Analysis wave propagation into the North Sea : memo

In some cases the swell wave forecasts based on the SWAN-North Sea model are much lower, in the order of 1 – 2 meters, than the observed swell waves, especially when waves come from a north-westerly direction. Since the underestimation is already seen in the northern locations close to the edge of the model, wave boundary conditions seem to be the main reason for this. Besides, it is advised to adjust the HE10 output (representing the low frequency wave height) of SWAN so that it is based on the absolute wave spectrum instead of the currently used relative wave spectrum, to ensure a fair comparison with measurements. This adaptation will only solve some 0.1 - 0.2 m of the underestimation. It is recommended to do a follow-up of this study to dive deeper into the comparison of SWAN model and measurement data and in particular the wave boundary conditions of SWAN-North Sea to improve the system understanding and look for possible solutions to improve the SWAN-North Sea model.