A Revised Motion and Tilt Correction for Direct Air-Sea Flux Measurements

Ship based measurements of wind speed and direct fluxes are affected by air flow distortion that can lead to a tilt of the wind vector as well as acceleration or deceleration of the wind speed. Direct flux measurements are additionally affected by the angular velocity of the platform. The classic approach is to first correct the wind speed for angular and translational platform velocities and thereafter rotate the wind vector into the mean flow. We find that for moving ships, this leads to an overestimation of the vector tilt and biased flux estimates. This may explain the common observation that flux estimates from moving ships have lower quality than measurements taken on station. Here we present an alternative approach where the flow distortion-induced tilt of the wind vector is estimated from the 3D-wind speed measurements and applied to the apparent wind vector. The tilt correction is carried out after correction for the platform angular velocity, but before removing the ship's mean translational velocity. We show that this new method significantly improves the momentum and scalar flux measurements made from a moving ship. We suggest that reanalysis of previous direct flux measurements with the proposed tilt-motion correction will improve our understanding of air-sea interaction.