Bubble Plumes, Bubbles, and Wave Characteristics from Wind-Steepened, Breaking Waves
Résumé
Breaking-wave bubbles are important to many geophysical processes; however, few measurements have related the generated bubble distribution and plume characteristics with the wave evolution and plume dynamics. As part of the LUMINY wind-wave experiment, measurements of breaking waves, breaking-wave generated bubbles, and bubble plume characteristics were made in a wind tunnel for wind-stress breaking waves. Bubble plumes exhibited a wide range of bubble distributions, physical extents, and dynamics. A classification scheme was developed and the plume formation rate, P, at different fetches measured for approximately 3500 plumes. Bubble plumes were classified as major, minor, or micro depending upon physical plume dimensions. Plumes were also classified as diffuse or dense based upon their ability to obscure the background. Waves and wave breaking characteristics were determined for the experiments. Wave height was weakly dependant on fetch with peak wave height at the fetch of maximum wave breaking, 22.5 m. Wave height then decreased with greater fetch. In contrast, wave breaking depended strongly on fetch. P followed wave development and for micro and minor plumes occurred at the greatest fetch surveyed. The ratio of dense to diffuse plumes also followed the trend in wave development. Using the bubble-size population distributions for each plume class and P, the global bubble plume injection size distribution, Yi(r), where r is radius, was calculated. Yi decreased as Yi ~ r-1.2, for r < 1700 µm and Yi ~ r-3.9 for larger r. Volume injection rate for the study area was 640 cm3 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.