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We used remote-sensing and in situ measurements of surface accumulation rate, ice surface velocity, thickness and elevation to evaluate the mass budgets of grounded ice-flow regimes that form the Lambert Glacier-Amery Ice Shelf system. Three distinct drainage regimes are considered: the western and eastern margins of the ice shelf, and the southern grounding line at the major outlet glacier confluence, which can be identified with drainage zones 9, 11 and 10 respectively of Giovinetto and Zwally (2000). Our findings show the entire grounded portion of the basin is approximately in balance, with a mass budget of -4.2_9.8 Gt a-1. Drainages 9, 10 and 11 are within balance to the level of our measurement uncertainty, with mass budgets of -2.5_2.8 Gt a-1, -2.6_7.8 Gt a-1 and 0.9_2.3 Gt a-1, respectively. The region upstream of the Australian Lambert Glacier basin (LGB) traverse has a net mass budget of 4.4_6.3 Gt a-1, while the downstream region has -8.9_9.9 Gt a-1. These results indicate that glacier drainages 9, 10 and 11, upstream and downstream of the Australian LGB traverse, are in balance to within our measurement error.

We used remote-sensing and in situ measurements of surface accumulation rate, ice surface velocity, thickness and elevation to evaluate the mass budgets of grounded ice-flow regimes that form the Lambert Glacier-Amery Ice Shelf system. Three distinct drainage regimes are considered: the western and eastern margins of the ice shelf, and the southern grounding line at the major outlet glacier confluence, which can be identified with drainage zones 9, 11 and 10 respectively of Giovinetto and Zwally (2000). Our findings show the entire grounded portion of the basin is approximately in balance, with a mass budget of -4.2_9.8 Gt a-1. Drainages 9, 10 and 11 are within balance to the level of our measurement uncertainty, with mass budgets of -2.5_2.8 Gt a-1, -2.6_7.8 Gt a-1 and 0.9_2.3 Gt a-1, respectively. The region upstream of the Australian Lambert Glacier basin (LGB) traverse has a net mass budget of 4.4_6.3 Gt a-1, while the downstream region has -8.9_9.9 Gt a-1. These results indicate that glacier drainages 9, 10 and 11, upstream and downstream of the Australian LGB traverse, are in balance to within our measurement error.

We used in situ measurements and remote-sensing data sets to evaluate the mass budgets of the Lambert, Mellor and Fisher Glaciers and the basal melting and freezing rates beneath their flowbands on the Amery Ice Shelf. Our findings show the Lambert and Mellor Glaciers upstream of the ANARE Lambert Glacier Basin (LGB) traverse may have positive imbalances of 3.9±2.1 Gt a−1 and 2.1±2.4 Gt a−1, respectively, while the Fisher Glacier is approximately in balance. The upstream region as a whole has a positive imbalance of 5.9±4.9 Gt a−1. The three same glaciers downstream of the ANARE LGB traverse line are in negative imbalance, where the whole downstream region has a negative imbalance of −8.5±5.8 Gt a−1. Overall the mass budgets of the Lambert, Mellor, and Fisher Glaciers are close to balance, and the collective three-glacier system is also nearly in balance with a mass budget of −2.6±6.5 Gt a−1. The significant positive imbalances for the interior basin upstream of the ice-movement stations established in the early 1970s (GL line) reported previously are possibly due to an overestimate of the total accumulation and an underestimate of the ice flux through the GL line.The mean melting rate is −23.0±3.5 m ice a−1 near the southern grounding line, which decreases rapidly downstream, and transitions to refreezing at around 300 km from the southern extremity of the Amery Ice Shelf. Freezing rates along the flowbands are around 0.5±0.1 to 1.5±0.2 m ice a−1. The percentage of ice lost from the interior by basal melting beneath the flowbands is about 80%±5%. The total basal melting and refreezing beneath the three flowbands is 50.3±7.5 Gt ice a−1 and 7.0±1.1 Gt ice a−1, respectively. We find a much larger total basal melting and net melting than the results for the whole Amery Ice Shelf derived from previous modeling and oceanographic measurements.

We used in situ measurements and remote-sensing data sets to evaluate the mass budgets of the Lambert, Mellor and Fisher Glaciers and the basal melting and freezing rates beneath their flowbands on the Amery Ice Shelf. Our findings show the Lambert and Mellor Glaciers upstream of the ANARE Lambert Glacier Basin (LGB) traverse may have positive imbalances of 3.9±2.1 Gt a−1 and 2.1±2.4 Gt a−1, respectively, while the Fisher Glacier is approximately in balance. The upstream region as a whole has a positive imbalance of 5.9±4.9 Gt a−1. The three same glaciers downstream of the ANARE LGB traverse line are in negative imbalance, where the whole downstream region has a negative imbalance of −8.5±5.8 Gt a−1. Overall the mass budgets of the Lambert, Mellor, and Fisher Glaciers are close to balance, and the collective three-glacier system is also nearly in balance with a mass budget of −2.6±6.5 Gt a−1. The significant positive imbalances for the interior basin upstream of the ice-movement stations established in the early 1970s (GL line) reported previously are possibly due to an overestimate of the total accumulation and an underestimate of the ice flux through the GL line.The mean melting rate is −23.0±3.5 m ice a−1 near the southern grounding line, which decreases rapidly downstream, and transitions to refreezing at around 300 km from the southern extremity of the Amery Ice Shelf. Freezing rates along the flowbands are around 0.5±0.1 to 1.5±0.2 m ice a−1. The percentage of ice lost from the interior by basal melting beneath the flowbands is about 80%±5%. The total basal melting and refreezing beneath the three flowbands is 50.3±7.5 Gt ice a−1 and 7.0±1.1 Gt ice a−1, respectively. We find a much larger total basal melting and net melting than the results for the whole Amery Ice Shelf derived from previous modeling and oceanographic measurements.

