Due to their motion, floating wind lidars overestimate turbulence intensity ( T I ) compared to fixed lidars. We show how the motion of a floating continuous-wave velocity–azimuth display (VAD) scanning lidar in all six degrees of freedom influences the T I estimates, and present a method to compensate for it. The approach presented here uses line-of-sight measurements of the lidar and high-frequency motion data. The compensation algorithm takes into account the changing radial velocity, scanning geometry, and measurement height of the lidar beam as the lidar moves and rotates. It also incorporates a strategy to synchronize lidar and motion data. We test this method with measurement data from a ZX300 mounted on a Fugro SEAWATCH Wind LiDAR Buoy deployed offshore and compare its T I estimates with and without motion compensation to measurements taken by a fixed land-based reference wind lidar of the same type located nearby. Results show that the T I values of the floating lidar without motion compensation are around 50 % higher than the reference values. The motion compensation algorithm detects the amount of motion-induced T I and removes it from the measurement data successfully. Motion compensation leads to good agreement between the T I estimates of floating and fixed lidar under all investigated wind conditions and sea states.
In the development, design and operation of wind farms the knowledge of wind turbulence intensity is of paramount importance, particularly at hub height. Given that measurements are rare, turbulence intensities are often determined using simplified formulations. These formulations only account for a dependence on wind speed and include a neutral stability assumption. There are, therefore, large uncertainties associated with the results of such formulations. In order to quantify these uncertainties we determine the dependence of turbulence intensity at different heights on the surface wind velocity, wave conditions and vertical temperature gradients from offshore LiDAR wind observations in the North Sea. The turbulence intensity is shown to depend strongly on the atmospheric stability and less strongly on the sea surface roughness. The lower turbulence intensity values are observed under stable atmospheric conditions. The dependence of the turbulence intensity on the surface roughness is higher at the lower levels, with the significant wave height being the sea surface roughness parameters with the stronger correlation with the turbulence intensity.
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