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Abstract: A comprehensive investigation is undertaken to discern the structure of momentum flux, turbulent kinetic energy, and scalar fluxes like heat, CO \(_2\) , and H \(_2\) O in the atmospheric surface layer (ASL) at the Thumba Equatorial Rocket Launching Station—a coastal station on the west coast of southern peninsular India. The vertical transport, transfer efficiency, and dissimilarity between flux transport are studied as a function of stability using data collected over 1 year. The transfer efficiency for heat fluxes and momentum exhibits a strong dependence on stability ( \(\zeta \) ). However, the transfer efficiency of passive scalars CO \(_2\) and H \(_2\) O displays no apparent dependence on \(\zeta \) . The correlation between fluxes and squared coherence estimates is evaluated to study the dissimilarity between flux transport. The correlation is strongest among momentum and heat fluxes and between CO \(_2\) and H \(_2\) O fluxes and shows a dependence on the prevailing stability conditions. However, the influence of stability is not evident for the various other combinations. The momentum and heat flux transport is dissimilar for unstable conditions, and it becomes similar during the transition from unstable to near-neutral conditions. The quadrant analysis is employed to study the contribution of different fluid motions to the aforementioned turbulent fluxes. Except for CO \(_2\) and H \(_2\) O fluxes, where all the quadrants have an equal contribution, ejections and sweeps are the dominating contributors for momentum and heat fluxes. The stability conditions greatly determine the ejection-sweep balance for heat flux, while some changes in duration and impact fraction are also detectable for momentum flux. Furthermore, contour maps of joint-probability function (JPDF) of vertical velocity fluctuations ( \(w'\) ) with streamwise velocity fluctuation ( \(u'\) ), temperature fluctuation ( \(T'\) ), and scalar fluctuations, respectively, are also presented. The dominance of the ejection and sweep cycles for turbulent fluxes provide evidence for the presence and importance of coherent structures in ASL. PubDate: 2023-07-01

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Abstract: This study developed a backward-Eulerian footprint modelling method based on an adjoint equation for atmospheric boundary-layer flows. In the proposed method, the concentration footprint can be obtained directly by numerical simulation with the adjoint equation, and the flux footprints can be estimated using the adjoint concentration based on the gradient diffusion hypothesis. We first tested the proposed method by estimating the footprints for an ideal three-dimensional boundary layer with different atmospheric stability conditions based on the Monin–Obukhov profiles. It was indicated that the results were similar to the FFP method (Kljun et al. in Boundary-Layer Meteorol 112:503–523, 2004, https://doi.org/10.1023/B:BOUN.0000030653.71031.96; Geosci Model Dev 8:3695–3713, 2015, https://doi.org/10.5194/gmd-8-3695-2015) for convective conditions and the K–M method (Kormann and Meixner in Boundary-Layer Meteorol 99:207–224, 2001, https://doi.org/10.1023/A:1018991015119) for stable conditions. The proposed method was then coupled with the Reynolds averaged Navier–Stokes model to calculate the footprints for a block-arrayed urban canopy. The results were qualitatively compared to the results from the Lagrangian-Large-Eddy-Simulation (LL) method (Hellsten et al. in Boundary-Layer Meteorol 157:191–217, 2015, https://doi.org/10.1007/s10546-015-0062-4). It was shown that the proposed method reproduced the main features of footprints for different sensor positions and measurement heights. However, it is necessary to simulate the adjoint equation with a more sophisticated turbulence model in the future to better capture turbulent effects in the footprint modelling. PubDate: 2023-07-01

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Abstract: Compared to conventional wind field measurement methods such as wind masts or wind towers, UAV-based measurement is a relatively new approach to making wind field measurements. In the present study, a method for measuring wind field by using a six-rotor UAV mounted with an ultrasonic anemometer was established, and the feasibility thereof in wind field measurement was tested. Firstly, the influence of the UAS fuselage attitude on the accuracy of wind measurement results was analysed by means of wind tunnel testing. The results show that the average wind speed obtained by the UAV anemometry system (UAS) was slightly larger, but the average wind speed obtained by the UAS was consistent with that obtained by the Cobra anemometer after the modification of the fuselage attitude coefficient. Secondly, the wind field measurement results obtained by the UAS and the wind tower were compared, and the revised wind speed, wind direction, turbulence intensity and other parameters obtained by the UAS were found to be consistent with those of the anemometers at the same height on the wind tower. The difference was within 5%, and the longitudinal fluctuating wind power spectra obtained by the two were almost the same, being in good agreement with the Von Karman spectrum. Finally, the UAS was used to measure the wind field characteristic parameters of a certain site, which were compared with the corresponding parameters of national regulations. The feasibility of the UAS in measuring the air wind field was verified. These research results provide a reference for further research into UAV wind measurement methods. PubDate: 2023-07-01

