Air-sea exchange of momentum, heat and water vapor over whitecap sea states

To understand air-sea energy and mass transfer over whitecap sea states, a detailed analysis of measurements of heat, water vapor and momentum fluxes in high wind conditions was made. These data were collected on the North Sea as part of the HEXOS (Humidity Exchange Over the Sea) Main Experiment, HEXMAX. Several modelling studies had predicted an enhancement of the air-sea heat and water vapor fluxes due to evaporation of sea spray, which would increase with increasing winds. Contrary to these predictions, the HEXMAX data reveal no increase in the sensible heat and water vapor transfer coefficients with increasing wind speed. The results indicate that any increase in water vapor flux due to evaporation of spray may be offset by a reduction in surface evaporation due to a decrease in water vapor density gradient.;The HEXMAX sea surface drag coefficients increase with increasing wind speed for the range 5-27 m/s in good agreement with measurements at similar water depth, but are higher than those derived from open ocean data. Some of the variability in the drag coefficient for a given wind speed, and the difference between measured drag coefficients at this shallow water site (15 m depth) and those derived at deep water sites can be explained by an additional dependence of the drag coefficient on wave age. The variation in drag coefficient across three surface frontal systems was also examined. No clear pattern emerged from this study.;Some of the variability in the water vapor exchange coefficient for a given wind speed can be explained by a dependence on the drag coefficient. These results indicate that the water vapor flux may also vary with sea state. The water vapor roughness length, an indicator of the efficiency of water vapor transfer, decreases with increasing wind speed, implying that this transfer efficiency also decreases in high winds. No similar dependence of the sensible heat flux exchange coefficient on drag coefficient was found for these data, which were dominated by very small sensible heat fluxes associated with near neutral density stratification.