Evaluation of a shallow water unstructured mesh model for the North Sea–Baltic Sea

A two-dimensional shallow water model is used to simulate tide and storm surges for the North Sea–Baltic Sea. In order to resolve the complex coastline, a finite element model using an unstructured mesh is applied. The resolution varies from 300 m in the shallow and narrow Danish straits to approximately 20 km in the deep parts of the domain. The model is forced by tidal elevations along open boundaries, and by atmospheric wind stress and mean sea level pressure obtained from a high resolution NWP model.The test consist of three simulations: (1) a 10-days simulation of the M2 tide only, (2) a one-year full tidal simulation, and (3) a one-year predictive simulation including both tides and atmospheric forcing. The simulations are validated against sea level data from a number of coastal tide gauges, using harmonic analysis and direct time series comparison. The tidal simulations are used to calibrate model bathymetry and bed friction. The last simulation is validated in terms of surface elevation, following procedures applied on the operational storm surge system run at the Danish Meteorological Institute. The model gives reasonable sea level predictions, with the quality matching that of an equivalent finite difference model.