High-quality tall mast and wind lidar measurements over the North and Baltic Seas are used to validate the wind climatology produced from winds simulated by the Weather, Research and Forecasting (WRF) model in analysis mode. Biases in annual mean wind speed between model and observations at heights around 100 m are smaller than 3.2% at offshore sites, except for those that are affected by the wake of a wind farm or the coastline. These biases are smaller than those obtained by using winds directly from the reanalysis. We study the sensitivity of the WRF-simulated wind climatology to various model setup parameters. The results of the year-long sensitivity simulations show that the long-term mean wind speed simulated by the WRF model offshore in the region studied is quite insensitive to the global reanalysis, the number of vertical levels, and the horizontal resolution of the sea surface temperature used as lower boundary conditions. Also, the strength and form (grid vs spectral) of the nudging is quite irrelevant for the mean wind speed at 100 m. Large sensitivity is found to the choice of boundary layer parametrization, and to the length of the period that is discarded as spin-up to produce a wind climatology. It is found that the spin-up period for the boundary layer winds is likely larger than 12 h over land and could affect the wind climatology for points offshore for quite a distance downstream from the coast.

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The advantages and limitations of the ZephIR®, a continuous-wave, focused light detection and ranging (LiDAR) wind profiler, to observe offshore winds and turbulence characteristics were tested during a 6 month campaign at the transformer/platform of Horns Rev, the world's largest wind farm. The LiDAR system is a ground-based sensing technique which avoids the use of high and costly meteorological masts. Three different inflow conditions were selected to perform LiDAR wind profiling. Comparisons of LiDAR mean wind speeds against cup anemometers from different masts showed high correlations for the open sea sectors and good agreement with their longitudinal turbulence characteristics. Cup anemometer mean wind speed profiles were extended with LiDAR profiles up to 161 m on each inflow sector. The extension resulted in a good profile match for the three surrounding masts. These extended profiles, averaged over all observed stabilities and surface roughness lengths, were compared to the logarithmic profile. The observed deviations were relatively small. Offshore wind farm wakes were also observed from LiDAR measurements where the wind speed deficits were detected at all LiDAR heights. Profile-derived friction velocities and roughness lengths were compared to Charnock's sea roughness model. These average values were found to be close to the model, although the scatter of the individual estimations of sea roughness length was large.where ū is the mean wind speed at height z; u * is the friction velocity; k is the von Karman constant (∼0.4); z o is the aerodynamic roughness length, and y M is the universal stability function. This last term depends on both the height and the Obukhov length L.For wind resource assessments, equation (1) has been used to extrapolate wind speed measurements normally performed at low heights, e.g. 10 m. The vertical wind profi le is determined from equation (1) by estimating u * and L, e.g. from measurements of turbulence fl uxes. The surface roughness length z o can be estimated in relation to the land cover or by using roughness models for the sea state. 2 Equation (1) has been validated from experiments at heights up to 32 m (e.g. the Kansas experiment 3 ) and 50-80 m in Gryning et al. 4 Beyond these levels, deviations have been reported in different boundary layer studies. The height of the boundary layer is introduced in Gryning et al. 4 as a length scale to correct vertical wind profi les measured up to 250 m. The inversion height is estimated in Lange et al. 5 from air density differences to correct offshore wind profi les at Rødsand (Denmark). These attempts to extend vertical profi les are important for the development of the wind energy because the knowledge of the wind resource at high levels in the atmosphere is still immature.Conventional techniques (e.g. cup and sonic anemometers) have been extensively used to observe winds and turbulence. They have reached a limit in the vertical range which is similar to the current turbine's hub height. This is due mainly to the costs of er...

Operational since 2004, the National Centre for Wind Turbines at Høvsøre, Denmark has become a reference research site for wind-power meteorology. In this study, we review the site, its instrumentation, observations, and main research programs. The programs comprise activities on, inter alia, remote sensing, where measurements from lidars have been compared extensively with those from traditional instrumentation on masts. In addition, with regard to wind-power meteorology, wind-resource methodologies for wind climate extrapolation have been evaluated and improved. Further, special attention has been given to research on boundary-layer flow, where parametrizations of the length scale and wind profile have been developed and evaluated. Atmospheric turbulence studies are continuously conducted at Høvsøre, where spectral tensor models have been evaluated and extended to account for atmospheric stability, and experiments using microscale and mesoscale numerical modelling.