Entrained bubble populations and their influence on the turbulence, dissipation, and stress beneath breaking and non-breaking waves

The development and collapse of the air-sea interface as a defined two-phase boundary of evolving surface waves entrains air and other gases and results in bubbles and bubble plumes, typically injected as jet intrusions or by crushing of the crest air cavity. These bubbles possess an initial downward momentum and a subsequent buoyant rise velocity, and have been reported to alter the turbulent kinetic energy (TKE) spectrum, as well as the energy balance between turbulence and dissipation as a function of depth. In July 2018, an extensive series of 35 air-sea gas transfer experiments using noble gases in natural seawater was conducted at the University of Miami’s SUrge STructure Atmosphere InteractioN (SUSTAIN) wind-wave tank laboratory; an imaging system and 3D acoustic Doppler current profiler captured the bubble populations of 30-1500 microns and sub-surface velocity measurements in a variety of wind and wave conditions. Wind conditions ranged from U10 of 10.6 – 50 ms-1, and both monochromatic and spectral mechanically generated waves were produced for at least one of three water temperatures between 20 and 32°C. Individual wave crests observed by conductive wave wires were analyzed using a breaking onset likelihood technique based on the crest steepness, skewness, and asymmetry. Preliminary results show that bubble populations become more widely distributed in the observed radii at higher wind speeds and the peak radius shifts from 150-250 micron bubbles to smaller (40-50 micron) bubbles in monochromatic waves. Conversely, an opposite trend is seen for the spectrum waves. Due to steepness-related instability of the wave crests, breaking is found to be most likely for the 20-degree Celsius monochromatic waves, and kinetic energy profiles are largest in magnitude beneath these conditions. TKE spectra show the expected frequency peak associated with the different wave conditions, however, the slope changes at small scales suggest differences in breaking and non-breaking conditions. Hence, through analysis of the bubble populations, turbulence, and dissipation rates at different significant-wave-height-scaled depths, our data provides insight into how sub-surface energy, dissipation rate, and Reynolds stress are altered by bubbles in different wind-wave conditions and at different water temperatures.