Browsing by Author "Ackley, Stephen F."
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Item Large-scale sea ice drift and deformation: Comparison between models and observations in the western weddell sea during 1992(American Geophysical Union, 1998) Geiger, Cathleen A.; Hibler, W. D., III; Ackley, Stephen F.; Geiger, Cathleen A., Hibler, W. D., Ackley, S. F.; Geiger, Cathleen A.Statistical comparisons between numerical sea ice models and an observed large-scale strain array in the western Weddell Sea during 1992 are used to evaluate the performance of three of the more generally utilized sea ice rheology formulations. Results show that sea ice velocity is reproduced with relatively high accuracy (90% coherence, >80% normalized cross correlation) in models having high-quality atmospheric forcing fields (e.g., the European Centre for Medium-Range Weather Forecasts). On the other hand, temporal and spatial variability of the velocity field, as exemplified by progressive vector plots and ice deformation, respectively, are reproduced less accurately (coherence and normalized cross correlation <50%). In terms of model sensitivity, this means that deformation and temporal variability are more discriminating in terms of elucidating specifics about the constitutive relation and mechanical properties of sea ice on a large scale. For example, inclusion of both compressive and shear stresses is important in attaining a proper probability distribution of deformation relative to observations. Additional analysis shows that adjustments to specific model parameters improve the model results for either drift or select deformation components, but no best solution could be found, given the models examined here. Results suggest that inclusion of more physically based processes, such as subdaily tidal and inertial oscillations; reconsideration of the boundary layer formulation, and consideration of anisotropy, may be necessary to include in next-generation sea ice models, especially those that are intended for coupling with high-resolution (eddy resolving) ocean models.Item The Ronne polynya of 1997/98: observations of air-ice-ocean interaction(International Glaciological Society, 2001) Ackley, Stephen F.; Geiger, Cathleen A.; King, J. C. (John Christopher), 1955-; Hunke, E. C.; Comiso, J.; Ackley, S. F., Geiger, Cathleen A., King, J. C., Hunke, E. C., Comiso, J.; Geiger, Cathleen A.The Ronne polynya formed in the Weddell Sea, Antarctica, during the period November 1997-February 1998 to an extent not seen previously in the 25 years of all-weather satellite observations. The vessel HMS Endurance traversed the polynya region and took sea-ice, physical oceanographic and meteorological measurements during January and early February 1998. These observations, together with satellite imagery and weather records, were analyzed to determine the causes of the anomalous condition observed and to provide comparisons for numerical modeling experiments. The polynya area, analyzed from satellite imagery, showed a linear, nearly constant, increase with time from mid-November 1997 through February 1998. It had a maximum open-water area of 3 X 10(5) km(2) and extended 500 km north of the Ronne Ice Shelf (at 76degrees S) to 70degrees S. The ice and snow structure of floes at (lie northern edge of the polynya showed the ice there had formed in the previous mid- to late winter October 1997 or earlier) and had been advected there either from the eastern Weddell Sea or from the front of the Ronne Ice Shelf. Analyses of the wind fields showed anomalous spring -summer wind fields in the polynya year, with a strong southerly to southwesterly component compared to the mean easterly winds typical of summer conditions. These southerly wind conditions, ill both magnitude and direction, therefore account for the drift of ice northward. The predominant summer easterly winds usually fill the southern Weddell Sea with ice from the east, and the high-albedo surfaces reflect (lie solar radiation, preventing warming of the surface ocean waters and consequent sea-ice melt. Instead, high incident solar radiation From November 1997 to February 1998 was absorbed by (lie open water, rather than being reflected, thereby both melting ice and preventing ice formation, and thereby sustaining the polynya. We conclude that open-water-albedo feedback is necessary to allow the observed polynya formation, since similar drift conditions prevail in winter (arising from southerly winds also) and usually, result in extensive new ice formation in front of the Ronne Ice Shelf. The strong southerly winds therefore have quite opposing seasonal effects, leading to high ice production ill winter as usually found, and extensive open water if they occur in spring and Summer, as seen in this atypical event in 1997/98. In this case, the atypical southerly winds may be associated with all El Nino-Southern Oscillation (ENSO) -induced atmospheric circulation pattern.Item Thickness distribution of Antarctic sea ice(American Geophysical Union, 2008) Worby, A. P. (Anthony P.); Geiger, Cathleen A.; Paget, M. J.; Van Woert, Michael L.; Ackley, Stephen F.; DeLiberty, Tracy L.; Worby, A. P., Geiger, Cathleen A., Paget, M. J., Van Woert, M. L., Ackley, S. F.,; Geiger, Cathleen A.; DeLiberty, Tracy L.Ship-based observations are used to describe regional and seasonal changes in the thickness distribution and characteristics of sea ice and snow cover thickness around Antarctica. The data set comprises 23,373 observations collected over more than 2 decades of activity and has been compiled as part of the Scientific Committee on Antarctic Research (SCAR) Antarctic Sea Ice Processes and Climate (ASPeCt) program. The results show the seasonal progression of the ice thickness distribution for six regions around the continent together with statistics on the mean thickness, surface ridging, snow cover, and local variability for each region and season. A simple ridge model is used to calculate the total ice thickness from the observations of level ice and surface topography, to provide a best estimate of the total ice mass, including the ridged component. The long-term mean and standard deviation of total sea ice thickness (including ridges) is reported as 0.87 +/- 0.91 m, which is 40% greater than the mean level ice thickness of 0.62 m. Analysis of the structure function along north/south and east/west transects revealed lag distances over which sea ice thickness decorrelates to be of the order of 100-300 km, which we use as a basis for presenting near-continuous maps of sea ice and snow cover thickness plotted on a 2.5 degrees x 5.0 degrees grid.Item Year-round pack ice in the Weddell Sea, Antarctica: Response and sensitivity to atmospheric and oceanic forcing(International Glaciology Society, 1997) Geiger, Cathleen A.; Ackley, Stephen F.; Hibler, W. D., III; Geiger, Cathleen A., Ackley, S. F., Hibler, M. D.; Geiger, Cathleen A.Using a dynamic-thermodynamic numerical sea-ice model, external oceanic and atmospheric forcings on sea ice in the Weddell Sea are examined to identify physical processes associated with the seasonal cycle of pack ice, and to identify further the parameters that coupled models need to consider in predicting the response of the pack ice to climate and ocean-circulation changes. In agreement with earlier studies, the primary influence on the winter ice-edge maximum extent is air temperature. Ocean heat flux has more impact on the minimum ice-edge extent and in reducing pack-ice thickness, especially in the eastern Weddell Sea. Low relative humidity enhances ice growth in thin ice and open-water regions, producing a more realistic ice edge along the coastal areas of the western Weddell Sea where dry continental air has an impact. The modeled extent of the Weddell summer pack is equally sensitive to ocean heat flux and atmospheric relative humidity variations with the more dynamic responses being from the atmosphere. Since the atmospheric regime in the eastern Weddell is dominated by marine intrusions from lower latitudes, with high humidity already, it is unlikely that either the moisture transport could be further raised or that it could be significantly lowered because of its distance from the continent (the lower humidity source). Ocean heat-transport variability is shown to lead to overall ice thinning in the model response and is a known feature of the actual system, as evidenced by the occurrence of the Weddell Polynya in the mid 1970s.