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Abstract: It is demonstrated that the vertical profile of gradient Richardson number, Ri, can be shaped by control of the working-section inlet temperature profile. In previous work (Hancock and Hayden in Boundary-Layer Meteorol 168:20–57, 2018; 175:93–112, 2020; 180:5–26, 2021) the inlet temperature profile had been specified but without control of the profile of \(Ri\) in the developed-flow region of the working section. Control of the inlet temperature profile is provided by 15 inlet heaters (spread uniformly across the height of the working section), allowing control of the temperature gradient over the bulk of the boundary layer, and the overall temperature level above that of the surface. The bulk Richardson number for the 11 cases covers the range 0.01–0.17 (there is no overlying inversion). In the upper \(\approx\) 2/3 of the boundary layer the Reynolds stresses and turbulent heat flux are controlled by the gradient in mean temperature, while in the lower \(\approx\) 1/3 they are controlled both by this gradient and by the level above the surface temperature. In three examples, \(Ri\) is approximately constant at 0.07, 0.10 and 0.13 across the bulk of the layer. The previous observation of horizontally homogenous behaviour in the temperature profiles in the top \(\approx\) 2/3 of the boundary layer but not in the lower \(\approx\) 1/3 is repeated here, except when, tentatively, Ri does not exceed 0.05 over the bulk of the boundary layer. Favourable validation comparisons are made against two sets of local scaling systems and field data over the full depth of the boundary layer, over the range 0.006 \(\le Ri \le\) 0.3, or, in terms of height and local Obukhov length, 0.005 \(\le z/L \le\) 1. PubDate: 2023-07-01

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Abstract: Modelling the horizontal and vertical variation of wind speed is crucial for wind energy applications. A model frequently used for this purpose is part of the Wind Atlas Analysis and Application program (WAsP). Here, we modify the model in WAsP to account for local atmospheric stability parameters. Atmospheric stability effects are treated by using the impact of a temperature scale on the geostrophic drag law and the diabatic logarithmic wind profile. Using this approach, wind-direction dependent mean and standard deviation of a surface-layer temperature scale and a mean boundary-layer height scale can be obtained from either numerical weather prediction model output or observations to improve vertical extrapolations of Weibull wind speed distribution parameters. The modified atmospheric stability model is validated at six flat sites in northwestern Europe. The surface-layer temperature scale is available from sonic anemometer measurements at three of the sites. At all sites the temperature scale is also estimated from reanalysis data and from mesoscale model output. The modified model improves the aggregated estimations of power density distributions when extrapolating to nearby locations from 5.2 to 3%, when using the temperature scale derived from either observations or mesoscale/reanalysis output compared to the current model. PubDate: 2023-07-01

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Abstract: We describe a new direct correction approach to accurately restore frequency attenuated eddy covariance (EC) measurements. The new approach utilizes the Wiener deconvolution method to optimally estimate the original signal from noisy atmospheric measurements. Key features over conventional EC spectral correction methods include (i) the use of physics-based response functions, (ii) the ability to account for the non-linear phase contributions, and (iii) the direct restoration of the original signal rather than simulating the effect on an ideal reference spectrum. The new correction approach is compared to conventional spectral correction methods in a numerical simulation where the magnitude of the key limitations of conventional methods is explored under conditions relevant to common EC set-ups. The simulation results showed that the spectral correction methods commonly used for calculating EC fluxes introduced systematic error up to 10% to the restored fluxes and substantially increased their random uncertainty. The errors are attributed to the effect of using inappropriate response functions, failing to account for the contribution of the non-linear phase, and due to the assumption of spectral similarity on the scale of averaging intervals. The Wiener deconvolution method is versatile, can be applied under non-ideal conditions, and provides an opportunity to unify analytical and “in-situ” spectral correction methods by applying existing transfer functions to directly restore attenuated spectra. Furthermore, the Wiener deconvolution approach is adaptable for use with various micrometeorological measurement techniques such as eddy accumulation and flux profile measurements. PubDate: 2023-07-01

