Spectral Energy Dissipation in Broad-banded Wave Fields

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This team has made significant progress toward remotely estimating energy dissipation by wave breaking in the open ocean in a previous NSF-funded study in which they validated remote observations of wind-wave breaking in the absence of swell. This project will extend that research to broad-banded wave fields (i.e., mixed seas). The specific objectives are to:1. Validate energy dissipation from the Phillips (1985) distribution using simultaneous in situ observations in broad-banded wave fields,2. Determine the dependence of energy dissipation on wave age and frequency-directional spread, and3. Test the validity of a commonly used equilibrium approximation, in which energy dissipation is balanced by wind input.Two field experiments will be conducted to estimate energy dissipation during wave breaking via simultaneous remote and in situ observations. The experiments will extend incrementally from recent observations of energy dissipation in narrow-banded breaking waves on Lake Washington and Puget Sound (Thomson et al., 2009). The first experiment will be in the Juan de Fuca Strait, and the second experiment will be in the North Pacific. A Fourier based method (Thomson and Jessup, 2009) that has performed well for remote observations of narrow-banded waves will be adapted to broad-banded waves. A structure function method (Gemmrich, 2009) developed for stationary Doppler profiles of in situ velocity will be adapted to observations from a new free drifting platform.Intellectual MeritQuantification of energy dissipation during wave breaking is essential for accurate modeling of waves and the evolution of sea states. The proposed research will continue testing and application of a hypothesis for remotely estimating dissipation using the distribution of wave crests [Phillips, 1985]. The remote estimates are spectral, and thus ideal as input to numerical models, however the estimates require determination of a scale-free breaking parameter b. The Phillips formulation recently has been validated, including determination of b, for narrow-banded wave fields (Thomson et al., 2009), but not for the broad-banded wave fields that are typical of open ocean conditions. The proposed research will expand the validation, while evaluating the dependence of breaking on frequency-directional spreading, wave age, and wind stress.Broader ImpactsThe proposed research will include training of a PhD student, who will participate in all aspects of the work. The proposed research will be incorporated into ongoing outreach activities, including demonstrations during Washington Weekend (University of Washington open house) and presentations at KCTS public television?s Science Café. Results from the proposed research will be presented at conferences and in peer-reviewed journals. Potential applications of the results include improved global wave models for climate predictions and shipping/naval operations.