Propagation of ocean surface waves on a spherical multiple-cell grid

Abstract

Satellite observations have established that the Arctic ice is retreating faster than expected and global ocean surface wave models have to be extended to cover the polar region in the future. The major obstacle preventing the wave model extension is that the diminishing longitude grid-length at high latitudes exerts a severe restriction on time steps and leads to polar singularity. A spherical multiple-cell (SMC) grid is installed in a global wave model to overcome the polar problems. A 2nd order upstream non-oscillatory advection scheme and a rotation scheme for wave spectral refraction are used. The unstructured SMC grid allows time step to be relaxed and land cells to be removed, saving over 1/3 of the total computation time in comparison with the original latitude–longitude grid model. It also allows multi-resolutions within one model domain so that coastlines and small islands can be resolved at refined resolutions. It also makes it possible to merge regional models into a single global model, replacing nested models in operational forecasting systems. Validations with satellite and buoy observations show that the SMC grid wave model performs as well as the latitude–longitude grid model and yields better swell predictions if coastlines and small islands are resolved at refined resolutions. Due to the ice coverage in the Arctic, an ideal wave spectral propagation in an ice-free Arctic is used to illustrate a map-east reference direction method for extension of the wave model over the whole Arctic.