Coherent structures beneath wind waves and their influence on air‐water gas transfer

Coherent structures generated beneath laboratory wind waves were investigated using particle image velocimetry. An algorithm was developed to detect these structures and to determine their size, vorticity, and kinetic energy. As the wind speed increased from 4.5 to 11.0 m s−1, the maximum vorticity of the coherent structures increased by 40%, their average size increased by 20%, their frequency of occurrence increased 400%, and the fraction of the water surface renewed by coherent structures increased from 0.12 to 0.33. Distributions of the total kinetic energy of the coherent structures as a function of size showed that the most energetic eddies occurred in the size range 0.8–1.6 cm in diameter. The near‐surface flow could be divided into areas with one of two distinct characteristics: energetic regions occupied by coherent structures and quiescent regions largely devoid of coherent structures. A surface renewal model for air‐water exchange was used to calculate the local transfer velocity in both types of regions. The model predicted that the gas transfer velocities in the energetic regions were 2.8 times larger than in the quiescent regions and that 60% of the total air‐water gas flux occurred across the energetic regions at all wind speeds. In addition, the rate of turbulent kinetic energy dissipation was ∼2.5 times higher in the energetic regions compared to the quiescent regions at all wind speeds.