The BSH's new operational circulation model using general vertical co-ordinates

At the Federal Maritime and Hydrographic Agency (BSH), a new model version of a circulation model is under development which will use not only an improved grid resolution but also a novel formulation of the vertical co-ordinate. The three-dimensional baroclinic circulation model (BSHcmod) is an important component of the operational model system, which has been running at the BSH for more than 20 years now. Important applications of the model system are the support of the BSH's water level prediction service, forecasting of oil drift paths and pollutant dispersion, as well as water quality studies. The circulation model simulates tidal, wind and density driven motion in the entire North Sea and Baltic Sea region. Two nested and interactively coupled grids are used for the German Bight and western Baltic Sea. The horizontal grid spacing of the new version is 900 m in the nested areas, and approx. 5 km in the other parts of the North Sea and Baltic. The vertical grid resolution has been improved as well. In addition, a new type of vertical co-ordinate representation has been implemented which allows layer thicknesses to be varied in time and space. With regard to the performance of general circulation models in a more general context, the new formulation provides a dynamic co-moving description of stratification and its maintenance. Internal dynamics are captured using weakly inclined flexible co-ordinate surfaces. The formulation is of the evolution type, and all common forms of vertical representation are recovered as special examples.