Analysis of wind profiles above the water surface in wind-wave interaction thanks to a scanning wind LiDAR

Abstract

With the aim of studying the momentum flux in wind-wave modulation situations, a 7 month field experiment was set-up in 2023 from the Belle-Ile-en-Mer island off the West coast of France. The site was selected for its exposure to dominant wind and swell, the proximity of a wave buoy, and the rapidly increasing water depth to allow a focus on deep to intermediate wave dispersion regimes. A scanning wind LiDAR [1,2] installed on the coast of the island was used to measure the vertical profile of the horizontal wind speed and direction from 1 to 3 kilometers from the coast. The configuration allowed for measurements of the wind speed and direction profiles starting at some meter above the water surface and going up to some 150m. This original approach enables to obtain quasi-instantaneous vertical planes of the wind speed as well as 30-min mean profiles simultaneously with a wave parameter. The wide range of wind and wave combinations observed during the deployment allows statistical analysis. Significant wave heights, wave peak periods, and U10 wind speeds were observed in the range 0-6.5 m, 2-20 s, and 0.5-18 m/s, respectively. From this rich database, the near-surface momentum flux estimated by the wind profile close to the water surface appears to match well with results from COARE 3.5 algorithm. The possibility of the scanning wind LiDAR to measure mean wind profiles allows an original point of view to analyze wind-wave interactions. It was observed that for young seas, the profile can be in equilibrium, following Monin-Obukov similarity theory from close to the water surface up to some 100m. In contrast, for fast-travelling waves, significant deviations of the wind profile are observed compared to the surface fluxes. These deviations are parametrized as function of height and analyzed as function of the wave age. [1] Paskin, L., Conan, B., Perignon, Y., & Aubrun, S. (2022). Evidence of Ocean Waves Signature in the Space–Time Turbulent Spectra of the Lower Marine Atmosphere Measured by a Scanning LiDAR. Remote Sensing, 14(13), 3007. [2] Conan, B., & Visich, A. (2023). Measurement and analysis of high altitude wind profiles over the sea in a coastal zone using a scanning wind LiDAR–application to wind energy. Wind Energy Science Discussions, 2023, 1-23.