Direct Covariance Flux Estimates from Mobile Platforms at Sea*

TLDR

This paper describes two methods for computing direct covariance fluxes from anemometers mounted on moving platforms at sea. The methods use either a strapped‑down or gyro‑stabilized system to correct for instantaneous tilt, angular velocities, and translational motion, and the paper compares fluxes from three strapped‑down systems in two field experiments. Comparisons show that direct covariance fluxes agree with bulk aerodynamic estimates, that flow distortion increases momentum flux by 15 %, and that applying motion and distortion corrections yields flux estimates with 10–20 % uncertainty.

Abstract

This paper describes two methods for computing direct covariance fluxes from anemometers mounted on moving platforms at sea. These methods involve the use of either a strapped-down or gyro-stabilized system that are used to compute terms that correct for the 1) instantaneous tilt of the anemometer due to the pitch, roll, and heading variations of the platform; 2) angular velocities at the anemometer due to rotation of the platform about its local coordinate system axes; and 3) translational velocities of the platform with respect to a fixed frame of reference. The paper provides a comparison of fluxes computed with three strapped-down systems from two recent field experiments. These comparisons shows that the direct covariance fluxes are in good agreement with fluxes derived using the bulk aerodynamic method. Additional comparisons between the ship system and the research platform FLIP indicate that flow distortion systematically increases the momentum flux by 15%. Evidence suggests that this correction is appropriate for a commonly used class of research vessels. The application of corrections for both motion contamination and flow distortion results in direct covariance flux estimates with an uncertainty of approximately 10%–20%.