This paper estimates the air/sea drag coefficient for first-year ice from recent aircraft measurements and reconciles the range of observed drag coefficients (103CD = 1.2–3.7 referenced to 10 m) for all sea ice types, based on ice roughness and seasonal meteorology. For the purpose of sea ice modeling, it is necessary to define an effective drag coefficient which relates regional stress to regional wind, because sea ice is heterogeneous on scales less than 20 km. Regional stress is influenced by the distribution of surface roughness, the buoyancy flux from quasi-periodic leads, and external atmospheric conditions, principally the inversion height. 103CD is 1.3–1.5 for smooth ice but is much greater for nonflat surfaces. For wind speeds greater than 5 m/s and air temperatures below freezing, the effective 103CD is 2.5–3.0 for nearly continuous pack ice, such as first-year ice in seasonal ice zones and central Arctic basin. The range of values of 103CD is 3.0–3.7 for unstable boundary layers typical of off-ice winds in the marginal ice zone (MIZ) or even greater if the ice has been broken by a recent storm. CD values at the lower end of these ranges are associated with low inversion heights. These coefficients are confirmed by 103CD of 2.9, 2.5, and 3.1 for first-year sea ice calculated from gust probe data collected by the NOAA P-3 aircraft, interior to the inner MIZ in the Bering Sea during the Marginal Ice Zone Experiment (MIZEX-West) in February 1983, and 103CD of 2.6 for the Arctic calculated from Arctic Ice Dynamics Joint Experiment (AIDJEX) aircraft data for February 1976. The effective drag coefficient with the presence of even a small concentration of sea ice is greater than the oceanic value as shown by a 103CD of 2.2 calculated from NOAA P-3 gust probe data over a 40-km track of 0.4 ice concentration in the outer MIZ of the Greenland Sea in June 1984 during MIZEX-84. The relation of surface wind and stress to the geostrophic wind for shallow inversion heights, typical of high latitudes, is reviewed with a turbulent closure atmospheric boundary layer model. The winter Arctic is typified by low inversions (<100 m), low geostrophic drag coefficients, and large geostrophic/surface wind turning angles, with low functional dependence on surface roughness. Seasonal ice zones are typified by a more modest influence of the inversion on boundary layer characteristics.

Atmospheric boundary layer structure and drag coefficients over sea ice

The surface of the Arctic Ocean in summer is a mix of sea ice and water in both leads and melt ponds. Here we use data collected at multiple sites during the year-long experiment to study the Surface Heat Budget of the Arctic Ocean (SHEBA) to develop a bulk turbulent flux algorithm for predicting the surface fluxes of momentum and sensible and latent heat over the Arctic Ocean during summer from readily measured or modelled quantities. The distinctive aerodynamic feature of summer sea ice is that the leads and melt ponds create vertical ice faces that the wind can push against; momentum transfer to the surface is thus enhanced through form drag. In effect, summer sea ice behaves aerodynamically like the marginal ice zone, which is another surface that consists of sea ice and water. In our bulk flux algorithm, we therefore combine our SHEBA measurements of the neutral-stability drag coefficient at a reference height of 10 m, CDN10, with similar measurements from marginal ice zones that have been reported in the literature to create a unified parametrization for CDN10 for summer sea ice and for any marginal ice zone. This parametrization predicts CDN10 from a second-order polynomial in ice concentration. Our bulk flux algorithm also includes expressions for the roughness lengths for temperature and humidity, introduces new profile stratification corrections for stable stratification, and effectively eliminates the singularities that often occur in iterative flux algorithms for very light winds. In summary, this new algorithm seems capable of estimating the friction velocity u∗ (a surrogate for the momentum flux) over summer sea ice with an absolute accuracy of 0.02 –0.03 m s −1 ; the sensible heat flux, with an accuracy of about 6 W m −2 ; and the latent heat flux, with an accuracy of 3.5 W m −2 .C opyright c � 2010 Royal Meteorological Society

https://people.nwra.com/resumes/andreas/publications/Andreas_etal_QJRMS_2010.pdf

Parametrizing turbulent exchange over summer sea ice and the marginal ice zone

[1] During our yearlong participation in the Surface Heat Budget of the Arctic Ocean experiment (SHEBA), we found the measured relative humidity, figured for saturation with respect to ice, to almost always be near 100%. Often, multiple humidity sensors even showed supersaturation. Four months of observations over sea ice in the Antarctic showed the same behavior. These frequent, ubiquitous, and reproducible measurements are too compelling to discount. We hypothesize that the high relative humidity is a consequence of plentiful water vapor given up by leads and polynyas. We thus develop a simple time-dependent vapor budget model that we solve analytically to assess the role of leads in supplying water vapor to the polar atmospheric boundary layer. The solution to that model shows that (1) because the polar marine boundary layer is generally thin, its timescale for reaching moisture equilibrium is much shorter than the timescale of the synoptic processes that tend to disrupt equilibrium, and (2) because they have relatively warm surfaces, open leads and polynyas supply water vapor more rapidly than the surrounding sea ice surface can remove it, despite an open water fractional area that may be only 5%. In concert, the two processes commonly lead to water vapor densities in the boundary layer over sea ice that are near the value for ice saturation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JC000411

