR wind maps are area-averaged by a simple footprint method assuming neutral stability and with three nonlinear weighting footprint methods including correction for stability. From a physical point of view, the latter is more correct. However, between in situ and SAR-derived wind-speed estimates comparison results of the nonlinear footprint values are statistically less correlated (R 2=0.73–0.77) and the standard error (SE) is larger (>1.5 m s−1) than results from the simple footprint (R 2=0.78–0.80 and SE=1.3 m s−1). The results are found with wind direction determined from wind streaks in the SAR images by Fast Fourier Transform. Using in sit...

In numerical weather prediction, climate and hydrologicalmodelling, the grid cell size is typically larger than the horizontal length scales of variations in aerodynamicroughness, surface temperature and surface humidity. These local land cover variations give rise to sub-gridscale surface flux differences. Especially the roughness variations can give a significantly differentvalue between the equilibrium roughness in each of the patches as compared to the aggregated roughness value,the so-called effective roughness, for the grid cell. The effective roughness is a quantity that secures thephysics to be well-described in any large-scale model. A method of aggregating the roughness step changesin arbitrary real terrain has been applied in flat terrain (Denmark) where sub-grid scale vegetation-drivenroughness variations are a dominant characteristic of the landscape. The aggregation model is a physicaltwo-dimensional atmospheric flow model in the horizontal domain based on a linearized version of theNavier Stoke equation. The equations are solved by the Fast Fourier Transformation technique, hence the codeis very fast. The new effective roughness maps have been used in the HIgh Resolution Limited Area Model(HIRLAM) weather forecasting model and the weather prediction results are compared for a number of casesto synoptic and other observations with improved agreement above the predictions based on currentstandard input. Typical seasonal springtime bias on forecasted winds over land of +0.5 m s-1 and-0.2 m s-1 in coastal areas is reduced by use of the effective roughness maps.

Friction velocity data from different heights above a forest are used to evaluate the influence from the surrounding landscape on forest micro-meteorological measurements under near-neutral conditions. Data are used from one field site and two forest sites. The field site data are used to estimate the magnitude of the scatter. Different theoretical friction velocity profiles for the Internal Boundary Layer (IBL) are tested against the forest data. The results yield information on the Internal Equilibrium Layer (IEL) growth and an equation for the IEL height for neutral conditions is derived. For stable conditions the results indicate that very long fetches are required in order to measure parameters in equilibrium with the actual surface.

The energy balance at most surface-atmosphere flux research sites remains unclosed. The mechanisms underlying the discrepancy between measured energy inputs and outputs across the global FLUXNET tower network are still under debate. Recent reviews have identified exchange processes and turbulent motions at large spatial and temporal scales in heterogeneous landscapes as the primary cause of the lack of energy balance closure at some intensively-researched sites, while unmeasured storage terms cannot be ruled out as a dominant contributor to the lack of energy balance closure at many other sites. We analyzed energy balance closure across 173 ecosystems in the FLUXNET database and explored the relationship between energy balance closure and landscape heterogeneity using MODIS products and GLOBEstat elevation data. Energy balance closure per research site (C-EBS)averaged 0.84 +/- 0.20, with best average closures in evergreen broadleaf forests and savannas (0.91-0.94) and worst average closures in crops, deciduous broadleaf forests, mixed forests and wetlands (0.70-0.78). Half-hourly or hourly energy balance closure on a percent basis increased with friction velocity (u.) and was highest on average under near-neutral atmospheric conditions. C-EBS was significantly related to mean precipitation, gross primary productivity and landscape-level enhanced vegetation index (EVI) from MODIS, and the variability in elevation, MODIS plant functional type, and MODIS EVI. A linear model including landscape-level variability in both EVI and elevation, mean precipitation, and an interaction term between EVI variability and precipitation had the lowest Akaike's information criterion value. C-EBS in landscapes with uniform plant functional type approached 0.9 and C-EBS in landscapes with uniform EVI approached 1. These results suggest that landscape-level heterogeneity in vegetation and topography cannot be ignored as a contributor to incomplete energy balance closure at the flux network level, although net radiation measurements, biological energy assimilation, unmeasured storage terms, and the importance of good practice including site selection when making flux measurements should not be discounted. Our results suggest that future research should focus on the quantitative mechanistic relationships between energy balance closure and landscape-scale heterogeneity, and the consequences of mesoscale circulations for surface-atmosphere exchange measurements. (C) 2012 Elsevier B.V. All rights reserved. (Less)

A system for measuring a velocity of tracerparticle motion in a fluid comprising at least one laser emitterconfigured to emit a continuous wave laser beam and a plurality of optical devices being configured to alternately receivea laser beam, focusing the laser beam onto a same probevolume comprising tracer particles, and receiving backscatteredlight from the tracer particles. The optical deviceshave a common optical input/output port for transmitting thereceived continuous-wave laser beam and for receiving thebackscattered radiation. The optical devices are provided tohave mutual pointing angles so that each of the plurality ofoptical devices points at the probe volume under a differentangle. A processor is configured to receive a part of the trans -mitted laser beam and the received backscattered radiationbeam to calculate a Doppler shift, and thereby determine aplurality of velocity components of the tracer particles usingcoherent detection.

