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

Abstract

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%.