Microbreaking and the enhancement of air-water transfer velocity

[1] The role of microscale wave breaking in controlling the air-water transfer of heat and gas is investigated in a laboratory wind-wave tank. The local heat transfer velocity, k(H), is measured using an active infrared technique and the tank-averaged gas transfer velocity, k(G), is measured using conservative mass balances. Simultaneous, colocated infrared and wave slope imagery show that wave-related areas of thermal boundary layer disruption and renewal are the turbulent wakes of microscale breaking waves, or microbreakers. The fractional area coverage of microbreakers, A(B), is found to be 0.1-0.4 in the wind speed range 4.2-9.3 m s(-1) for cleaned and surfactant-influenced surfaces, and k(H) and k(G) are correlated with A(B). The correlation of k(H) with A(B) is independent of fetch and the presence of surfactants, while that for k(G) with A(B) depends on surfactants. Additionally, A(B) is correlated with the mean square wave slope, [S-2], which has shown promise as a correlate for k(G) in previous studies. The ratio of k(H) measured inside and outside the microbreaker wakes is 3.4, demonstrating that at these wind speeds, up to 75% of the transfer is the direct result of microbreaking. These results provide quantitative evidence that microbreaking is the dominant mechanism contributing to air-water heat and gas transfer at low to moderate wind speeds.