C22A-01 INVITED 10:20h
GLAS observations of Antarctic ice shelf features: The vertical dimension
Since the launch of ICESat in January 2003, the Geoscience Laser Altimeter System (GLAS) has provided glaciologists with unprecedented topographic information for Antarctica, delivering elevation profiles containing exquisite detail. GLAS's small footprint (65m) reveals many important ice shelf features which had been heretofore unresolved by large-footprint satellite radar altimeters. For example, not only are we able to measure accurately the width of rifts that cut across ice shelves, but by careful re-tracking of individual GLAS waveforms we can also measure the thickness of the ice m\'{e}lange that fills these rifts and estimate how this changes with time. We illustrate this using several rifts of various ages on the Ross and the Amery Ice Shelves. GLAS also yields substantial advances over previous measurements of the topography of ice shelf fronts: the same re-tracking procedure allows us not only to locate the ice front along the ICESat ground track very precisely, but also to resolve topographic signatures across the front of some ice shelves which are consistent with significant basal melting.
C22A-02 10:35h
Combining ICESat and Aircraft Laser Altimetry Observations to Examine Recent Changes in Canadian Ice Caps
Precise repeat airborne laser surveys were conducted over the major ice caps in the Canadian Arctic Archipelago during the spring of 1995 and 2000 to measure elevation changes in the region. Our observations reveal thinning at lower elevations (below 1600 m) on most of the ice caps and glaciers, but either very little change or thickening at higher elevations in the ice cap accumulation zones. The behavior of the ice caps in the north on the Queen Elizabeth Islands can be explained by recent temperature and precipitation anomalies, but this is not the case for the more southern ice caps on Baffin Island, which appear to be still shrinking in response to the Little Ice Age. The regional characteristics of elevation change as a function of elevation enables an assessment of the Canadian ice caps' contribution to sea level during the 1995-2000 time period. Our estimates place them among the more significant sources of eustatic sea level rise, though they are not as substantial as Greenland ice sheet, Alaskan glaciers, or the Patagonian ice fields. The spring 2004 campaign of the Ice Cloud and land Elevation Satellite (ICESat) mission provides a means of examining the character of changes since 2000. Comparisons between the ICESat data and the earlier aircraft campaigns where the ICESat ground tracks intersect the aircraft flight lines reveal significant changes in ice cap behavior between the late 1990s and the last four years. The results of these comparisons will be discussed along with the differences in the 1995-2000 and 2000-2004 climate conditions that affect the mass balance and elevation characteristics in those time periods.
C22A-03 10:50h
Southern Alaska Glaciers: Spatial and Temporal Variations in Ice Volume
Although temperate mountain glaciers comprise less than 1% of the glacier-covered area on Earth, they are important because they appear to be melting rapidly under present climatic conditions and, therefore, make significant contributions to rising sea level. In this study, we use ICESat observations made in the last 1.5 years of southern Alaska glaciers to estimate ice elevation profiles, ice surface slopes and roughness, and bi-annual and/or annual ice elevation changes. We report initial results from the near coastal region between Yakutat Bay and Cape Suckling that includes the Malaspina and Bering Glaciers. We show and interpret ice elevations changes across the lower reaches of the Bagley Ice Valley for the period between October 2003 and May 2004. In addition, we use off-nadir pointing observations to reference tracks over the Bering and Malaspina Glaciers in order to estimate annual ice elevation change. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Shuttle Radar Topography Mission (SRTM) derived DEMs are used to estimate across track regional slopes between ICESat data acquisitions. Although the distribution and quantity of ICESat elevation profiles with multiple, exact repeat data is currently limited in Alaska, individual ICESat data tracks, provide an accurate reference surface for comparison to other elevation data (e.g. ASTER and SRTM X- and C-band derived DEMs). Specifically we report the elevation change over the Malaspina Glacier's piedmont lobe between a DEM derived from SRTM C-band data acquired in Feb. 2000 and ICESat Laser #2b data from Feb.-March 2004. We also report use of ICESat elevation data to enhance ASTER derived absolute DEMs. Mountain glaciers generally have rougher surfaces and steeper regional slopes than the ice sheets for which the ICESat design was optimized. Therefore, rather than averaging ICESat observations over large regions or relying on crossovers, we are working with well-located ICESat footprint returns to estimate glacier ice elevations and surface characteristics. To obtain the optimal ICESat results, we are reprocessing the ICESat data from Alaska to provide a well-calibrated regional ICESat solution. We anticipate that our ICESat results combined with earlier data will provide new constraints on the temporal and spatial variations in ice volume of individual Alaskan mountain ranges. These results allow us to address how recent melting of the southern Alaska glaciers contribute to short-term sea-level rise. Our results will also enable us to quantify crustal stress changes due to ice mass fluctuations and to assess the influence of ice mass changes on the seismically active southern Alaskan plate boundary zone.