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Abstract: Lagrangian stochastic models (LSM) are widely used to model the dispersion of sea spray droplets injected from the water surface into the marine atmospheric boundary layer (MABL) and for evaluation of the spray impact on the exchange fluxes between the atmosphere and the ocean. While moving through the MABL the droplets pass through the region of high gradients of air velocity, temperature and humidity occurring in the vicinity of the air–water interface. In this case, the applicability of LSMs constructed under the assumption of weakly inhomogeneous flows is questionable. In this work, we develop a Lagrangian stochastic model taking into account the strongly inhomogeneous structure of the airflow in MABL and, in particular, the anisotropy of turbulence dissipation rate. The model constants and the diffusion matrix coefficients are calibrated by comparison of the LSM prediction for the profiles of droplet concentration and the exchange fluxes of sensible and latent heat against the results of direct numerical simulation of turbulent, droplet-laden airflow over a waved water surface. PubDate: 2023-07-01

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Abstract: The pioneering work of Raupach et al. (Boundary-Layer Meteorol 78:351–382, 1996) proposed that the character of turbulent flow in a homogeneous plant canopy is analogous to that of a planar mixing-layer. This proposal was based on qualitative arguments, as well as scaling arguments including the observation that the stream-wise integral turbulent length scales linearly with the shear length scale \(L_s\) . These results have been confirmed for numerous additional canopy flow cases. However, many studies have reported that as canopies become “sparse” (i.e., low vegetation density), the flow transitions to that of a rough wall boundary-layer, and linear mixing-layer scaling apparently breaks down. The work presented here re-examines the original formulation of the shear length scale, particularly the fact that the original formulation did not account for the non-negligible lower canopy velocity characteristic of sparse canopies. A new formulation is suggested and evaluated that accounts for this velocity, which appeared to recover mixing-layer scaling for a wide range of “sparse”, homogeneous canopies. Results suggested that, even in very sparse canopies without a clear inflection in the velocity profile, mixing-layer scaling appears to hold provided that the majority of the shear stress is due to the canopy and not the ground/wall below. PubDate: 2023-07-01

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Abstract: In this work, wakes of wind farms are investigated using large-eddy simulation with an actuator disk model for the wind turbine. The effects of streamwise turbine spacings, number of wind turbine rows and roughness lengths of ground surface on the characteristics of wind farm wakes are examined. The simulation results showed that the effects of \(S_x\) (streamwise turbine spacings) are mainly located in the near wake of wind farm (less than 20 rotor diameters downstream from the last row of the wind farm), where the turbulence intensity is higher for smaller values of \(S_x\) . In the far wake of wind farms (more than 90 rotor diameters downstream from the last row of the wind farm), the streamwise velocity deficit as well as the Reynolds stresses from cases with different streamwise turbine spacings are close to each other. For cases with more wind turbine rows ( \(N_{row}\) ) and larger roughness length of ground surface ( \(k_0\) ), faster velocity recovery and higher turbulence intensity are observed. Terms in the budget equation for mean kinetic energy (MKE) are examined. The analyses showed that the vertical MKE transport via mean convection and turbulence convection plays a dominant role in the velocity recovery in wind farm wakes, being different from the wind farm region where streamwise MKE flux due to mean convection also plays a role. Lastly, an analytical model for the velocity deficit in wind farm wake is proposed based on the Emeis model. Improvements on the model predictions are observed for all the simulated cases. The velocity deficit at one downstream location of the wind farm needs to be given is one major limitation of the analytical model of this type. PubDate: 2023-06-02

