Geodesy [G]

G21A   BCC:325   Tuesday 

Observations of Polar Ice With Spaceborne Altimeters: Recent Results and Future Prospects I

Presiding: R Scharroo, Altimetrics; D C McAdoo, NOAA Laboratory for Satellite Altimetry; H Fricker, Scripps Institution of Oceanography


Estimates of sea ice Freeboard and Thickness From Space-borne Altimetry : Past Present and Future

* Laxon, S W ( , Centre For Polar Observation and Modelling, University College London, Gower St., London, WC1E 6BT United Kingdom
Farrell, S L ( , Centre For Polar Observation and Modelling, University College London, Gower St., London, WC1E 6BT United Kingdom
Giles, K A ( , Centre For Polar Observation and Modelling, University College London, Gower St., London, WC1E 6BT United Kingdom
Ridout, A L ( , Centre For Polar Observation and Modelling, University College London, Gower St., London, WC1E 6BT United Kingdom

Satellite altimeters provide the only method for directly measuring sea ice freeboard from space from which sea ice thickness, a critical parameter in climate modeling and monitoring, can be estimated. Past estimates from ERS radar altimetry show good agreement with in-situ data and reveal a high inter-annual and regional variability in ice thickness across the Arctic. However further efforts are required to understand the uncertainties involved through in-situ and aircraft observations as well as inter-comparisons between radar and laser altimetry over sea ice. We will describe results from the ERS mission and their use in understanding the factors which control sea ice thickness. We will use results from recent research using radar/laser altimetry on both aircraft and satellite platforms to explore current uncertainties particularly in relation to sea surface elevation and snow depth. Further we will outline the complexities involved in determining errors on different temporal and spatial scales in terms of the covariance of the different terms in the error budget. Finally we will describe future activities, in particular those relating to the recently announced European Space Agency's CryoSat-2 mission, due for launch in 2009.


Arctic sea level change from satellite altimetry

* Scharroo, R ( , Altimetrics LLC, 330a Parsonage Road, Cornish, NH 03745 United States
Ridout, A L ( , University College London, Center for Polar Observation and Modelling Gower Street, London, WC1E 6BT United Kingdom
Laxon, S W ( , University College London, Center for Polar Observation and Modelling Gower Street, London, WC1E 6BT United Kingdom

The near-polar orbiting altimeter satellites, ERS-1, ERS-2 and Envisat, have been used widely to study ice thickness in the Arctic, but little use has been made of the same data to determine Arctic sea level change. This is not an easy feat, as the ice cover obstructs the view of part of the sea surface and affects the measurements in mixed ocean/sea-ice conditions. Thus considerable effort has been put in the separation of the radar returns from leads and from sea ice. Moreover, microwave radiometer measurements of wet tropospheric delay can not be used since those measurements are also affected by sea ice. The Arctic altimeter data were retracked using an OCOG retracking algorithm, and the diffuse returns from the leads and open ocean were combined with a host of instrumental corrections and geophysical models to determine instantaneous mean sea level. Such measurements were compared with conventional altimeter products to ensure consistency. The sea level time series obtained from this study form a unique data set for this region where in-situ measurements are virtually non-existent and altimeter data from other satellites are not available or otherwise rejected.


Bubbling Under: An accidental view of Antarctic subglacial drainage

* Shepherd, A ( , University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP United Kingdom
Wingham, D ( , University College London, Gower Street, London, WC1E 6BT United Kingdom

While 145 subglacial lakes demonstrate that much of the East Antarctic Ice Sheet base is melting, the hydrology beneath this huge ice mass is poorly understood. Such knowledge is critical to understanding ice flow, basal water transfer to the ice margin, glacial landform development and subglacial lake habitats. Furthermore, little attention has been given to the possibility that the very existence of subglacial lakes determines the mode of basal drainage through rapid discharges. Here we record a rapid discharge from a subglacial lake in central East Antarctica. In 16 months, 1.8 km3 of water was transferred 290 km to at least two other subglacial lakes. While the lake's ice roof may moderate discharge, the intrinsic instability of pressurising subglacial lakes makes such events a likely normal mode of basal drainage. If large lakes, such as Lake Vostok or Lake Concordia1, are pressurising, substantial discharges that could reach the coast may be foreseen. The recorded discharge conflicts with expectations of subglacial lakes having long residence times and slow circulations; indeed entire subglacial drainage basins may be flushed periodically. The rapid transfer of water between lakes will result in large-scale solute and microbe relocation. Drainage system contamination from in situ exploration is, therefore, a distinct risk.


NASA's Ice Cloud and Land Elevation Satellite (ICESat): Providing New Insights into the Changing Cryosphere

* Abdalati, W ( , Cryospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, MD 20771 United States
Zwally, H (

Launched on January 12, 2003, NASA's Ice Cloud and land Elevation Satellite (ICESat) provides a new three- dimensional view of the Cryosphere with unprecedented accuracy and precision. This capability has enabled detailed measurements of ice sheet, ice stream, and ice shelf topography, ice elevation changes, and sea ice freeboard in ways that have never been possible before. On the ice sheet margins, where surfaces are rough and slopes are generally steep, ICESat's small footprint enables the detection of major changes that has not been achievable with conventional radar altimeters. ICESat's unprecedented accuracy in measuring surface elevation enables the detection of small elevation changes in the central regions of the vast ice sheets, where changes of a just a few centimeters are of great consequence to the overall ice sheet mass balance. The high precision of the ICESat measurements along with its dense along-track sampling has resulted in the first detailed satellite-derived sea ice thickness estimates, and provides an important new data set for understanding changes in this critical component of the cryosphere. Now into its ninth operations period of 33- day observations in winter, spring, and fall ice cover, the ICESat mission is providing meaningful information for relating changes in the cryosphere to climate conditions that affect those changes. The most recent results of ice sheet characteristics and changes, as well as sea ice thickness observations will be presented, along with a summary of the current state of the ICESat mission and its prospects for continued operation.


Mapping Elevation Changes in the Ross Embayment with ICESat Cross-Overs and Repeat- Track Analysis

* Smith, B E ( , University of Washington, JHN 070 /Box 351310, Seattle, WA 98195 United States

With the conclusion of the March 2006 measurements, ICESat laser operations provide elevation data extending over a period of 36 months. ICESat cross-over measurements have already shown significant elevation changes in the Ross Embayment of West Antarctica. Because ICESat has been flying the same repeat orbit for the most recent 30 months of this, it is now possible to recover mean rates of elevation change from points along the ground tracks between cross-overs by correcting elevation measurements for the across- track slope at each point. Calculations of mean elevation-change rates over large regions using along-track measurements require careful treatment of data covariances, because a significant portion of the error in each measurement is correlated for points on the same pass. We present an updated set of recovered elevation rates calculated form cross-over measurements including all data from 2005 and early 2006, and incorporate elevation rates calculated from along-track elevation differences into mean elevation rates recovered form cross-over analysis. Along-track differences are also used to find the boundaries of regions undergoing rapid elevation changes. We find that a large area in the catchment of Kamb ice stream is undergoing rapid uplift that cannot easily be explained by instrumental errors or by accumulation variations.