An intercomparison of three-dimensional tidal hydrodynamic models of the Irish Sea

This paper deals with an intercomparison of two three-dimensional hydrodynamic numerical models based upon very different approaches in the vertical. In the horizontal domain, both models use an identical grid, open boundary conditions, water depths and coastal topography and hence results of the comparison can truly be attributed to variations in the vertical. Moreover, since both models use σ-coordinates in the vertical, changes in topography are represented in a similar manner. A range of algebraic eddy viscosity profiles are used in the models, and the sensitivity of tidal current profiles to variations in this parameter are considered in detail. One model uses a functional or modal approach in the vertical and resolves the bottom boundary layer (a no slip condition is applied at the sea bed). The other model uses a finite difference grid in the vertical with a slip condition at the bed. The roughness height is derived from the Chezy coefficient used in depth averaged calculations. The advantages/disadvantages and the correspondence between the modal approach and the finite difference approach for the vertical are considered. The sensitivity of model results to changes in the representation of internal friction and bed friction are considered in detail by comparing model results with observations of M2 tidal elevations and currents. The results show that both types of modelling approaches are capable to reproduce the water levels equally well. This also holds for the currents and the vertical current variation, although the current speeds tend to be overpredicted. The type of flow considered is modelled equally well with application of a slip condition and a no slip condition at the bed. Notwithstanding their conceptual differences, the behaviour of the models is very similar when compared with the measurements: the model results tend to be clustered.