UC San Diego

Wave fields in the open ocean evolve according to the radiative transfer equation of wave energy or action, which has three source terms, wind input, non-linear wave- wave interactions, and dissipation. Of these, dissipation is thought to be the least well understood, but is expected to be dominated by wave breaking. This dissertation is an investigation of the physical processes associated with the wave breaking and dissipation. Data were taken during three field experiments on R/P FLIP. These experiments took place in September 2009 south of Hawaii (Radiance in a Dynamic Ocean experiment), off the coast of Northern California in June of 2010 (High Resolution Air-Sea Interaction experiment, HIRES), and in the Southern California bight in December 2010 (an extension of HIRES). Between the three campaigns, winds of 0 to 18 m/s and significant wave heights of 0.5 to 5 m were experienced. Stereo Long Wave Infra Red (LWIR) video cameras mounted on one of FLIP's booms were used to reconstruct the 3D structure of an approximately 3x4 m patch of sea surface. Using surface temperature structure as a passive tracer, pattern imaging velocimetry (PIV) was applied to consecutive video frames to extract the velocity field of this patch. An important statistic of breakers is [Lambda](c), the distribution of crest length per unit area of sea surface as a function of breaker velocity c. A new technique, based on image texture, was developed to track breaking waves on the stereo IR reconstructed surface. These waves ranged from large air- entraining breakers to micro breakers that would be undetectable in visible imagery. This allowed measurements of [Lambda](c) that also cover the high-wavenumber gravity wave spectrum. Stress (or wave momentum flux) and dissipation can be related to the fourth and fifth moments of [Lambda](c), and comparisons of these moments with wind stress and wave field dissipation showed that micro- breaking without air entrainment is dynamically significant. A new technique was developed, whereby irrotational surface waves can be separated from rotational turbulence using a Helmholtz decomposition. Turbulent kinetic energy (TKE) dissipation at the sea surface was then estimated using this rotational velocity field. Synchronized subsurface velocity measurements from an array of profiling pulse-coherent acoustic Doppler profilers allowed the calculation of the dissipation profile to depths O(10) significant wave heights. Tying surface and subsurface measurements together allows estimation of total TKE dissipation in the surface wave zone of the marine boundary layer. Turbulence measurements were supported by wind and wave data, allowing us to measure the wave coherence of TKE dissipation and relate it to wind and wave conditions, especially wave breaking