Numerical problems associated with coupling hydrodynamic models in shelf edge regions: the surge event of February 1994

Abstract

A coarse grid three dimensional model of the north west European Continental shelf, and a finer grid model of the region off the west coast of Scotland and the Irish Sea are used to examine flow through the North Channel of the Irish Sea. The period of the storm event of February 1994 is considered. Comparisons of computed and observed currents in the North Channel show that the shelf wide model can reproduce the main features of the flow, although the grid of the model is too coarse to resolve the detailed topography in the region. Calculations with the fine grid model alone, show that at times of strong meteorological forcing in the North Channel region (at the start of the storm) this model can accurately reproduce the flow. However at times of reduced wind forcing (later in the storm period) when shelf wide elevation gradients are important in determining flow along the west coast of Britain, the limited area fine grid model fails to reproduce the flow, to such an extent that the current is in the wrong direction. Nesting the fine grid model within the shelf wide model, using a conventional radiation condition, produces a slight improvement, although in the shelf edge region there are problems. Also, the nested model fails to reproduce the correct current direction in the North Channel to a similar extent to that found using the west coast model alone. A simple adjustment to the radiation condition to take account of the transport computed with the shelf wide model is developed. When this condition is applied to nest the west coast model to the shelf model, there is a significant improvement in the currents in the shelf edge region. Also, the flow through the North Channel is correctly reproduced. Results from the calculations show that at times of strong winds off the west coast, the North Channel flow can be explained in terms of local wind forcing. However, at times of weak winds following a strong wind period the flow is related to shelf wide elevation gradients. Including their effect in a limited area model where the open boundary crosses steep topography (namely the shelf edge) is shown to produce some problems, although these can be overcome.