A guide to the Nedwam wave model

Nedwam is a numerical model for making North Sea wave forecasts. The Nedwam software is a KNMI product. The model physics of Nedwam is the same as that of the global wave model WAM. WAM has been developed by an international group of wave modellers, including people from the KNMI. Nedwam models the evolution of the two-dimensional wave-variance spectrum at each point of a stereographic grid which covers the North Sea and a large part of the Norwegian sea. The geographic grid contains 612 gridpoints. The grid spacing is approximately 75 km. The wave-variance spectrum is defined on a wave-vector grid of 25 frequencies by 12 directions.

The various terms in the central equation of wave physics have direct counterparts in the Nedwam model. Therefore, it will be relatively easy to change Nedwam if better representations for the various terms in this equation become available or if one wants to include data assimilation. As the total number of model variables is rather large, 612x25x12=183600, the Nedwam mode requires much more computer time and memory than previous KNMI wave models. On the other hand, it produces more detailed information, as in principle the full 2-D wave spectrum is available for all times and places. One can ask e.g. the significant wave heights, the peak periodds, the wave direction and the spectra at any grid point.

The first four chapters of this guide give a comprehensive description of the Nedwam wave model.. The basic equation for the evolution of the wave spectrum is discussed and all the formulae which are used by the numerical model are given explicitly. The fifth chapter treats the Nedwam computer program and its implementation at the KNMI.