Modelling ice conditions in the easternmost Gulf of Finland in the Baltic Sea

Abstract

To model ice conditions in the eastern Gulf of Finland, a high-resolution three-dimensional hydrodynamic model is coupled with the advanced sea-ice model HELMI ( Haapala et al., 2005). To test the model in extreme situations, the ice pattern in the eastern Gulf of Finland was simulated for a mild ice winter (2007–2008) and for a moderate one (2003–2004). The reference runs were performed on the assumption that the ice in the model domain is fast ice if the sea depth is less than 10 m. Using this assumption, the ice thickness averaged over the Neva Bay (the easternmost part of the Gulf of Finland) is overestimated by the model for almost the entire wintertime in the mild winter and during the ice formation and melting periods in the moderate winter, as compared with the thickness reported in ice charts. For both winters, the model solution without the assumption of a fast ice region is in better agreement with averaged ice thicknesses from ice charts during the ice formation and ice melting periods. The addition of snow on the ice surface in the model without a fast ice region led to a significant decrease of ice thickness during the entire wintertime period for the moderate winter, so that the model agrees well with the ice thickness estimated from observations. At the same time, the decrease of ice thickness in the mild winter was not strong enough, and the model overestimated the observed ice thickness in the Neva Bay in January–March by about 10 cm. We will show that possible reasons for this are heat sources (warm wastewater) within the Neva Bay and uncertainties in the assumed river temperature. In the case of no fast ice region and snow on ice, the model correctly simulates the spatial distribution of ice thickness seen on ice charts for the mild winter. Despite the absence of irregularities in wind forcing – which was assumed to be homogeneous over the model domain – the modelled ice thickness distributions included meso-scale features such as the ridged and rafted ice regions with increased ice thickness, and the regions of small ice thicknesses having a resemblance to polyniyas.