The sensitivity of tidal current profiles in the North Channel of the Irish Sea to the parameterization of momentum diffusion

A high resolution (horizontal grid of order 1 km) three-dimensional non-linear model of the North Channel of the Irish Sea is used to examine the spatial variability of the M2 and M4 tidal elevations and currents in the region. The sensitivity of computed tidal elevations and velocity profiles to a range of parameterizations of vertical eddy viscosity, from simple flow-dependent viscosities to those involving two equation turbulence energy models is examined in detail, and the accuracy of the various approaches is considered by comparing observed and computed tidal current profiles in a range of water depths. No significant differences in the computed profiles were obtained using the various approaches, with the simple flow-dependent viscosities yielding solutions of equivalent accuracy to those computed with the turbulence energy models. A sensitivity study to open boundary forcing showed that the position of the M2 amphidrome in the region was particularly sensitive to input along the western open boundary of the model. Calculations of the M4 tidal current showed that the current was significant only in a small number of coastal regions where the advective terms were important. However, there was no detailed observational data set with which the M4 tidal currents could be compared. A high-resolution grid was essential to resolve these generation regions. A detailed study of the temporal and spatial distribution of turbulence energy, its dissipation rate, and eddy viscosity showed a close correlation with tidal current magnitude, with turbulence energy and its dissipation rate decaying rapidly with height above the sea bed.