The temporal and spatial variability of the annual accumulation rate and the mass budgets of five sub-basins of the Lambert GlacierAmery Ice Shelf system (LAS), East Antarctica, at high elevations are assessed using a variety of datasets derived from field measurements and modeling. The annual temporal variations of the accumulation rate for four cores from the west and east sides of the LAS are around ±34%. Decadal fluctuation of the accumulation from the DT001 firn core drops to ±10%, and the 30 year fluctuation to ±5%, which is assumed to contain the information about the regional and long-term trend in accumulation. The 15-point running mean of the annual accumulation rate derived from stake measurements can remove most of the high-frequency spatial variation so as to better represent the local accumulation. Model simulations show that the spatial variability of erosion/ deposition of snow by the wind has a noticeable impact on the surface mass balance at the higher parts of the LAS. Mass-budget estimates at high-elevation sub-basins of the LAS suggest drainage 9 has a negative imbalance of 0.7±0.4 Gt a1, Lambert and Mellor Glaciers have a positive imbalance of 3.9±2.1 and 2.1±2.4 Gt a1 respectively, and Fisher Glacier and drainage 11 are approximately in balance. The higher-elevation region as a whole has a positive mass imbalance of 4.4±6.3 Gt a1, which is consistent with the most recent radar altimetry assessment that shows an overall thickening over this region.

The temporal and spatial variability of the annual accumulation rate and the mass budgets of five sub-basins of the Lambert GlacierAmery Ice Shelf system (LAS), East Antarctica, at high elevations are assessed using a variety of datasets derived from field measurements and modeling. The annual temporal variations of the accumulation rate for four cores from the west and east sides of the LAS are around ±34%. Decadal fluctuation of the accumulation from the DT001 firn core drops to ±10%, and the 30 year fluctuation to ±5%, which is assumed to contain the information about the regional and long-term trend in accumulation. The 15-point running mean of the annual accumulation rate derived from stake measurements can remove most of the high-frequency spatial variation so as to better represent the local accumulation. Model simulations show that the spatial variability of erosion/ deposition of snow by the wind has a noticeable impact on the surface mass balance at the higher parts of the LAS. Mass-budget estimates at high-elevation sub-basins of the LAS suggest drainage 9 has a negative imbalance of 0.7±0.4 Gt a1, Lambert and Mellor Glaciers have a positive imbalance of 3.9±2.1 and 2.1±2.4 Gt a1 respectively, and Fisher Glacier and drainage 11 are approximately in balance. The higher-elevation region as a whole has a positive mass imbalance of 4.4±6.3 Gt a1, which is consistent with the most recent radar altimetry assessment that shows an overall thickening over this region.

This study examines differences in the normalized radar cross section, derived from ground-based versus spaceborne radar data. A simple homogeneous half-space model, indicates that agreement between the two improves as 1) the distance from the scatterer is increased; and/or 2) the extinction coefficient increases.

Measurements of ice thickness on the Antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database. From these data, a seamless suite of digital topographic models have been compiled for Antarctica and its surrounding ocean. This includes grids of ice sheet thickness over the grounded ice sheet and ice shelves, water column thickness beneath the floating ice shelves, bed elevation beneath the grounded ice sheet, and bathymetry to 60°S, including the sub-ice-shelf cavities. These grids are consistent with a recent high-resolution surface elevation model of Antarctica. While the digital models have a nominal spatial resolution of 5 km, such high resolution is justified by the original data density only over a few parts of the ice sheet. The suite does, however, provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps. The total volume of the Antarctic ice sheet calculated from the BEDMAP grid is 25.4 million km3, and the total sea level equivalent, derived from the amount of ice contained within the grounded ice sheet, is 57 m, comprising 52 m from the East Antarctic ice sheet and 5 m from the West Antarctic ice sheet, slightly less than earlier estimates. The gridded data sets can be obtained from the authors.

