The Atmospheric Boundary Layer Over Baltic Sea Ice

The temporal and spatial variability of the atmospheric boundary layer (ABL) over Baltic Sea ice is presented. The focus is on the properties of the underlying surface (temperature and albedo), the heat fluxes at the air–ice/water interface and the vertical ABL structure (inversion layer and low-level jet). The study uses data from two field campaigns, BASIS 1998 and BASIS 2001, applying ice stations, a ship and aircraft over various surface types (land-fast ice, drift ice, and open water). Sensible heat flux ranges from −100 W m−2 under warm-air advection conditions to 300 W m−2 under cold-air advection conditions. On average, it is negative over land-fast ice and positive over open water. An ABL inversion is almost permanently present. It is surface-based 2/3 of the time and extends to 165 m on average. The average base of the elevated inversion is 343 m. The mean temperature increase across all inversions is 2.1 K and is clearly smaller than for Arctic sea-ice inversions. A low-level jet occurs about 86% of the time with an average height of 245 m and speed of 13.3 m s−1, which is 7 m s−1 higher than the surface-layer wind. On average, the low-level jet is located near the top of the surface-based inversion or near the base of the elevated inversion.