TSA - A Two Scale Approximation for Wind-Generated Ocean Surface Waves

Abstract

Abstract : The rationale for this project is that nonlinear 4-wave interactions, represented by FBI (Full Boltzmann Integral) have been shown to be central to wave forecast models. However, these FBI formulations are too slow for operational wave forecasting. The discrete interaction approximation (DIA) is the basis for operational wave models WAM, and it has substantial deviations from FBI in selected parts of the spectrum. Two Scale Approximation (TSA) offers a new approach to replace DIA, outperforming DIA in accuracy, with potentially fast run times for DIA. Therefore TSA is a candidate for implementation in operational wave models. The meaningful technical results achieved in this fiscal year are: a) In Resio et al. (2011) we examine nonlinear fluxes of energy and momentum through wave spectra via an exact integration of the Full Boltzmann Integral. b) We implemented TSA into WW3 and completed a number of tests for fetch- and duration-limited wave growth. Results are acceptable and compare well with observed data, and runs, using the exact FBI code in WW3. The real challenge has been turing winds, such as generated by a moving cyclone or hurricane, and the generalization of WW3 to accommodate spectral evolution in turning wind cases. This challenge was met be generalizing the manner in which the broad scale of TSA was defined, so that in complicated rapidly changing wave spectra cases, a second broad scale could be defined, in order to handle double peaks, and to let TSA downshift appropriately.