Measuring bubbles, boundary layers, and related processes in the ocean surface layer

Abstract

Bubbles entrained by breaking waves are important in the active and passive acoustics of the ocean surface layer, and also play an important role as tracers for acoustic Doppler techniques. Bubbles are not just passive tracers but may also be dynamically significant, thereby affecting the structure of the wave-zone boundary layer. The processes affecting the distribution of entrained air and the distribution of bubble sizes are reviewed in the context of the available measurement techniques, models, and observations of the surface boundary layer. In many respects, the knowledge of the fluid dynamic structure of the surface layer is no better than the knowledge of the acoustics and bubble phenomena, and the talk will emphasize those processes which are important for both fields. These include wave breaking, convection, Langmuir circulations, and turbulent fluxes and dissipation. Indeed, acoustic instrumentation plays an important role in probing the surface layer, and a better understanding of bubble-related phenomena is necessary to better understand the performance of these instruments.