Simulation of snowbands in the Baltic Sea area with the coupled atmosphere-ocean-ice model COSMO-CLM/NEMO

Wind-parallel bands of snowfall over the Baltic Sea area are common during late autumn and early winter. This phenomenon occurs when cold air flows over the warm water surface, enhancing convection and leading to heavy snow fall. Six snowband events from 1985 to 2010 are simulated by using the coupled atmosphereocean- ice model COSMO-CLM/NEMO. The model resolution is reasonably high to capture the snowbands; the atmospheric model COSMO-CLM has a horizontal grid-spacing of approximately 25 km and the ocean sea-ice model NEMO has a horizontal grid-spacing of approximately 3 km. The model results show that the coupled system COSMO-CLM/NEMO successfully reproduced the snowband events with a high contrast of temperatures between the surface and the atmosphere, sharp bands of precipitation over the sea, as well as the enormous heat fluxes released by the ocean to the atmosphere during the days when snowbands occurred. In the two cases when radar data are available, the model precipitation is shown to be in satisfactory agreement. The precipitation patterns closely follow the cloud shapes on satellite images. When not coupled with the ocean model, the atmospheric stand-alone model provided acceptable results if forced by high-quality sea surface temperatures (SSTs) from reanalysis data. However, COSMO-CLM forced with lower quality SSTs could not recreate the snowbands. The results indicate the need of an atmospheric model with high SST skill or a coupled ocean model when extreme event climatology is the primary aim in the Baltic Sea area.