C22A-04 11:05h
ICESat: Thin ice and open water detection for validation of freeboard retrievals
In an examination of ICESat data, {\em Kwok et al. }[2004] show that the retrieved elevations from the laser altimeter are sensitive to new openings (containing thin ice or open water) in the sea ice cover as well as to surface relief of old and first-year ice. Uncertainties in the elevation of the sea surface topography however require the availability of surface references for estimation of local freeboard - the height of the air-snow surface above sea level. Here, we use the relative reflectivity signature of thin ice and open water to identify areas suitable as sea level reference. Before being covered by a snow layer, laser returns from thin ice in newly opened areas typically have lower reflectivities in contrast to that of the surrounding ice cover. Returns from open water and perhaps grease ice, however, are frequently saturated and unreliable for surface estimation. These observations are validated by analysis of near-coincident ICESat data and openings in RADARSAT imagery. With these reference levels, and an estimate of the sea surface profile, we show that freeboard can be determined relatively consistently by comparing the estimates at adjacent tracks and crossovers. Results from the three ICESat laser-on periods (Feb-Mar, 2003; Oct-Nov, 2003; Mar-Apr, 2004) will be shown. These datasets allow us to examine the interannual as well as seasonal behavior of the freeboard. In the conversion to ice thickness, the uncertainties in thickness due to snow loading remain. Ref: Kwok, R., H. J. Zwally, and D. Yi. (2004) ICESat observations of Arctic sea ice: A first look., {\em Geophys. Res. Lett.}, 31, L16401, doi:10.1029/2004GL020309.
C22A-05 11:20h
Mapping Arctic Sea-Ice Freeboard-Height Distributions and Ice Thicknesses With ICESat
The state of the sea ice pack can be well described by distributions of sea-ice thickness obtained over spatial scales such as 50 km by 50 km. Information that can be derived from thickness distributions includes the mean sea-ice thickness and the ratio of thicker multiyear ice to thinner ice types. We use the capability of ICESat to measure the mean surface elevation over 70 m footprints spaced at 170 m with a range precision of less than 3 cm to detect the freeboard height (i.e., snow cover plus sea ice above sea level). Probability-density functions (PDF) of freeboard heights are constructed using data within 25 km of each measurement point along track. A single or double dual-sigma gaussian is fitted to each PDF to derive an ocean reference level for the PDF. The dual-sigma guassian accounts for the typical asymmetry of the distributions and the double gaussian accounts for the bi-modal distributions observed in areas of first-year and multiyear ice mixtures. The methodology is based on the likelihood of the existence of at least 1 to 2 percent open water and/or thin ice within each 50 km segment and/or the inclusion of sufficient information in the PDF to allow extrapolation to the reference level. Sea-ice thickness is calculated using snow-cover climatology and estimates of densities. Maps of sea-ice thickness show similarities to those from satellite radar altimetry. The spatial distributions for the winters (February/March) of 2003 and 2004 show significant interannual variations. In 2004, thicker ice is more compacted in its more usual distribution near the Canadian Arctic than it was in 2003, with a larger area of thinner ice in the Beaufort and Chukchi Seas where the summer ice cover has been rapidly decreasing. In 2003, the multiyear ice pack extends farther southward in the direction of 45 degrees East than normal.
