Bubbles generated from wind‐steepened breaking waves: 2. Bubble plumes, bubbles, and wave characteristics

Measurements of breaking‐wave‐generated bubble plumes were made in fresh (but not clean) water in a large wind‐wave tunnel. To preserve diversity, a classification scheme was developed on the basis of plume dimensions and “optical density,” or the plume's ability to obscure the background. Optically dense plumes were due to the presence of a peak at large radius in the plume bubble size distribution. For each class, the plume formation rate, P , was measured at different fetches. The relationship between wave‐breaking characteristics and the bubble plume evolution is examined in detail for these experiments. The wave‐breaking rate and intensity were strongly fetch‐dependent as the mechanically steepened wind waves rapidly evolved with fetch because of wind, dissipation, and nonlinear wave‐wave interactions. P followed the trend in wave breaking, reaching a maximum at the fetch of maximum wave breaking. The ratio of dense to diffuse plumes was more sensitive to the wave‐breaking intensity. Using P and the bubble population size distributions for each class, the global bubble plume injection size distribution, Ψ i ( r ), where r is radius, was calculated. Ψ i decreased as Ψ i ∼ r −1.2 for r < 1700 μm and Ψ i ∼ r −3.9 for larger r . Total volume injection was 640 cm 3 s −1 , divided approximately equally between bubbles smaller and larger than 1700‐μm radius. Using plume volumes at maximum penetration for each class, a concentration distribution was calculated and showed plume concentrations greater than the background population by one to several orders of magnitude, depending upon r .