A study of the inertial‐dissipation method for computing air‐sea fluxes

Abstract

The inertial‐dissipation method has long been used to estimate air‐sea fluxes from ships because it does not require correction for ship motion. A detailed comparison of the inertial‐dissipation fluxes with the direct covariance method is given, using data from the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. In this experiment, inertial‐dissipation packages were deployed at the end of a 17 m boom, in a region relatively free of flow distortion; and on a mast 7 m above the platform (26 m above the sea surface) in a region of considerable flow distortion. An error analysis of the inertial‐dissipation method indicates that stress is most accurately measured in near‐neutral conditions, whereas scalar fluxes are most accurately measured in near‐neutral and unstable conditions. It is also shown that the inertial‐dissipation stress estimates are much less affected by the flow distortion caused by the platform as well as by the boom itself. The inertial‐dissipation (boom and mast) and boom covariance estimates of stress agree within ±20%. The latent heat flux estimates agree within approximately ±45%. The sensible heat flux estimates agree within ±26% after correction for velocity contamination of the sonic temperature spectra. The larger uncertainty in the latent heat fluxes is due to poor performance of our Lyman‐α hygrometers in the sea spray environment. Improved parameterizations for the stability dependence of the dimensionless humidity and temperature structure functions are given. These functions are used to find a best fit for effective Kolmogorov constants of 0.55 for velocity (assuming a balance of production and dissipation of turbulent kinetic energy) and 0.79 for temperature and humidity. A Kolmogorov constant of 0.51 implies a production‐dissipation imbalance of approximately 12% in unstable conditions.