C22A-06 11:35h
Sea-ice free-board heights in the Arctic Ocean from ICESAT and airborne laser
Sea-ice free-board heights, and thus ice thickness, may be readily determined by lowest level-filtering of Icesat or airborne lidar data in combination with a precise geoid model. In the paper we outline sea-ice freeboard height results in the Arctic Ocean from ICESAT, using an updated geoid model from combined ArcGP, GRACE and ICESAT-derived gravity field data. We compare the results to airborne scanning laser data from recent airborne campaigns in the Arctic Ocean north of Greenland, Ellesmere Island and Svalbard. The high-resolution airborne swath laser data, with a resolution of 1 m and a relative vertical accuracy of few cm, provide detailed mapping of ridges and leads, and thus provides an opportunity for understanding ICESAT waveform characteristics for different sea-ice regimes.
C22A-07 11:50h
Tabular Icebergs in the South Atlantic: Melt Ponding, Melt Pond Geometry and Margin Evolution.
In December of 2003, several massive icebergs originating from the Ronne-Filchner Ice Shelf in the souther Weddell Sea, Antarctica, arrived off the coast of South Georgia Island. MODIS (Moderate Resolution Imaging Spectroradiometer) imagery and photography obtained by astronauts aboard the international space station revealed that one of these icebergs was covered by and extensive meltwater pond. The main curiosity associated with the pond was its arrangement as elongate edge-parallel shallow water body about 1 km inboard from the iceberg edge. An "edge levee" appeared to keep this meltwater pond from spilling over the edge of the iceberg. ICESAT (Ice, Cloud, and Land Elevation Sattelite) profiles of this and "dry" icebergs of a similar size revealed surface elevation maxima near the iceberg edges that may represent the same kind of topographic expression that becomes the edge levees on the melt covered iceberg. Margin ridges seen in the ICESAT data were 1-2 m high and 400-1200 m wide. The one iceberg that displayed the levee-contained melt ponds disintegrated in late summer, 2003, in a pattern very similar to therapidbreak-up observed for the Larsen A and Larsen B ice shelves in 1995 and 2002, respectively. In this presentation, we shall consider the possible origins of this levee surface meltwater pond geometry. Among the hypotheses to be tested are: a. that the flexural effects of the iceberg in response to both the bending moment introduced at the iceberg edge by sea water pressure and the loading associated with surface melt ponding can explain the "edge levees", their size and location, and b. variable ice thickness, in particular thickening at the edges of the iceberg, is necessary to explain the "edge levees". The presentation will cover both the observational record of curious iceberg meltwater pond geometry and efforts to model iceberg flexural profiles using a simple approach involving the thin-elastic plate approximation. Where possible, model results will be compared to GLAS altimeter profiles of icebergs encountering strong surface melting.
C22A-08 12:05h
Ocean Elevation and Wave Heights from GLAS
The Geoscience Laser Altimeter System (GLAS) was designed to range to ice, land and atmosphere, but measures ocean surfaces as well. The footprints of the laser (70 m diameter, 170 m spacing, 40 Hz temporal resolution) contrast sharply with traditional radar altimetry (6-8 km diameter, 6-7 km spacing, 1 Hz temporal resolution). Consequently, the two systems observe different ocean phenomena. For calibration, laser and radar data can be similarly compared to each other with the application of ~1-Hz smoothing to GLAS data. The relative ocean height biases between GLAS, Topex, and Jason-1 are estimated using dual-satellite crossovers, along-track analyses, and by comparing to a common Mean Sea Surface (MSS). The Root-Mean-Square (RMS) statistics of these height comparisons, as well as internal crossover statistics, provide a measure of the relative precision of these biases and the current performance of the GLAS data collection campaigns. These relative biases and precisions are compared over global ocean surfaces and are subdivided into groups considering pass direction and local solar time. Ocean waves manifest themselves in the radar waveform as a slope on the leading and trailing edges, which are used to estimate Significant Wave Height (SWH) and sigma0 (wind speed). The wave relationships used for radar altimetry are reexamined for the very different laser altimeter geometry. Comparisons are made between GLAS, Topex, and Jason-1 of SWH, mesoscale variability, and other parameters.