Browsing by Author "Geiger, Cathleen A."
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Item Antarctic sea-ice thickness and volume estimates from ice charts between 1995 and 1998(International Glaciological Society, 2015-10-01) Bernstein, E. Rachel; Geiger, Cathleen A.; DeLiberty, Tracy L.; Lemcke-Stampone, Mary D.; E. Rachel BERNSTEIN, Cathleen A. GEIGER, Tracy L. DeLIBERTY, Mary D. LEMCKE-STAMPONE; Bernstein, E. Rachel; Geiger, Cathleen A.; DeLiberty, Tracy L.This work evaluates two distinct calculations of central tendency for sea-ice thickness and quantifies the impact such calculations have on ice volume for the Southern Ocean. The first calculation, area-weighted average thickness, is computed from polygonal ice features and then upscaled to regions. The second calculation, integrated thickness, is a measure of the central value of thickness categories tracked across different scales and subsequently summed to chosen regions. Both methods yield the same result from one scale to the next, but subsequent scales develop diverging solutions when distributions are strongly non-Gaussian. Data for this evaluation are sea-ice stage-ofdevelopment records from US National Ice Center ice charts from 1995 to 1998, as proxy records of ice thickness. Results show regionally integrated thickness exceeds area-weighted average thickness by as much as 60% in summer with as few as five bins in thickness distribution. Year-round, the difference between the two calculations yields volume differences consistently >10%. The largest discrepancies arise due to bimodal distributions which are common in ice charts based on current subjective-analysis protocols. We recommend that integrated distribution be used for regional-scale sea-ice thickness and volume estimates from ice charts and encourage similar testing of other large-scale thickness data archives.Item Coincident buoy- and SAR-derived surface fluxes in the western Weddell Sea during Ice Station Weddell 1992(American Geophysical Union, 2005) Drinkwater, M. R.; Geiger, Cathleen A.; Geiger, Cathleen A., Drinkwater, M. R.; Geiger, Cathleen A.We examine sea ice kinematics relevant to surface fluxes using ERS-1 synthetic aperture radar (SAR) images coincident with buoys in the western Weddell Sea in austral autumn of 1992. Careful matching of temporal and spatial scales shows that buoy- and SAR-derived velocities differ in root-mean-square error (RMSE) by 0.6 cm s(-1) and 7.80 degrees in magnitude and direction, respectively. These values represent agreements of 91.3% and 92.7%, respectively, and correspond to instrument uncertainties. Scaling analysis shows that shear matching is best at the smallest scales (<= 5 km), while divergence is better represented at scales of 40 km and larger. Sensitivity to error propagation shows lower agreement for divergence (47.4%; RMSE = 7.46 x 10-(8) s(-1)), but we find these results sufficient for integrated surface flux comparisons. Using a toy model, we test the effects of aliasing in surface flux determination. The results show that variability associated with storms, ocean tides, inertial oscillations, and other high-frequency forcing affects integrated sea ice growth rates along this continental slope location. Integrated salt and new ice production rates computed from buoys are found to be two times larger than those computed from ERS-1 SAR motion products. We show that these differences in salt and ice production rates result primarily from inadequate temporal resolution of heat flux variability and sea ice divergence. Comparison with other studies shows that the problem is widespread, thereby impacting the modeling of sea ice mass balance and variability. The small-scale processes cited here have significant ramifications for larger scales and the global thermohaline circulation.Item Full-physics 3-D heterogeneous simulations of electromagnetic induction fields on level and deformed sea ice(International Glaciological Society, 2015-10-01) Samluk, Jesse P.; Geiger, Cathleen A.; Weiss, Chester J.; Jesse P. SAMLUK, Cathleen A. GEIGER, Chester J. WEISS; Samluk, Jesse P.; Geiger, Cathleen A.In this paper we explore simulated responses of electromagnetic (EM) signals relative to in situ field surveys and quantify the effects that different values of conductivity in sea ice have on the EM fields. We compute EM responses of ice types with a three-dimensional (3-D) finite-volume discretization of Maxwell’s equations and present 2-D sliced visualizations of their associated EM fields at discrete frequencies. Several interesting observations result: First, since the simulator computes the fields everywhere, each gridcell acts as a receiver within the model volume, and captures the complete, coupled interactions between air, snow, sea ice and sea water as a function of their conductivity; second, visualizations demonstrate how 1-D approximations near deformed ice features are violated. But the most important new finding is that changes in conductivity affect EM field response by modifying the magnitude and spatial patterns (i.e. footprint size and shape) of current density and magnetic fields. These effects are demonstrated through a visual feature we define as ‘null lines’. Null line shape is affected by changes in conductivity near material boundaries as well as transmitter location. Our results encourage the use of null lines as a planning tool for better ground-truth field measurements near deformed ice types.Item Impact of spatial aliasing on sea-ice thickness measurements(International Glaciological Society, 2015-10-01) Geiger, Cathleen A.; Müller, Hans-Reinhard; Samluk, Jesse P.; Bernstein, E. Rachel; Richter-Menge, Jacqueline A.; Cathleen