Microbreaking and the enhancement of air-water transfer velocity

TitleMicrobreaking and the enhancement of air-water transfer velocity
Publication TypeJournal Article
Year of Publication2004
AuthorsZappa, CJ, Asher, WE, Jessup, AT, Klinke, J, Long, SR
JournalJ. Geophys. Res.

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, , 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.

Citation Keyzappa2004