Geostrophic drag coefficients for the central Arctic derived from Soviet drifting station data*

Based on 4 years of central arctic atmospheric sounding data and 10 years of surface wind data from Soviet drifting stations, combined with geostrophic winds from the arctic buoy program, the following relation between air-ice stress, tau, and the surface geostrophic wind speed, g, can be recommended tau = rhoC(g)2gamma2g2, rohC(g)2 = 0.85 x 10(-3) (nt m-2) (m2 s-2)-2, C(g) = 0.024, November-March, rhoC(g)2 = 1.10 x 10(-3) (nt m-2)(m2 s-2)-1, C(g) = 0.029, June-August. If the spacing between sea level pressure values from buoys is greater than 400 km, such as in the arctic buoy array, a speed enhancement factor of gamma = 1.3 should be applied to correct for insufficient sampling and smoothing in generating the geostrophic wind field. If the spacing is of order 100 km or less, then gamma = 1.0. An inflow angle alpha, the angle between the geostrophic wind and the surface wind, of 33-degrees- is recommended for winter and 23-degrees- for summer. Values for the transition months April, May, September and October can be interpolated between winter and summer values. The winter value of Cg is calculated 3 ways: from the Soviet station surface wind-geostrophic wind speed ratio using suitable 10 m drag coefficients, from regression equations based on surface-900 mb stability, and from the AIDJEX analyses. The summer values are based on the surface wind-geostrophic wind ratio, model, and AIDJEX derived values. There is considerable day-to-day variability in atmospheric stability and geostrophic coefficients, but no statistically significant variation in the within-season monthly mean and median values; month-to-month variability is within 5 % for C(g) and 5-degrees- for alpha for the winter and summer seasons. Neglect of stability variations for daily cases can contribute an error of +/- 40 % in the relation between surface stress and geostrophic wind speed squared compared with using constant winter values. Use of atmospheric temperature profiles from satellites may increase the accuracy of C(g) for daily cases by providing an estimate of lower-atmospheric inversion strength.