Measurements of ice thickness on the Antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database. From these data, a seamless suite of digital topographic models have been compiled for Antarctica and its surrounding ocean. This includes grids of ice sheet thickness over the grounded ice sheet and ice shelves, water column thickness beneath the floating ice shelves, bed elevation beneath the grounded ice sheet, and bathymetry to 60°S, including the sub-ice-shelf cavities. These grids are consistent with a recent high-resolution surface elevation model of Antarctica. While the digital models have a nominal spatial resolution of 5 km, such high resolution is justified by the original data density only over a few parts of the ice sheet. The suite does, however, provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps. The total volume of the Antarctic ice sheet calculated from the BEDMAP grid is 25.4 million km3, and the total sea level equivalent, derived from the amount of ice contained within the grounded ice sheet, is 57 m, comprising 52 m from the East Antarctic ice sheet and 5 m from the West Antarctic ice sheet, slightly less than earlier estimates. The gridded data sets can be obtained from the authors.

An ultra-wideband radar and a plane wave antenna have been used to measure the high resolution scattering response of bare saline ice, snow covered ice and pancake ice during the winter seasons of 1994 and 1995 at the US Army Cold Regions Research and Engineering Laboratory (CRREL). The objectives of these experiments were to study various mechanisms for simulating roughness and to understand scattering mechanisms better. These backscatter measurements were made at 2-18 GHz and 0.5-16.5 GHz during the 1994 and 1995 experiments respectively, and were made for incidence angles ranging from 0 to 55 deg. These broadband measurements provide a vigorous test for models at many frequencies using a single system instead of many single frequency systems.

Radar backscatter measurements from sea ice during the fall freeze-up were performed from aboard the United States Coast Guard Icebreaker Polar Star as a part of the International Arctic Ocean Expedition (IAOE'91) in August-September, 1991. The US portion of the experiment took place on board the Polar Star and has been referred to TRAPOLEX'91 (Transpolar expedition) by some investigators. Before prematurely aborting its mission because of mechanical failure of her port shaft the Polar Star reached 84 degrees 57' N. latitude at 35 degrees E. longitude. The ship was in the ice (>50% coverage) from 14 August until 3 September and was operational for all but 6 days due to two instances of mechanical problems with the port shaft. The second was fatal to the ship's participation in the expedition., Prepared in cooperation with Ohio State University, Columbus, OH.

Radar backscatter measurements were performed on artificially grown sea ice on an outdoor pond at the US. Army Cold Regions Research and Engineering Laboratory (CRREL) during the 1990 winter season. Measurements of microwave backscatter from simulated sea ice demonstrate that the dominant backscatter mechanism is the surface scatter. Both the co-polarized and cross-polarized measurements compare favorably with the predictions of surface scattering models at two frequencies. The surface parameters used in the models were measured directly from photographs of cross sections of the saline ice. Therefore, since two frequencies of backscatter measurements were predicted accurately by surface scatter models, the predominate backscatter must be from the surface., Prepared in cooperation with Ohio State Univ, Byrd Polar Research Center, Columbus, Ohio.

In June 1991, the NASA/Jett Propulsion Laboratory airborne SAR (AIRSAR) collected the first calibrated multi-channel SAR observations of the Greenland ice sheet. Large changes in radar scattering are detected across different melting zones. In the drysnow zone, Rayleigh scattering from small snow grains dominates at C-band. In the soaked-snow zone, surface scattering dominates, and an inversion technique was developed to estimate the dielectric constant of the snow. The radar properties of the percolation zone are in contrast unique among terrestrial surfaces, but resemble those from the icy Galilean satellites. The scatterers responsible for the percolation zone unusual echoes are the massive ice bodies generated by summer melt in the cold, dry, porous firm. An inversion model is developed for estimating the volume of melt-water ice retained each summer in the percolation zone from multi-channel SAR data. The results could improve current estimates of the mass balance of Greenland, and could help monitor spatial and temporal changes in the strength of summer melt in Greenland with a sensitivity greater than that provided by altimeters.

In June 1991, the NASA/Jett Propulsion Laboratory airborne SAR (AIRSAR) collected the first calibrated multi-channel SAR observations of the Greenland ice sheet. Large changes in radar scattering are detected across different melting zones. In the drysnow zone, Rayleigh scattering from small snow grains dominates at C-band. In the soaked-snow zone, surface scattering dominates, and an inversion technique was developed to estimate the dielectric constant of the snow. The radar properties of the percolation zone are in contrast unique among terrestrial surfaces, but resemble those from the icy Galilean satellites. The scatterers responsible for the percolation zone unusual echoes are the massive ice bodies generated by summer melt in the cold, dry, porous firm. An inversion model is developed for estimating the volume of melt-water ice retained each summer in the percolation zone from multi-channel SAR data. The results could improve current estimates of the mass balance of Greenland, and could help monitor spatial and temporal changes in the strength of summer melt in Greenland with a sensitivity greater than that provided by altimeters.

This report describes two new methods for computing borehole geometry from discrete measurements of borehole inclination (theta) and azimuth (alpha). In the first method theta and alpha are assumed to vary linearly with arc length. This results in an analytic model of the borehole that is continuous but not smooth. The second model, which takes theta and alpha to vary quadratically with arc length between three measuring points, improves the smoothness of the model but the analysis must be carried out numerically. These models were applied to the borehole inclinometry data collected at DYE-3, Greenland. The methods were found convenient to use and it is claimed that the results represent physically reasonable approximations to the borehole geometry. (Author)