Improved estimates of nearshore wave conditions in the Gulf of Finland

The heterogeneous coastline of the Gulf of Finland can result in widely varying wave conditions in a small geographical area. It is therefore challenging to obtain comprehensive information about the wave spectrum, which is needed to accurately quantify, e.g. the wave-bottom interactions. In this study, we implemented the wave model WAM to the coastal waters off Helsinki using a high-resolution 0.1 nmi grid and found that the model mostly predicts the wave field well. However, WAM overestimated the wave energy for south-westerly winds blowing over the peninsula sheltering the study area. This spurious behaviour was not caused by inaccuracies in the wind forcing, the boundary wave field or the available bathymetric grids. We present two methods for improving the prediction of the near-shore wave field. The first approach, which is also used to describe the structure of the wave field, is based on e.g. wave growth relations and models the fetch-limited and the longer waves separately. The second method determines a so-called effective wind forcing for the wave model by comparing the observed and modelled non-dimensional wave spectra. Both methods were validated using wave buoy observations and were found to clearly improve the predictions for the significant wave height and wave spectrum at the study site for south-westerly winds. Because some prior information on the wave conditions is required to implement the methods, they are best suited for expanding the usability of any limited measurement dataset available for a study. The method based on the effective wind forcing can also be implemented for operational forecasting or be used to gather statistics from hindcasts.