Climate studies show that the frequency of heat wave events and above-average high temperatures during the summer months over Europe will increase in the coming decades. Such climatic changes and long-term meteorological conditions will impact the seasonal development of vegetation and ultimately modify the energy distribution at the land surface. In weather and climate models it is important to represent the vegetation variability accurately to obtain reliable results. The weather research and forecasting (WRF) model uses green vegetation fraction (GVF) time series to represent vegetation seasonality. The GVF of each grid cell is additionally used to scale other parameters such as LAI, roughness, emissivity and albedo within predefined intervals. However, the default GYP used by WRF does not reflect recent climatic changes or change in management practices since it was derived more than 20 years ago. In this study, a new high resolution GYP product based on MODIS images is applied in a high resolution WRF simulation over Denmark during the 2006 heat wave year. It is found that the GYP is very different in a heat wave year compared to the default GVF. The simulation is compared to a control run using the default GVF data and their performances are quantified against gridded data. The verification includes 2-m temperature and precipitation. The results show that although the simulation using the new GYP product performs well, it does not significantly improve performance compared to the default GYP, despite significant differences in vegetation fractions.

Six to nine years of net ecosystem carbon exchange (NEE) data from forests in Hyytiälä in Finland, Sorø in Denmark and Norunda in Sweden were used to evaluate the interannual variation in the carbon balance. For half-monthly periods, average NEE was calculated for the night-time data. For the daytime data parameters were extracted for the relationship to photosynthetic active radiation (PAR). The standard deviation of the parameters was highest for Norunda where it typically was around 25% of the mean, while it was ca. 15% for Hyytiälä and Sorø. Temperature was the main controller of respiration and photosynthetic capacity in autumn, winter and spring but explained very little of the interannual variation in summer. A strong correlation between respiration and photosynthesis was also revealed. The start, end and length of the growing season were estimated by four different criteria. The start date could explain some of the variation in yearly total NEE and gross primary productivity (GPP) in Hyytiälä and Sorø, but the average maximum photosynthetic capacity in summer explained more of the variation in annual GPP for all sites than start, end or length of the growing season.DOI: 10.1111/j.1600-0889.2006.00324.x

Data on net CO 2 exchange from eight forests in Denmark, Sweden, Finland and Iceland were used to analyse which factors were controlling photosynthesis and respiration. The forests consisted of different species ranging in climatic condition from temperate to subarctic. Only well mixed conditions were analysed ( u * > 0.3 m s -1 ). The parameters of a light response function showed strong seasonal variations with similar behaviour for all stands except for a beech forest where the development of a vigorous ground vegetation in spring affected the photosynthesis parameters differently as compared to the other forests. The beech forest also showed the highest respiration rates in the earlier part of the growing season in contrast to the other forests that showed maximum values in late part of July. The mean half-monthly nighttime respiration rates were well explained by an equation with one fitting parameter, the respiration rate at 10 °C, with an r 2 = 0.864 for all stands together. The difference between the stands concerning both photosynthesis and respiration parameters were largely explained by the differences in LAI. After normalizing for LAI, the only remaining correlation was between respiration and stand age. These results are promising for application of remote sensing for estimation of respiration as well as gross primary productivity from forests. DOI: 10.1111/j.1600-0889.2007.00330.x

The height and rotor diameter of modern wind turbines are so extensive, that the wind conditions they encounter often are well above the surface layer, where traditionally it is assumed that wind direction and turbulent fluxes are constant with respect to height, if the surface is homogenous. Deviations from the requirement of homogeneity are often the focus of micro-scale studies in forested areas. Yet, to explain the wind climate in the relevant height range for turbines, it is necessary to also account for the length scales that are important parameters for the meso-scale flow. These length scales are the height of the planetary boundary layer and the Monin-Obukhov length, which both are related to the energy balance of the surface. Examples of important micro- and meso-scale effects of forested terrain are shown using data and model results from recent and ongoing experiments. For micro-scale modeling, the issue of model resolution is discussed.