Effect of Microscale Wave Breaking on Air-Water Gas Transfer

Abstract

Measurements of simultaneous and co-located infrared and wave slope imagery of laboratory wind waves show that the wave-related areas of thermal boundary layer disruption and renewal are the turbulent wakes of microscale breaking waves or microbreakers. These signatures of disruption are associated with waves that have a steep forward face and a dimpled crest, and can be quantified by infrared imaging techniques. The fractional area coverage, A B , of the surface affected by these renewal features is significant (0.25 - 0.40) and found to be linearly correlated with the transfer velocity, k. Furthermore, this correlation is insensitive to the presence of surfactants and independent of fetch. Using the controlled flux technique (CFT) to measure k locally, the renewal within the wakes of microscale breaking waves was found to enhance the transfer by a factor of 3.5 on average compared to that outside the wakes. Moreover, up to 75% of the transfer across the air-water interface under moderate wind speeds is the direct result of microbreaking. The roughness features associated with microscale breaking waves are shown to contribute significantly to the mean square slope, , and may explain the observed correlation between k and . The correlation between k and A B regardless of surfactant concentration, combined with the enhanced local k and wave slope results, provides quantitative laboratory evidence that microbreaking is an important physical process contributing to gas transfer at low to moderate wind speeds.