Mass transfer velocity across the breaking air–water interface at extremely high wind speeds

Abstract

Mass transfer velocity k L across the wind-driven air–water interface was estimated at extremely high wind speeds (up to U 10=70 m s−1) in a high-speed wind-wave tank by measuring changes in CO2 concentration in the water. In addition, the volume flux of dispersing droplets lost from the tank and the wave height were measured. k L increases drastically with wind speed at extremely high wind speeds. The volume flux of dispersing droplets begins to increase drastically and the mean height of significant waves changes its rate of increase at almost the same wind speed as that at which the rate of increase of k L changed. These results suggest that intense wave breaking occurs at extremely high wind speeds and it has significant effects on mass transfer. k L is well correlated with the free-stream wind speed for both present laboratory and previous field measurements in the low and moderate wind speed regions. Present k L agrees well with the conventional correlation curves proposed by Wanninkhof (1992), Wanninkhof and McGillis (1999) and Wanninkhof et al. (2009) for low and moderate free-stream wind speeds. However, for extremely high free-stream wind speeds, the present data deviate upward from the correlation curves of Wanninkhof (1992) and Wanninkhof and McGillis (1999) and approach to that of Wanninkhof et al. (2009) as the wind speed increases. This indicates that the correlation curve of Wanninkhof et al. (2009) is more appropriate for the correlation between k L and free-stream wind speed than those of Wanninkhof (1992) and Wanninkhof and McGillis (1999) in extremely high wind speed region.