You have to prepare a data file containing the ice concentration before you set up the spectral wave simulation. The file must be a 2D unstructured data file (dfsu) or a 2D grid data file (dfs2). The area in the data file must cover the model area. If a dfsu-file is used piecewise constant interpolation is used to map the data, while if a dfs2-file is used bilinear interpolation is used. The data must cover the complete simulation period. The time step of the input data file does not, however, have to be the same as the time step of the spectral wave simulation. A linear interpolation will be applied if the time steps differ.