Measurement and analysis of high altitude wind profiles over the sea in a coastal zone using a scanning doppler LiDAR: Application to offshore wind energy

• A scanning doppler LiDAR are measuring wind profile from 20 m to 450 m with 27 levels using an original approach. • A 7 month database of the 10 min wind profile was acquired at 1.5 km offshore. • Low-level jets are observed with an occurrence of 15. • Low-level jets are observed to contribute to high shear events. • The power production of offshore several wind turbines at the site is analysed. The lack of observations at heights relevant to the wind energy industry constitutes a major challenge for the development of the next generation of offshore wind turbines (over 10 MW), which are expected to operate within the first few tens kilometres from the coast with turbine tips extending more than 250 m. Observations within the coastal zone, which proves to be complex by its nature because of sea breezes, low-level jets and land/sea transition are key both to understanding the marine atmospheric boundary layer processes interacting with the turbine and to parameterizing the wind profile well above the surface layer. These needs introduce difficulties associated with measuring wind properties in the region 150–500 m above the sea surface. This paper uses the virtual mast method to reconstruct the 10-min averaged wind profile 1.5 km offshore using a scanning Doppler LiDAR (Light Detection And Ranging) device installed on the coastline. A novel method is proposed to increase the vertical resolution to 27 levels, from the sea surface to 500 m above sea level. Being mainly based on the homogeneity hypothesis, this method’s sensitivity to wind direction is evaluated, while results are compared with reanalysis data at several heights. This method is then implemented during a 7-month test campaign on the northeastern Atlantic coast, yielding a dataset of wind profile measurements. An analysis of this dataset shows a 15.4 % proportion of low-level jets, mainly originating from land at night, with a core within the rotor area of > 10 MW wind turbines. The power production is subsequently analysed for 10 MW, 15 MW and 22 MW wind turbines. Wind shear events above the design values are observed in 30 % of all profiles, accounting for more than a third of the total power production. High shear events are found to be more probable during low-level jets (56 % of the time), compared to no low-level jet events (26 %). A description of low-level jets and high-shear events is thus key since these are situations where the wind profile differs from the standard values used for wind turbine design, and may affect both the load and fatigue predictions.