The wind energy farming in the offshore environment is characterized by ever-increasing structures and costs, for which reducing structural damage and maximizing production become an imperative. Such challenge is faced by site planning, intelligent design and active control systems that ultimately require a fine Atmospheric Boundary Layer (ABL) description. Contributing to refine the wind flow description in the offshore environment for engineering purposes, this work considers: (i) The interaction between turbulent and Wave Induced fluctuations, pronounced in the lower portion of the Marine ABL (MABL): Region so-called Wave Boundary Layer (WBL); (ii) The impact of the WBL in the flow above it. Focusing in the MABL sustained by non-equilibrium old-seas in neutral atmospheric conditions, the free-surface position and velocities are here prescribed into a Large Eddy Simulation (LES) according to a fifth order Stokes solution. The swell disturbances on the WBL are explored through mean profiles and spectral analyses. An original definition of the Wave Induced flow is presented, considering correlated turbulent and Wave Induced motions thus accessing the coupled dynamics between those fields and allowing the evaluation of the WBL height. Employing the proposed decomposition, the turbulent flow characteristics are recovered as expected in a flat bottom ABL, though some of its scales change considerably, forced by the WBL existent below.

To achieve more accurate weather and climate forecasting, and propose efficient engineering solutions for exploiting offshore renewable energies, it is imperative to accurately describe the atmospheric turbulent flow in the offshore environment. The ocean’s dynamics raise specific challenges for the aforementioned applications, as they significantly alter the atmospheric flow through complex wind–wave interactions. These interactions are important in fairly common situations and notably in old-sea conditions, where ocean waves travel fast, under comparatively slow wind velocities. In the present study, a scanning LiDAR (sLiDAR) was deployed on the shore to study micro-scale wind–wave interactions by performing horizontal scans 18 m above the ocean, and as far as 2 km from the coast. In the proposed configuration, and in the test cases presented in old seas, the sLiDAR captures wave-induced disturbances propagating into the lower part of the marine atmospheric boundary layer. Based on measurements of high-resolution space–time maps of the Radial Wind Speed, an original two-dimensional spectral analysis of the space–time auto-correlation functions was performed. Unlike more conventional data-processing techniques, and as long as the waves travel sufficiently (∼twofold) faster than the mean wind at the measurement height, the upward transfer of motions from the waves to the wind can be clearly distinguished from the atmospheric turbulence in the wave-number–angular-frequency (k–w) turbulent spectra. These are the first space–time auto-correlation functions of the wind velocity fluctuations obtained at micro-scales above the ocean. The analyses demonstrate sLiDAR systems’ applicability in measuring k–w-dependent turbulent spectra in the coastal environment. The findings present new perspectives for the study of micro-scale wind–wave interactions.

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