Turbulent fluxes and air-ice coupling in the Baltic Air-Sea-Ice Study

Turbulent surface eddy fluxes of heat and momentum as well as the vertical profiles of wind speed and air temperature were measured on the sea ice in the northern Baltic Sea, in the BALTEX-BASIS experiment, in March 1998. The fluxes were measured by a sonic anemometer and compared with the ones derived from the Monin-Obukhov theory based flux-profile relationships. For the latter, observations from an on-site 10-m high meteorological mast were used. The neutral drag coefficient, C_D10N, showed no apparent dependence on the wind speed and was 1.0 x 10^-3 for smooth snow-covered ice and 1.5 x 10^-3 for deformed ice. Actually, the local drag coefficient depended strongly on the wind direction, due to orography and geometry of  the  archipelago in the coastal observation site. The sonic measured and profile based fluxes agreed mutually well and supported the validity of the M-O similarity theory.

The roughness length for temperature ( z_T ) revealed a greater apparent dependence on wind speed and  z_T was slightly larger than the aerodynamic roughness  ( z_o ) for low wind speeds, and vice versa for moderate and high winds. We give a new empirical expression that predicts how the scalar roughness depends on the aerodynamic roughness (drag coefficient) and wind speed. In definition of the roughness Reynolds, number we use a diffierent approach compared with the "ordinary" one used e.g. by Andreas (1987). 

As the third independent alternative, the fluxes were modelled by a coupled thermodynamic air-ice-sea model and those compared well with the eddy-flux and gradient methods.  The surface (skin) temperature estimation by the three methods agreed mutually well also.

In stably stratified conditions, the observations suggested the fluxes and turbulence to be less suppressed by stability than estimated by the current universal functions.