Résumé de la publication

Keywords: wind resource, extreme events, near-offshore region, scanning LiDAR,

The near-offshore region, which extends from the coast up to 100 km offshore, produces complex local atmospheric flow phenomena, such as boundary-layer transitions, coastal low-level jets and sea breeze, causing a wide range of wind conditions and a significant deviation from the classical description of well-mixed atmospheric boundary layer conditions (e.g. Monin-Obukhov Similarity Theory, power law). For the development of offshore wind energy, a fine understanding of wind conditions in the near-offshore region is crucial (Archer et al, 2014), yet observation of the marine atmospheric boundary layer is still very limited compared to onshore conditions. Since several
years, it mostly relies on floating LiDAR measurements. For the wind industry, extreme wind shear events and low-level jets are of special interest as they directly influence the wind power extraction, increase the loads and, finally, can significantly affect lifetime of the turbines (Debnath et al, 2020, Moller et al 2020).
The present study focuses on the Northeast Atlantic coastal region of France (Southern Brittany), who received the first French offshore wind turbine (https://sem-rev.ec-nantes.fr/), and where a total of 1-GW fixed-bottom offshore wind parks are being installed and several floating pilot and commercial wind farm are planned but where very limited observations are publicly available.
During a 9-month field experiment, a scanning LiDAR (Fig. 1) was used to measure the offshore wind resource. The scanning LiDAR was positioned on the shore at Le Croisic (France) and configured to measure a sequence of six PPI (plane position indicator) planes with a 40° azimuth opening above the sea at 6 different elevation angles. The advantage of this configuration compared to floating LiDAR installation is the simplicity of deployment (cost, accessibility) and the absence of LiDAR motion and related questions (compensation, correction etc). In a 10min time period, each of the six elevation angles is scanned six times. The hypothesis of horizontal homogeneity of the wind above the sea surface allows for the reconstruction of the horizontal wind speed and wind
direction at a remote distance from the shore making a remote virtual mast with six heights. Here, data are processed at 1500 m) from 20 m to 350 m. These heights are relevant for wind energy applications. The reconstruction of the horizontal wind speed was done using a cosinus fitting function similar to the approach by Shimada et al 2020. This database is then processed to detect and analyze two particular events of interest for the wind energy sector:
extreme vertical wind shear and the presence of low-level jets. Results show that it is quite common that the vertical wind profile is way outside the classical MOST and that extreme shear values across the rotor are way above classically used norms. These results are important for the further deployment of offshore wind farms.