DIRECT COVARIANCE FLUX ESTIMATES FROM MOBILE PLATFORMS AT SEA

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