A comparison of two spectral wave models in the Southern California Bight

Two models, a spectral refraction model (Longuet-Higgins) and a parabolic equation method ( PEM) refraction-diffraction model (Kirby), are used to simulate the propagation ofsurface gravity waves across the Southern California Bight. The Bight contains numerous offshore islands and shoals and is significantly larger ( ~ 300 km by 300 km) than regions typically studied with these models. The effects of complex bathymetry on the transformation f incident wave directional spectra, So (f,00), which are very narrow in both frequency and direction are difficult o model accurately. As So(f, Oo) becomes broader in both dimensions, agreement between the models improves and the spectra pre-dicted at coastal sites become less sensitive to errors in the bathymetry grid, to tidal changes in the mean water depth, and to uncertainty inSo (f,0o) itself. The smoothing associated with even relatively narrow (0.01 Hz-5 ° bandwidth) So(f, Oo) is usually sufficient to bring the model predictions of shal-low water energy into at least qualitative agreement. However, neither model is accurate at highly sheltered sites. The importance ofdiffraction degrades the predictions of the refraction model, and a positive bias [O(10%) of the deep ocean energy] in the refraction-diffraction model estimates, be-lieved to stem from numerical "noise " (Kirby), may be comparable tothe low wave energy. The best agreement between the predicted spectra generally occurs at moderately exposed locations in deeper waters within the Bight, away from shallow water diffractive ffects and in the far-field of the islands. In these cases, the differences between the models are small, comparable tothe errors caused by tidal fluctuations in water depth as waves propagate across the Bight. The accuracy of predicted energies at these sites is likely to be limited by the uncertainty in specifying So(fOo