Microscale wave breaking and air-water gas transfer

Laboratory results showing that the air-water gas transfer velocity k is correlated with mean square wave slope have been cited as evidence that a wave-related mechanism regulates k at low to moderate wind speeds [Jahne et al,, 1987; Beck et al., 1999]. Csanady [1990] has modeled the effect of microscale wave breaking on air-water gas transfer with the result that k is proportional to the fractional surface area covered by surface renewal generated during the breaking process. In this report we investigate the role of microscale wave breaking in gas transfer by determining the correlation between k and Ag, the fractional area coverage of microscale breaking waves. Simultaneous, colocated infrared (IR) and wave slope imagery is used to verify that Ag detected using IR techniques corresponds to the fraction of surface area covered by surface renewal in the wakes of microscale breaking waves. Using measurements of k and Ag made at the University of Washington wind-wave tank at wind speeds from 4.6 to 10.7 m s(-1), we show that k is linearly correlated with Ag, regardless of the presence of surfactants. This result is consistent with Csanady's [1990] model and implies that microscale wave breaking is likely a fundamental physical mechanism contributing to gas transfer.