Obstruction grids for spectral wave models

An algorithm has been developed to construct obstruction grids for spectral wave models to properly account for wave attenuation by unresolved islands. The algorithm uses the GSHHS coastline dataset, and accounts for islands overlapping grid cells as well as the orientation of islands in neighboring cells. The algorithm has been validated by conducting numerical experiments with the wave model WAVEWATCH III. The numerical experiments consisted of a swell propagating through French Polynesia with grids of various spatial resolutions (2′, 4′, 8′, 15′ and 30′). The systematic patterns of energy attenuation behind the islands in the highest resolution grid, with a limited number of unresolved islands represented by obstructions, were well reproduced by the lower resolution grids where most islands are represented by obstruction grids. The algorithm is part of a larger software package that has been developed to build grids for spectral wave models, in particular WAVEWATCH III.