Oceanic application of the active controlled flux technique for measuring air-sea transfer velocities of heat and gases

Detailed understanding of the hydrodynamic mechanisms controlling the air-sea exchange of heat and gas requires a method for rapid measurement of the associated transfer velocities. The active controlled flux technique ( ACFT), where the temperature decay of a small patch of water heated by an infrared laser is tracked using an infrared imager, has been proposed as a method for making these fast noninvasive measurements of the heat and gas transfer velocities. Here, we report on ACFT measurements of the transfer velocity of heat, k(H), made in the ocean during the Fluxes, Air-sea Interactions and Remote Sensing ( FAIRS) experiment (September/October, 2000) and GasEx-01 (January/February, 2001). The results for kH from both FAIRS and GasEx-01 compare favorably when plotted versus wind speed. However, when scaled to a Schmidt number of 660, the measured kH values were found to be a factor of two larger than gas transfer velocities measured during GasEx-01. The ACFT-derived k(H) values were combined with direct measurements of the bulk-skin oceanic temperature difference to calculate net air-sea heat fluxes during both experiments. Comparison of these values with heat fluxes determined by direct measurements of the latent, sensible, and radiative heat fluxes showed that the ACFT measurements are a factor of seven larger than the direct measurements. One possible theory explaining both the overprediction of the gas transfer velocities and the scale factor between the measured and calculated net heat fluxes is that air-sea exchange is best described by surface penetration rather than surface renewal.