Laboratory Observations of Foam Generation from Field-Scale Breaking Waves

Abstract

Wave generated bubbles are a primary mechanism for air-sea flux of heat, gas, and momentum, aerosol production, ambient noise generation, and energy dissipation due to breaking in the open ocean. An actively breaking wave, or whitecap, entrains air, leaving behind a bubbly wake. Bubbles rise to the air-sea interface, and residual foam develops from the receding wake. Previous small-scale laboratory research by Masnadi et al. (2021) has shown that under conditions of outward heat flux from the ocean to the atmosphere, the thermal time scale of cooling surface foam from wave breaking is related to bubble plume time scales and energy dissipation. This project involves further investigation of the thermal signature of cooling residual foam and its relationship to bubble formation and wave energy dissipation. To extend the hypothesis from Masnadi et al. (2021), an experiment with field-scale waves was conducted at the O.H. Hinsdale Wave Research Laboratory’s Large Wave Flume at Oregon State University (OSU). In the experiment, one-meter plunging breakers were created from a solitary wave propagating over an individual shoal to initiate depth-limited breaking. Surfactant (Triton X-100) was added to the flume’s freshwater to simulate the effects of saltwater on bubble formation and longevity of surface foam. In contrast to the conditions tested by Masnadi et al. (2021), inward heat flux conditions that generated a warm skin were measured at OSU. At different locations along the wave flume, a combination of optical and thermal remote sensing, acoustic, and in-situ instruments were used to measure and map horizontal variability of bubble plume geometry, bubble size distribution, and whitecap foam coverage. The analysis focuses on time scales associated with visible foam, subsurface bubble plumes, onset times from breaking to warming of residual foam, and estimates of the energy dissipation rate, and the results show both similarities and differences to those found previously.