Taking the motion out of floating lidar: Turbulence intensity estimates with a continuous-wave wind lidar

Abstract

Due to their motion, floating wind lidars overestimate turbulence intensity (TI) 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 TI 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 TI 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 TI 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 TI and removes it from the measurement data successfully. Motion compensation leads to good agreement between the TI estimates of floating and fixed lidar under all investigated wind conditions and sea states.