Introducing Observation-Based Physics Into the WAM Wave Model

Abstract

Third generation spectral wave models predict the growth, decay and transformation of wind-generated waves. WAM is one of the two dominant third-generation spectral wave models covering the global ocean. The standard representation of deep-water wave physics in WAM is the semi-empirical approach of Ardhuin et al. (2010; hereafter ARD). The other modern package of wave physics for spectral models, available in WAVEWATCH-III and SWAN, but not in WAM, is the observation-based approach developed by A. V. Babanin, I. R. Young, M. A. Donelan, M. L. Banner and W. E. Rogers (hereafter BYDBR). Its source functions were measured and therefore are not subject to tuning (within confidence limits of the measurements). Furthermore, the observations revealed and/or quantified physical phenomena missing or neglected in other source packages for spectral wave models such as non-linear airflow separation (hence relative reduction of wind input at strong winds), steepness-dependent wave growth (making the wind input a weakly non-linear function of the wave spectrum), negative wind input under adverse winds, two-part wave dissipation consisting of a local in wavenumber space term and a cumulative term (which is an integral of the spectrum), a wave breaking threshold below which no breaking occurs, and swell decay due to interaction with and production of oceanic turbulence. This work documents the implementation of BYDBR physics into WAM. We find that the performance of WAM using BYDBR physics is comparable to the standard with respect to the mean-based metrics. The mean differences between BYDBR and ARD are most prominent in Boreal winter, with widespread reductions in the N. H. of ∼10cm and ∼0.2s for significant wave height and mean wave period respectively.