Recent observational and modeling studies indicate that the Arctic sea-ice cover is undergoing significant climate-induced changes, affecting both its extent and thickness. The thickness or, more precisely, the mass balance of the ice cover is a key climate-change indicator since it is an integrator of both the surface heat budget and the ocean heat flux. Accordingly, efforts are underway to develop and deploy in situ observing systems which, when combined with satellite remote-sensing information and numerical models, can effectively monitor and attribute changes in the mass balance of the Arctic sea-ice cover. As part of this effort, we have developed an autonomous ice mass-balance buoy (IMB), which is equipped with sensors to measure snow accumulation and ablation, ice growth and melt, and internal ice temperature, plus a satellite transmitter. The IMB is unique in its ability to determine whether changes in the thickness of the ice cover occur at the top or bottom of the ice cover, and hence provide insight into the driving forces behind the change. Since 2000, IMBs have been deployed each spring from the North Pole Environmental Observatory and in several other areas, including a few in the Beaufort Sea and Central Basin. At this point, the collective time series is too short to draw significant and specific conclusions regarding interannual and regional variability in ice mass balance. Comparisons of available data indicate that ice surface ablation is greater in the Beaufort region (67-80 cm), relative to the North Pole (0-30 cm), consistent with a longer period of melt in the more southerly location. Ablation at the bottom of the ice (22 cm), maximum ice thickness (235 cm) and maximum snow depth (28 cm) were comparable in the two regions.

The von Kármán constant k relates the flow speed profile in a wall-bounded shear flow to the stress at the surface. Recent laboratory studies in aerodynamically smooth flow report k values that cluster around 0.42-0.43 and around 0.37-0.39. Recent data from the atmospheric boundary layer, where the flow is usually aerodynamically rough, are similarly ambiguous: k is often reported to be significantly smaller than the canonical value 0.40, and two recent data sets suggest that k decreases with increasing roughness Reynolds number Re *. To this discussion, we bring two large atmospheric data sets that suggest k is constant, 0.387 ± 0.003, for 2 6 Re * 6 100. The data come from our yearlong deployment on Arctic sea ice during SHEBA, the experiment to study the Surface Heat Budget of the Arctic Ocean, and from over 800 h of observations over Antarctic sea ice on Ice Station Weddell (ISW). These were superb sites for atmospheric boundary-layer research; they were horizontally homogeneous, uncomplicated by topography, and unobstructed and uniform for hundreds of kilometres in all directions. During SHEBA, we instrumented a 20 m tower at five levels between 2 and 18 m with identical sonic anemometer/thermometers and, with these, measured hourly averaged values of the wind speed U (z) and the stress τ (z) at each tower level z. On ISW, we measured the wind-speed profile with propeller anemometers at four heights between 0.5 and 4 m and measured τ with a sonic anemometer/thermometer at one height. On invoking strict quality controls, we gleaned 453 hourly U (z) profiles from the SHEBA set and 100 from the ISW set. All of these profiles reflect nearneutral stratification, and each exhibits a logarithmic layer that extends over all sampling heights. By combining these profiles and our measurements of τ , we made 553 independent determinations of k. This is, thus, the largest, most comprehensive atmospheric data set ever used to evaluate the von Kármán constant.

The Arctic is undergoing significant environmental changes due to climate warming. The most evident signal of this warming is the shrinking and thinning of the ice cover of the Arctic Ocean. If the warming continues, as global climate models predict, the Arctic Ocean will change from a perennially ice-covered to a seasonally ice-free ocean. Estimates as to when this will occur vary from the 2030s to the end of this century. One reason for this huge uncertainty is the lack of systematic observations describing the state, variability, and changes in the Arctic Ocean

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%.

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