Wave forecasting in coastal archipelagos

The Archipelago Sea, located between the Baltic Proper and the Gulf of Bothnia, has countless small islands. The southern edge of the Archipelago Sea is also characterized with shoals that cause wave refraction and depth-induced wave breaking. To assist safe and efficient shipping in this area wave forecasts, with resolution high enough to solve the islands and small scale features in the bathymetry, are needed. Wave modelling in the Archipelago Sea was studied with two high-resolution, 0.1 nmi and 0.5 nmi grids using wave model WAM. To evaluate the accuracy of the wave forecast in this area and to estimate the properties of wave field inside the archipelago, a-posteriori forecasts were made for summer 2013 using wind forcing from the HIRLAM model. Comparison of modeled significant wave height against wave measurements made at Finnish Meteorological Institute's wave buoy close to Utö islands showed that waves propagating from the Northern Baltic Proper towards the Archipelago Sea were modelled with relatively good accuracy, being slightly underestimated. When the wave field was propagating from the north to the Utö station the modelled significant wave height was overestimated. Preliminary analysis indicated that the overestimation resulted from slight overestimation of the forcing wind speed from northerly direction and from the overestimation of the growth of wave energy at short fetch by the WAM model. Further studies and more measurements are needed in order to improve the accuracy of wave forecasts in this area.