The effect of boundary field accuracy on high-resolution coastal wave modelling

The coastal area off Helsinki in the Gulf of Finland has both a complex shoreline and bathymetry which makes modelling the wave field especially challenging. Experience has shown, that these types of areas generally benefits from the use of a high-resolution model grid. Even though the coastal area is very sensitive to the resolution, this is not necessarily true for the boundary field used to force the nested grid. The area outside Helsinki was modelled with the third generation spectral wave model WAM, using a high-resolution 0.1 nmi grid. This nested grid was forced with boundary fields with two different resolutions (1 nmi and 4 nmi), and the difference in significant wave height inside the nested grid due to the change of boundary field is discussed. In comparison to the coarser 4 nmi boundary grid, the finer 1 nmi version was able to model the wave field with higher accuracy near the coastline. This led to an improved accuracy of the modelled significant wave height inside the nested grid also. However, the differences inside the coastal archipelago and at the two wave buoy sites inside the nested grid were insignificant.