FY 1985

Atmospheric boundary layer structure and drag coefficients over sea ice

Overland, J.E.

J. Geophys. Res., 90(C5), 9029–9049, doi: 10.1029/JC090iC05p09029 (1985)

This paper estimates the air/sea drag coefficient for first-year ice from recent aircraft measurements and reconciles the range of observed drag coefficients (10³C_D = 1.2–3.7 referenced to 10 m) for all sea ice types, based on ice roughness and seasonal meteorology. For the purpose of sea ice modeling, it is necessary to define an effective drag coefficient which relates regional stress to regional wind, because sea ice is heterogeneous on scales less than 20 km. Regional stress is influenced by the distribution of surface roughness, the buoyancy flux from quasi-periodic leads, and external atmospheric conditions, principally the inversion height. 10³C_D is 1.3–1.5 for smooth ice, but is much greater for non-flat surfaces. For wind speeds greater than 5 m/s and air temperatures below freezing, the effective 10³C_D is 2.5–3.0 for nearly continuous pack ice, such as first-year ice in seasonal ice zones and central arctic basin. The range of values of 10³C_D is 3.0–3.7 for unstable boundary layers typical of off-ice winds in the marginal ice zone (MIZ) or even greater if the ice has been broken by a recent storm. C_D values at the lower end of these ranges are associated with low inversion heights. These coefficients are confirmed by 10³C_D of 2.9, 2.5, and 3.1 for first-year sea ice calculated from gust probe data collected by the NOAA-P3 aircraft, interior to the inner MIZ in the Bering Sea during the Marginal Ice Zone Experiment (MIZEX-West) in February 1983, and 10³C_D of 2.6 for the Arctic calculated from Arctic Ice Dynamics Joint Experiment (AIDJEX) aircraft data for February 1976. The effective drag coefficient with the presence of even a small concentration of sea ice is greater than the oceanic value as shown by a 10³C_D of 2.2 calculated from NOAA-P3 gust probe data over a 40-km track of 0.4 ice concentration in the outer MIZ of the Greenland Sea in June 1984 during MIZEX-84. The relation of surface wind and stress to the geostrophic wind for shallow inversion heights, typical of high latitudes, is reviewed with a turbulent closure atmospheric boundary layer model. The winter Arctic is typified by low inversions (<100 m), low geostrophic drag coefficients, and large geostrophic/surface wind turning angles, with low functional dependence on surface roughness. Seasonal ice zones are typified by a more modest influence of the inversion on boundary layer characteristics.