This paper uses the Winter Weddell Sea Project 1986, winter Antarctic data set to (1) describe the nature of observed sea ice drift and momentum exchange and (2) determine relevant drag coefficients (linear and quadratic) and parameter values for three formulations of the momentum balance. The large‐scale mean divergence of the ice justifies, with some penalty, use of the steady free drift equation in which the air‐ice stress is balanced by ice‐ocean drag and the Coriolis force. Three forms of the free drift equation are considered: (1) stresses are parameterized with a quadratic drag law, (2) stresses are parameterized with a linear drag law (useful because of its analytically manageable form), and (3) the Coriolis force is ignored (owing to the thin, 0.6‐m ice), so ice speed is proportional to wind speed at a specified angle. All three formulations simulate the observed ice drift with the same degree of accuracy. The linear drag law is an excellent approximation to the quadratic law over a broad range of forcing only when the air‐ice and ice‐water stresses are both parameterized using the linear law (otherwise the ice‐water drag coefficient is a nonconstant function of wind speed). The linear drag coefficient and constant of proportionality relating ice speed to wind speed can both be computed directly from knowledge of the quadratic values. These calculations result in estimates within ≤2% of the optimum fitted values. Because of the ∼95% ice coverage, ice interaction is frequently significant. During such periods, the ice‐water drag coefficient represents an “effective” drag, artificially inflated to include the forces arising from this interaction. We break the ice drift data into 6‐hour nonoverlapping windows to allow isolation of periods of true free drift. Both true drag coefficient values and effective values are then estimated. The effective values show a strong correlation to the 4‐day average large‐scale ice divergence. They also show that the ice‐water stress is typically ∼1/3 the air‐ice stress, indicating a significant role of ice interaction (free drift still provides an excellent parameterization of ice drift but at the expense of neglecting the details of these physics). The optimum quadratic drag coefficient is 1.62×10−3 with turning angle 15.2°; the effective value is 3.22×10−3 with turning angle 18.1°; the linear drag coefficient is 0.80×10−3 with turning angle 18.1°; the effective value is 1.48×10−3 with turning angle 19.6°, and in general, the ice drifts at ∼3% of the wind speed, ∼23° to its left.

The aerodynamic drag of Arctic sea ice is calculated using surface data, measured by an airborne laser altimeter and a digital camera in the marginal ice zone of Fram Strait. The influence of the surface morphology on the momentum transfer under neutral thermal stratification in the atmospheric boundary layer is derived with the aid of model concepts, based on the partitioning of the surface drag into a form drag and a skin drag. The drag partitioning concept pays attention to the probability density functions of the geometric surface parameters. We found for the marginal ice zone that the form drag, caused by floe edges, can amount to 140% of the skin drag, while the effect of pressure ridges never exceeded 40%. Due to the narrow spacing of obstacles, the skin drag is significantly reduced by shadowing effects on the leeward side of floe edges. For practical purposes, the fractional sea-ice coverage can be used to parameter&e the drag coefficient Cdn, related to the 10 m-wind. Cdn increases from 1.2. 10e3 over open water to 2.8 t 10V3 for 55% ice coverage and decreases to 1.5 . 10m3 for 100% ice coverage.Aircraft turbulence measurements are used to compare the model values of Cdn with measurements. The correlation between measured and modelled drag coefficients results in r2 = 0.9 1, where r is the correlation coefficient.

Abstract. The influence of a single pressure ridge of 4.5 m height on the structure of the atmospheric surface layer is studied. The field of the mean wind velocity is governed by typical features of a Bernoulli effect with a speedup over the crest and a shadowing effect downwind of the ridge. It is found that the turbulence generated by the ridge compensates for the deformation of the flow field by mixing momentum downward. Both mean and turbulent fields are restored to their upwind values at a distance of -• 300 m downwind of the ridge, which is equivalent to an aspect ratio of -• 0.015. The level of maximum turbulence generated by the ridge is characterized by a linear relationship. A formulation for the determination of the form drag of a single ridge is proposed and generalized toward an ensemble of ridges. We estimate that the form drag contributes < 50 % to the total drag exerted by a typically ice covered sea surface on the atmospheric flow.

This paper presents air‐ice and ice‐water drag coefficients referenced to 10‐m‐height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP‐86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS‐89). The optimal values of the air‐ice drag coefficients, made from turbulent flux measurements, are C10 = (1.79 ± 0.06) × 10−3 for WWSP‐86 and (1.45 ± 0.09) × 10−3 for WWGS‐89. Neutral drag coefficient values are CN10 = 1.68 × 10−3 for WWSP‐86 and 1.44 × 10−3 for WWGS‐89. The slightly lower values for WWGS‐89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median z¯0=0.47 mm for WWSP‐86 and 0.27 mm for WWGS‐89). These values are consistent with Arctic measurements for 80–100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice‐water drag coefficient for both WWSP‐86 and WWGS‐89, estimated from periods of ice drift thought to represent free‐drift conditions (air‐ice stress balanced by ice‐water drag and Coriolis force), is (1.13 ± 0.26) × 10−3, and the ice‐water turning angle β¯=18±18° . This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first‐year Arctic ice. Consistent with previous findings for WWSP‐86, the free‐drift form of the momentum balance can be used to describe the observed WWGS‐89 ice drift observations by using an “effective” drag coefficient and turning angle that subsume the influence of ice‐ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by ∼33%.

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