On determining the role of wind wave turbulence and grid resolution upon computed storm driven currents

A three-dimensional coarse grid (of resolution 12 km) hydrodynamic model co vering the European Continental shelf with eddy viscosity depending upon cu rrent intensity and wind-wave significant height and period, together with a higher resolution (of order 0.924 km) limited area model of the sea regio n around the Shetland Islands are developed. These models are used to inves tigate the influence of a source of wind wave turbulence, and local grid re finement upon the three-dimensional currents during a major storm, namely t he storm of January 93. This period was chosen because it contains a number of storm events with significant wave heights exceeding 10 m. Also, it is the time when the tanker Braer went aground on the Shetland Islands, and se rves to illustrate the importance of high-resolution three-dimensional flow s in the region of an oil spill. The spatial distribution of the significant wave height and period over the continental shelf at the time of the Braer spill is computed using the WAM wave model running on the same finite difference grid as the continental s helf hydrodynamic model. This suite of models is used to investigate the spatial distribution of cur rents and waves at the time of the Braer oil spill on the Shetland Islands. The importance of including a high-resolution grid in the region of the Sh etland Isle and taking account of 'far field' effects computed with the she lf wide model within the limited area model, together with the accuracy of local winds in determining the circulation in the vicinity of the Shetland Isle is considered. Calculations show that including an additional source of wave-dependent vis cosity has a significant influence on surface currents particularly in deep water. However, this is restricted to a high shear surface layer having a thickness of a few meters. Circulation fields from the high-resolution model, show the importance of h aving accurate topography and a fine grid in the region of the Shetland Isl e, and input from the larger area model. Details of the flow field to the s outh of the Shetlands, and the time variability of an eddy in the region ar e shown to be influenced by the far field input to the area, which in turn depends upon the shelf wide wind fields. (C) 2000 Elsevier Science Ltd. All rights reserved.