Air-ice drag coefficients in the western Weddell Sea: 1. Values deduced from profile measurements

From 197 hourly averaged, four-level wind speed profiles collected on Ice Station Weddell (ISW) in February and March 1992, we compute the neutral stability, 10-m, air-ice drag coefficient, CDN10. Values range from 1.3×10−3 to 2.5×10−3 for the multiyear ice floe that was ISW. Individual CDN10 values depend critically on how well the mean wind is aligned with the dominant snowdrift patterns. On ISW, 20% of the time, we experienced drifting or blowing snow; when the wind speed at 5 m exceeded 8 m s−1, such wind-driven snow was a virtual certainty. Consequently, the surface was continually changing, drifts were building, drifts were eroding. As the wind continued from a constant direction and the building drifts streamlined the surface, CDN10 could decrease by as much as 30% in 12 hours. If the wind direction then shifted by as little as 20°, CDN10 would immediately increase significantly. The implications are that snow-covered sea ice does not present an isotropic surface; it has a preferred direction dictated by the wind's history. Consequently, computing surface stress using an average value for CDN10 will produce errors of up to 30%.