Inertial-Dissipation Air-Sea Flux Measurements: A Prototype System Using Realtime Spectral Computations

Abstract

A prototype system for the measurement and computation of air?sea fluxes in realtime was tested in the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. The system used a sonic anemometer/thermometer for wind speed, surface stress and sensible heat flux measurements and a Lyman-α fast hygrometer for latent beat flux. A small desktop computer combining both fast analog to digital (A/D) capabilities, external bus (IEEE-488) operation of a slow voltmeter/scanner unit, and a plug-in board for computation of turbulence spectra by Fast Fourier Transform was used for acquisition of 17 channels of data. At the end of a ten-minute averaging period, air?sea fluxes were computed from the velocity, temperature, and humidity variance spectra using the inertial-dissipation method. A second computer and data acquisition system was used for simultaneous computations of covariance fluxes for comparison. The sonic anemometer/thermometer proved to be well suited for this application: the velocity data appear to be of good quality and the temperature data wore unaffected by salt contamination. We suggest an infrared hygrometer as a replacement for the Lyman-α. For the six week HEXMAX period the inertial-dissipation flux estimates agreed with covariances computed from the same instruments with a typical average root-mean-square difference of ± 10% for stress and ± 25% for sensible and latent heat.