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Abstract: Large-eddy simulation is a formidable method for predicting winds around complex terrain, but predictions can be highly dependent on the lateral boundary conditions used in the computations. When periodic boundary conditions are not an option because of terrain complexity, inflow and outflow boundary conditions must be adopted. A common practice in micro-scale wind simulations with incompressible flow solvers is to orient the terrain such that the incoming wind is always orthogonal to the inflow face and impose constant pressure outlet boundary conditions on a flat terrain far away from the region of interest. However, terrain reorientation becomes computationally expensive to ingest meandering winds as inflow. In the present work, we demonstrate shortcomings of this existing practice when oblique inflow angle is imposed at the inlet faces. To address these shortcomings, we pursue a Neumann-type pressure boundary condition at the outflow boundaries with a global mass conservation correction step on the momentum field. Additionally, we revise the so-called box perturbation method to generate evenly distributed turbulence at inlet faces with oblique inflow direction. We use the canonical channel flow, the Perdigão terrain, and the Askervein hill examples to demonstrate the effectiveness of our proposed fixes. The major benefits of our proposed approach are savings in computational cost due to the ability to use a smaller simulation domain and elimination of laborious terrain reorientation and tapering, and mesh generation steps for every new wind direction. We expect our approach to be beneficial, particularly, for model-chain approaches for arbitrarily complex terrain simulations. PubDate: 2023-06-01

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Abstract: A new model is proposed for the so-called scalar footprint and flux footprint in the atmospheric boundary layer. The underlying semi-analytical model allows computing the scalar concentration and flux fields related to turbulent diffusion of heat, water-vapor or to the dispersion of any scalar (e.g. passive pollutant) in the framework of K-theory. It offers improved capabilities regarding the representation of the gradual stratification in the boundary layer. In this model, the boundary layer is split in a series of sublayers in which the aerodynamic inertivity (a compound parameter aggregating wind-speed and eddy-diffusivity) is approximated by a sum of two power-law functions of a new vertical scale corresponding to the height-dependent downwind extension of the plume. This multilayer approach allows fitting with vanishing error any boundary-layer stratification, in particular those described by the Monin–Obukhov similarity theory (MOST) in the surface layer, while keeping the computation time of the footprint to low values. As a complement, a fully analytical surrogate model is presented for practical applications. For MOST profiles, the flux (resp. concentration) footprint is, to a RMS difference less than 1% (resp. 1.2%), equal (resp. equal to a constant multiplicative factor) to the inverse Gamma distribution. The optimal parameters of this distribution were evaluated for a broad range of atmospheric conditions and height. Regression formulas were also provided to compute the crosswind-integrated flux footprint distribution easily and with less than 1.6% RMS residual error. A comparison with the well-known footprint approximate model by Kormann and Meixner and the one by Hsieh, Katul and Chi has allowed quantifying their performances and limitations. PubDate: 2023-06-01

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Abstract: The spatial structures of turbulent flow in the atmospheric boundary layer (ABL) are complex and diverse. Multi-point spatial correlation measurements can help improve our understanding of these structures and their statistics. In this context, we investigate Taylor’s hypothesis and the statistics of spatial structures on the microscale. For the first time, simultaneous horizontally distributed wind measurements with a fleet of 20 quadrotor UAS (unmanned aerial systems) are realized. The measurements were taken at different heights and under different atmospheric conditions at the boundary layer field site in Falkenberg of the German Meteorological Service (DWD). A horizontal flight pattern has been specifically developed, consisting of measurements distributed along and lateral to the mean flow direction with separation distances of \(5\ldots 205\) m. The validity of Taylor’s hypothesis is studied by examining the cross-correlations of longitudinally distributed UAS and comparing them with the autocorrelations of single UAS. To assess the similarity of flow structures on different scales, the lateral and longitudinal coherence of the streamwise velocity component is examined. Two modeling approaches for the decay of coherence are compared. The experimental results are in good agreement with the model approaches for neutral atmospheric conditions, whereas in stable and convective ABL, the exponential approaches are not unconditionally valid. The validation results and the agreement with the literature on coherence in the ABL underline the potential of the UAS fleet for the purpose of spatial turbulence measurements. PubDate: 2023-06-01

