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

[ 1] 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, Psi(i)( r), where r is radius, was calculated. Psi(i) decreased as Psi(i) similar to r(-1.2) for r < 1700 mu m and Psi(i) similar to 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-mu m 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.