Observations and modelling of the on-ice and off-ice air flow over the Northern Baltic Sea

Two cases of on-ice and off-ice air flow characterizing the opposite weather situations over the ice-edge zone in the northern Baltic Sea are analysed on the basis of aircraft observations, and modelled using a two-dimensional mesoscale model. The stable boundary layer (SBL) during the on-ice flow exhibited little thermal modification, but a low-level jet (LLJ) was generated at the 250-m high top of the SBL. In the model, the LLJ was associated with inertial oscillations in space, while the baroclinicity explained the shape of the wind profile well above the SBL. Although the observed LLJ was most pronounced over the ice, the modelling suggests that it was not generated by the ice edge but by the coastline some 400 km upwind of the ice edge, where a much more drastic change in the thermal stratification and surface roughness took place. The generation, maintenance, and strength of the LLJ were very sensitive to the parameterization of turbulent mixing in the SBL. In the case of the off-ice flow, the modification of the air mass and the development of a convective boundary layer (CBL) both over the ice and open sea were reasonably well modelled. Sensitivity runs suggested that it was essential to take into account the effects of subgrid-scale leads, a forest in the archipelago (which was crossed by the air flow), and water vapour condensation into ice crystals. The heat flux from leads was particularly important for the heat budget of the CBL, and the observed growth of the CBL was partly due to the effective mixing over the rough and relatively warm forest.