Near-surface water vapor over polar sea ice is always near ice saturation

A coupled aerosol/climate model, elaborated on the basis of a general circulation model of the atmosphere, is used to study the features of desert dust and sea-salt aerosols in the Antarctic region. Some of the observed seasonal characteristics of the two tracers are well simulated, and they are interpreted in terms of their relations with components of the atmospheric circulation. The model exhibits a strong influence of the boundary layer stability on the aerosols' vertical distributions near the surface. Observation at surface level may therefore be very misleading with respect to concentrations and seasonal variability higher in the atmosphere. An ice age experiment is run with the same aerosol/climate model, but fails to reproduce an expected large increase of dust and sea-salt concentrations in surface snow. The simulated enhancements of the production rates and atmospheric transport efficiency are weak. Changes in the distribution of the sources of dust, ignored in the experiment, could therefore have largely contributed to the last ice age dust increase. Sea-salt results suggest that the aerosol/climate model shortcomings are nevertheless not confined to source uncertainties, unless the complexity of the sources of sea-salt aerosol is also overlooked, for instance with respect to their relations with partial sea-ice cover. DOI: 10.1034/j.1600-0889.1992.00014.x

https://www.tandfonline.com/doi/pdf/10.3402/tellusb.v44i4.15464

Simulations of desert dust and sea-salt aerosols in Antarctica with a general circulation model of the atmosphere

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.

Ice drift and momentum exchange in winter Antarctic pack ice

During a traverse of the Antarctic marginal ice zone (MIZ) near the Greenwich Meridian in October 1981, we launched a series of radiosondes along a 150-km track starting at the ice edge. Since the wind was from the north, off the ocean, these radiosonde profiles showed profound modification of the atmospheric boundary layer (ABL) as the increasing surface roughness decelerated the flow. The primary manifestation of this modification was a lifting of the inversion layer with increasing distance from the ice edge by the induced vertical velocity. But there was also a cooling of the stably stratified mixed layer below the inversion and a consequent flux of sensible heat to the surface that averaged over 200 W/m2. The magnitude of this flux suggests that atmospheric heat transport plays a significant role in the destruction of ice in the Antarctic MIZ. Using the rising of the inversion and ABL similarity theory, we estimated the change in the neutral stability drag coefficient, CD, across the MIZ. CD increased from its open ocean value, 1.2×10−3, at the ice edge to 4.0×10−3 at 80–90% ice concentration. We present an equation for this dependence of drag on ice concentration that should be useful for modeling the surface stress in marginal ice zones.

Atmospheric boundary‐layer modification, drag coefficient, and surface heat flux in the Antarctic marginal ice zone

The polar regions are a very important part of the global climate, as they are a sink for the energy created in the Tropics. An integral component of the polar system is sea ice. Its presence transforms the exchange of energy between the atmosphere and the ocean. Sea ice also effects climate more generally through the ice-snow albedo feedback mechanism (Curry et al. 1995) and through the oceanic feedbacks involving ice growth and melt and the freshwater balance at the ice-ocean interface (Stocker et al. 2001). Several global models have simulated under increased-CO2 scenarios that warming is greatest in the Arctic enhanced by the retreat and thinning of the sea ice (Randall et al. 1998, Houghton et al. 1990). These models show large differences between the warming of the temperatures in this region (Houghton et al. 1990, Gates et al. 1996). These models are even problematic in their control simulations in this region. For instance, the newest version of the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM) is too warm by 8 K in the polar regions and has sea ice that is too thin in the Arctic (Kiehl and Gent 2004). These problems could be partially due to their parameterizations of the surface energy balance. Small changes in this have been shown to effect model ice thickness (Bitz and Lipscomb 1999). The net surface energy flux

Edgar L. Andreas

Papers

Atmospheric boundary‐layer modification, drag coefficient, and surface heat flux in the Antarctic marginal ice zone

An intercomparison of bulk aerodynamic algorithms over sea ice during the Surface Heat Budget of the Arctic Ocean experiment