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Abstract: We present a flexible solution for the representation of forest edges in windtunnel experiments that can be easily adapted to match observed wind statistics from fullscale field experiments. The solution is based on an incremental approach in which layers of coarse mesh are wrapped over a matrix of cylinders. The mesh layers increase the drag of the model canopy, while they also realistically simulate the strong damping of turbulent fluctuations inside the canopy. In addition to adding layers of mesh, the original 90 degree angle of the forest edge is tapered such that it more closely resembles the profile of the forest edge from a field experiment. The high vertical resolution of the wind-tunnel observations for the calibrated forest model shows both more detail and confirms previous findings from the field experiment. For example, in the region just above the canopy, where the slower wind flow inside the canopy mixes with the faster flow aloft, the streamwise skewness shows a strong zigzag pattern as a function of height and an increase in the streamwise velocity standard deviation. PubDate: 2023-06-01

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Abstract: Modelling the flow and mass transport in canopies requires knowledge of how roughness heterogeneity impacts them. We present wind-tunnel experiments that enable quantifying the (time-space) average flow statistics of a two-height model canopy. We use two-dimensional particle image velocimetry measurements of the flow in multiple planes inside the canopy layer (CL), the roughness sublayer, and above it. The canopy comprises thin metal plates of two heights, h and h/2, which form a specific heterogeneity that can be viewed as two horizontal layers, “top” and “bottom”. The overall frontal index ( \(\lambda =0.5\) ) resides in a less-studied dynamical zone between “dense” ( \(> 0.5\) ) and “sparse” ( \(0.1<\lambda < 0.5\) ). The effects of the canopy’s heterogeneity are highlighted by comparing the flow statistics, with published results from dense and sparse uniform canopies made of similar element shapes. The plane mixing layer analogy was reflected in the first-, second-, and third-order moments of the velocity fluctuations, as well as in the quadrant analysis of the Reynolds shear stress, with few reservations: (i) the shear length scale at canopy height, which quantifies the inverse of the mean shear, is relatively smaller than would have been expected from the momentum penetration depth \(h-d\) ; (ii) An additional inflection point, obeying the inviscid Fjørtoft’s criterion, is observed in the average streamwise velocity at the bottom-layer height (h/2), where a local increase in sweep/ejection events of the Reynolds shear stress are prominent; (iii) unlike typical dense canopies, the estimated contribution from wake to the production of TKE near the top CL is smaller relative to the contributions from shear; (iv) the dispersive fluxes of the shear stress and kinetic energy differ from that of uniform canopies that are either denser or sparser. These observations provide new insights into the possible impact that vertical roughness heterogeneity has on the flow statistics in plant and urban canopies. PubDate: 2023-06-01

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Abstract: Large-eddy simulations (LESs) of inversion-capped neutral atmospheric boundary layers (ABLs) are augmented to earlier small-domain LESs of a sparse forest and field observation to evaluate the budgets of all non-negligible resolved-scale Reynolds stress components. The focus is on the atmospheric surface layer comprised of the roughness sublayer (RSL) in and above horizontally homogeneous forests and the inertial sublayer (ISL) above the RSL over flat terrain. The greater LES domain and ABL depths result in greater depths of both the RSL and the ISL. A key result is that in the upper portions of the canopy and above, pressure redistribution is a major sink of normal stress in the horizontal direction with mean shear production as a major source, whereas in the horizontal direction absent of mean shear production and in the vertical direction, pressure redistributions are major sources of normal stresses. In the lower portions of the canopy where mean shear production and turbulent transport are much reduced, pressure redistributions are major sources of horizontal velocity variances but a major sink of vertical velocity variance. Pressure transport is a greater source of vertical velocity variance than turbulent transport from the ground level to just under the treetops where it transitions to a major sink up to about 1.5 times the canopy height. This greater significance of pressure transport over turbulent transport increases with increased vegetation area index (VAI). The impact of increased value of geostrophic wind speed is negligible compared to that of increased value of VAI on enhancing normalized budget terms in the vicinity of treetops. PubDate: 2023-06-01