GEIGER, Hans-Reinhard MÜLLER, Jesse P. SAMLUK, E. Rachel BERNSTEIN, Jacqueline RICHTER-MENGE; Geiger, Cathleen A.; Samluk, Jesse P.; Bernstein, E. RachelWe explore spatial aliasing of non-Gaussian distributions of sea-ice thickness. Using a heuristic model and >1000 measurements, we show how different instrument footprint sizes and shapes can cluster thickness distributions into artificial modes, thereby distorting frequency distribution, making it difficult to compare and communicate information across spatial scales. This problem has not been dealt with systematically in sea ice until now, largely because it appears to incur no significant change in integrated thickness which often serves as a volume proxy. Concomitantly, demands are increasing for thickness distribution as a resource for modeling, monitoring and forecasting air–sea fluxes and growing human infrastructure needs in a changing polar environment. New demands include the characterization of uncertainties both regionally and seasonally for spaceborne, airborne, in situ and underwater measurements. To serve these growing needs, we quantify the impact of spatial aliasing by computing resolution error (Er) over a range of horizontal scales (x) from 5 to 500 m. Results are summarized through a power law (Er = bxm) with distinct exponents (m) from 0.3 to 0.5 using example mathematical functions including Gaussian, inverse linear and running mean filters. Recommendations and visualizations are provided to encourage discussion, new data acquisitions, analysis methods and metadata formats.Item Large-scale comparison between buoy and SSM/I drift and deformation in the eurasian basin during winter 1992-1993(American Geophysical Union, 2000) Geiger, Cathleen A.; Zhao, Y. H.; Liu, A. K.; Hakkinen, S.; Geiger, Cathleen A., Zhao, Y. H., Liu, A. K., Hakkinen, S.; Geiger, Cathleen A.A method for comparing sea ice velocity, divergence, and shear at the large-scale between buoys and Special Sensor Micro wave Imager (SSM/I) is presented. For initial testing, the method is applied in the Eurasian Basin because of its relatively simple circulation dominated by the wind. Using eight Argos buoys, 11 strain rate arrays 100-600 km in size are constructed. Daily 100 km resolution sea ice motion derived from SSM/I 85 GHz brightness temperatures is sampled 100-1000 km from the center of the buoy arrays. Over this range of possible scales, a minimum RMS difference (RMSD) for deformation is used to identify an optimal inclusion radius of 600 km corresponding to a length scale of 1000 km. This length scale is typical of local storms confirming a strong connection between wind and observed sea ice motion. On the basis of all II arrays, an average RMSD of 2.48 +/- 0.05 cm s(-1) for velocity vector and 8.8 +/- 0.9 x 10(-8) s(-1) using all four deformation components (partial derivative u(i)/partial derivative x(j)) is found at the optimal inclusion radius corresponding to average correlation coefficients of 0.896 +/- 0.002 and 0.729 +/- 0.030, respectively. RMSD are found to scale with the temporal and spatial uncertainties of the SSM/I suggesting that even better results can be achieved with higher resolution instruments.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 On the uncertainty of sea-ice isostasy(International Glaciological Society, 2015-10-01) Geiger, Cathleen A.; Wadhams, Peter; Müller, Hans-Reinhard; Richter-Menge, Jacqueline A.; Samluk, Jesse P.; DeLiberty, Tracy L.; Corradina, Victoria; Cathleen GEIGER, Peter WADHAMS, Hans-Reinhard MÜLLER, Jacqueline RICHTER-MENGE, Jesse SAMLUK, Tracy DELIBERTY, Victoria CORRADINA1; Geiger, Cathleen A.; DeLiberty, Tracy L.; Corradina, Victoria; Samluk, Jesse P.During late winter 2007, coincident measurements of sea ice were collected using various sensors at an ice camp in the Beaufort Sea, Canadian Arctic. Analysis of the archived data provides new insight into sea-ice isostasy and its related R-factor through case studies at three scales using different combinations of snow and ice thickness components. At the smallest scale (<1 m; point scale), isostasy is not expected, so we calculate a residual and define this as ��� (‘zjey’) to describe vertical displacement due to deformation. From 1 to 10m length scales, we explore traditional isostasy and identify a specific sequence of thickness calculations which minimize freeboard and elevation uncertainty. An effective solution exists when the R-factor is allowed to vary: ranging from 2 to 12, with mean of 5.17, mode of 5.88 and skewed distribution. At regional scales, underwater, airborne and spaceborne platforms are always missing thickness variables from either above or below sea level. For such situations, realistic agreement is found by applying small-scale skewed ranges for the R-factor. These findings encourage a broader isostasy solution as a function of potential energy and length scale. Overall, results add insight to data collection strategies and metadata characteristics of different thickness products.Item The relation between Arctic sea ice surface elevation and draft: A case study using coincident AUV sonar and airborne scanning laser(American Geophysical Union, 2011) Geiger, Cathleen A.; Wadhams, Peter; Skourup, H.; Doble, M. J.; Doble, M. J., Skourup, H., Wadhams, P., Geiger, Cathleen A.; Geiger, Cathleen A.Data are presented from a survey by airborne scanning laser profilometer and an AUV-mounted, upward looking swath sonar in the spring Beaufort Sea. The air-snow (surface elevation) and water-ice (draft) surfaces were mapped at 1 x 1 m resolution over a 300 x 300 m area. Data were separated into level and deformed ice fractions using the surface roughness of the sonar data. The relation (R = d/f) between draft, d, and surface elevation, f, was then examined. Correlation between top and bottom surfaces was essentially zero at full resolution, requiring averaging over patches of at least 11 m diameter to constrain the relation largely because of the significant error (similar to 15 cm) of the laser instrument. Level ice points were concentrated in two core regions, corresponding to level FY ice and refrozen leads, with variations in R attributed primarily to positive snow thickness variability. Deformed ice displayed a more diffuse "cloud," with draft having a more important role in determining R because of wider deformed features underwater. Averaging over footprints similar to satellite altimeters showed the mean surface elevation (typical of ICESat) to be stable with averaging scale, with R = 3.4 (level) and R = 4.2 (deformed). The "minimum elevation within a footprint" characteristic reported for CryoSat was less stable, significantly overestimating R for level ice (R > 5) and deformed ice (R > 6). The mean draft difference between measurements and isostasy suggests 70 m as an isostatic length scale for level ice. The isostatic scale for deformed ice appears to be longer than accessible with these data (>300 m).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 Simulated low-frequency modes of circulation in the Arctic Ocean(American Geophysical Union, 2000) Hakkinen, S.; Geiger, Cathleen A.; Hakkinen, S., Geiger, Cathleen A.; Geiger, Cathleen A.The variability of the Arctic circulation is investigated for a 43 year period (1951-1993) from a coupled ice-ocean model. Empirical orthogonal function (EOF) analysis shows that the variability of the sea surface height (SSH) and vertically integrated transport is organized so that in the leading mode the whole Arctic operates as a single gyre. The mode is associated with the Arctic Oscillation (AO) [Thompson and Wallace, 1998], and it explains over 70% of the variance in the vertically integrated transport and 25% of the SSH variability. The physical interpretation of this mode is derived to arise from its close connection to the Atlantic inflow to the Arctic. The mode shows a major shift toward cyclonic circulation in the end of the 1980s which is associated with a large multiyear pulse of Atlantic water to the Arctic. Thus this event appears as the likely initiation of the Atlantic laver warming observed during the recent years [Carmack et al., 1995]. Overall, the first mode shows strong decadal variability as reported by Proshurinsky and Johnson [1997]. The second mode of the oceanic circulation, which explains 9% of the variance in the transport, contains mio gyres with opposing cyclonicity in the Eurasia and Canada basins. It projects onto the North Atlantic Oscillation (NAO) pattern and displays a 14 year cycle which is known to exist in the midlatitude North Atlantic surface temperatures [Deser and Blackmon, 1993]. A further examination reveals that this mode describes the variability of the flow through the Barents Sea, which is modulated by the water mass modification due to the local heat flux variability. The apparent NAO connection is provided by a simultaneous correlation between the time series of this second mode and the leading heat flux mode in the North Atlantic which is associated with NAG.Item Spatial and temporal characterization of sea-ice deformation(International Glaciological Society, 2011) Hutchings, J. K.; Roberts, A.; Geiger, Cathleen A.; Richter-Menge, Jacqueline A.; Hutchings, J. K., Roberts, A., Geiger, Cathleen A., Richter-Menge, J.; Geiger, Cathleen A.In late March 2007 an array of GPS ice drifters was deployed in the Beaufort Sea as part of the Sea Ice Experiment: Dynamic Nature of the Arctic (SEDNA). The drifters were deployed in an array designed to resolve four, nested spatial scales of sea-ice deformation, from 10 to 140 km, with the arrays maintaining appropriate shape for strain-rate calculation until mid-June. In this paper, we test whether sea-ice deformation displays fractal properties in the vicinity of SEDNA. We identify that deformation time series have different spectral properties depending on the spatial scale. At the scales around 100 km, deformation is a red-noise process, indicating the importance of the ice-pack surface forcing in determining the deformation rate of sea ice at this scale. At smaller scales, the deformation becomes an increasingly whiter process (it has pink noise properties), which suggests an increasing role of dissipative processes at smaller scales. At spatial scales of 10-100 km, and sub-daily scales, there is no deformation coherence across scales; coherence only becomes apparent at longer scales greater than 100 km. The lack of coherence at small scales aids in understanding previous observations where correlation between 10 km regions adjacent to each other varied widely, with correlation coefficients between 0.3 and 1. This suggests it is not appropriate to think of sea ice as having a decorrelation length scale for deformation. We find that lead scale observations of deformation are required when estimating ice growth in leads and ridging time series. For the two SEDNA arrays, we find coherence between 140 and 20 km scale deformation up to periods of 16 days. This suggests sea-ice deformation displays coherent deformation between 100 km scale and the scale of the Beaufort Sea (of order 1000 km), over synoptic time periods (daily to weekly timescales). Organization of leads at synoptic and larger scales is an emergent feature of the deformation field that is caused by the smooth variation of surface forcing (wind) on the ice pack.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.