An eddy covariance (EC) flux system was integrated onto a Navy Oceanographic Meteorological Automatic Device (NOMAD) floating lidar buoy fitted for measuring the offshore wind resource. The EC flux system consisted of two sonic anemometers and an inertial measurement unit that together provided high-frequency (10 Hz) motion-corrected wind measurements. To assess possible impacts of flow distortion, EC momentum fluxes (10 min) were calculated in gravity-aligned, natural, and planar fit (PF) coordinate frames. The buoy was anchored near the Martha’s Vineyard Coastal Observatory Air-Sea Interaction Tower (ASIT; near-shore) for validation of the buoy-based EC fluxes against the fixed tower measurements before deployment within the Third Wind Forecast Improvement Project (WFIP3; offshore) zone. Drag coefficients from the direct turbulent EC flux measurements were compared to the National Oceanic and Atmospheric Administration Coupled Ocean-Atmosphere Response Experiment (NOAA COARE) bulk air-sea flux algorithm, which was run with measurements of mean (10 min) air- and ocean-side bulk variables. At both the ASIT and WFIP3 deployment locations, COARE’s wave-dependent surface roughness parameterization gave drag coefficient estimates in closer agreement with the EC-based drag than its wind-dependent roughness formulation did. There was less agreement between the EC and COARE roughness at low wind speeds, where air-sea momentum transfer and motion correction are more nuanced. The range of wind speeds with the largest differences between the EC-measured and COARE-parameterized momentum flux overlaps with the range at which turbine power curves are most sensitive to wind speed. During unstable conditions, Monin-Obukhov extrapolations of the wind profile using the PF and COARE wave-based friction velocity match the shear, shape, and magnitude of lidar observations better than when using the COARE wind-based friction velocity.
David Marcial (Wed,) studied this question.