Diffraction of irregular ocean waves measured by altimeter in the lee of islands

Abstract

Despite the importance of diffraction of irregular ocean waves, there is a lack of reported measurements over scales larger than a few tens of wavelengths. In the coastal zone, Satellite Altimetry is hindered because the backscatter characteristics of waveforms deviate from Brown's theoretical model, specially up to 20 km from the coastline. Here, we combine a novel set of retracked — reprocessed — altimeter data with directional buoy spectra and simulations with a spectral numerical model where diffraction is computed with an approximation based on the Mild Slope Equation. The Channel Islands, off the coast of California, is an ideal spot for the analysis because of the sharp variations in bathymetry in the vicinity of the archipelago, increasing the relative importance of diffraction over refraction. Spatial variations of wave energy in the lee of the islands are investigated along and across the wave propagation direction. For the first time the lateral rate of energy spreading — across the wave propagation direction — was computed in oceanic conditions and are of the same magnitude as those found in previously published experiments in a wave tank. The importance of diffraction as a non-dissipative process during wave propagation in situations normally encountered in the ocean is also discussed.