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Abstract: Friction velocity is one of the important scaling parameters in atmospheric boundary layer studies. However, several definitions of friction velocity exist in the literature: e.g. estimated from the total drag force or used only pressure drag, etc. In this study, a series of large-eddy simulation (LES) calculations were carried out to evaluate the impact of various definitions on the friction velocity for idealized urban geometries, i.e. staggered array of cubes with different packing densities and wind directions. We first compared the normalized velocity fields with the literature data for the case with a packing density of \(25\%\) . The results show that the LES data normalized by the friction velocity derived from the Reynolds stress using the extrinsic spatial average is more consistent with the direct numerical simulation data. Furthermore, when varying the wind direction, the distribution of Reynolds stress and pressure drag show significant change in streamwise and spanwise directions. We further found that for packing density of \(44.44\%\) , the frictional drag accounts for more than a quarter of the total drag, and even higher than the pressure drag in parallel wind direction. This leads to the deviation of friction velocity estimated from the pressure drag and that calculated from the total drag force up to 33%. Such characteristic of viscous effect challenges the assumption widely used in wind tunnel experiments and urban canopy parameterizations that the contribution of viscous force is negligible, especially for ultra-dense arrays. PubDate: 2023-06-01

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Abstract: We test the hypothesis that internal waves observed in flow over forest canopies are generated by Kelvin–Helmholtz instability. The waves were observed at night, under stably stratified and weak wind conditions, with a horizontally scanning aerosol lidar and an instrumented tower. The lidar images are used to determine the salient wavelength and phase propagation velocity of each episode. Time series data measured at the tower are then used to form vertical profiles of background velocity and buoyancy just before each observed wave event. The profiles are input to the Taylor–Goldstein equation to predict the phase velocity, wavelength and period of the fastest-growing linear instability, and the results compared with the lidar observations. The observed wavelengths tend to be longer than predicted by the Taylor–Goldstein theory, typically by a factor of two. That discrepancy is removed when the theory is extended to account for the effects of ambient, small-scale turbulence. PubDate: 2023-06-01

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Abstract: Dust devils are organized convective vortices with pressure drops of hundreds of pascals that spirally lift surface material into the air. This material modifies the radiation budget by contributing to the atmospheric aerosol concentration. Quantification of this contribution requires good knowledge of the dust devil statistics and dynamics. The latter can also help to understand vortex genesis, evolution and decay, in general. Dust devil-like vortices are numerically investigated mainly by large-eddy simulation (LES). A critical parameter in these simulations is the grid spacing, which has a great influence on the dust devil statistics. So far, it is unknown which grid size is sufficient to capture dust devils accurately. We investigate the convergence of simulated convective vertical vortices that resemble dust devils by using the LES model PALM. We use the nesting capabilities of PALM to explore grid spacings from 10 to 0.625 m. Grid spacings of 1 m or less have never been used for the analysis of dust devil-like vortices that develop in a horizontal domain of more than 10 km \(^2\) . Our results demonstrate that a minimum resolution of 1.25 m is necessary to achieve a convergence for sample-averaged quantities like the core pressure drop. This grid spacing or smaller should be used for future quantifications of dust devil sediment fluxes. However, sample maxima of the investigated dust devil population and peak velocity values of the general flow show no convergence. If a qualitative description of the dust devil flow pattern is sufficient, we recommend a grid spacing of 2.5 m or smaller. PubDate: 2023-06-01

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Abstract: Wind energy potential in complex terrain is still poorly understood and difficult to quantify. With Switzerland’s current efforts to shift to renewable energy resources, it is now becoming even more crucial to investigate the hidden potential of wind energy. However, the country’s topography makes the assessment very challenging. We present two measurement campaigns at Lukmanier and Les Diablerets, as representative areas of the complex terrain of the Swiss Alps. A general understanding of local wind flow characteristics is achieved by comparing wind speed measurements from a near-surface ultra-sonic anemometer and from light detection and ranging (LiDAR) measurements further aloft. The measurements show how the terrain modifies synoptic wind for example through katabatic flows and effects of local topography. We use an artificial neural network (ANN) to combine the data from the measurement campaign with wind speed measured by weather stations in the surrounding area of the study sites. The ANN approach is validated against a set of LiDAR measurements which were not used for model calibration and also against wind speed measurements from a 25-meter mast, previously installed at Lukmanier. The statistics of the ANN output obtained from multi-year time series of nearby weather stations match accurately the ones of the mast data. However, for the rather short validation periods from the LiDAR, the ANN has difficulties in predicting lowest wind speeds at both sites, and highest wind speeds at Les Diablerets. PubDate: 2023-05-11