Earth's Ice Mass Fluxes
Programme Background
The CPOM science programmes in ice sheet, glacier and sea ice modelling are underpinned by the generation, principally from radar altimetry, of time-series, that now extend over a decade, of fluctuations in the mass of the Earth's ice sheets. This programme is concerned with the maintenance and extension in time and space of these time-series. The mass imbalance of 63% of the Antarctic Ice Sheet interior has been determined to ± 63 Gt yr-1 with ERS altimeter data. The time series is presently sufficient to constrain the mass imbalance of the entire interior. The largest changes at basin scales are already apparent. As the time series is extended, more subtle flow-related features will appear above the error over successively larger areas of the ice sheets. (In addition to their scientific importance, these measurements provide information in the prioritising of ice sheet field logistics.) Estimating sea-ice mass fluctuations and fresh water transport requires knowledge of the velocity, concentration and thickness of sea ice. The measurement of sea-ice extent, concentration, and motion using passive microwave imagery is well established, and CPOM has developed, uniquely, estimates of ice thickness using measurements of sea-ice elevation by space-borne altimetry. A central purpose of CPOM is the extension of these time-series using the ENVISAT (2001-), ICESAT (2001-) and CryoSat (2003-) altimeters. CPOM will also use the new capabilities of the ICESAT and CryoSat altimeters to provide the first complete survey of the world s small ice bodies. These are presently expected to dominate the 21st century cryospheric contribution to sea level rise.
The CPOM time-series of ice fluctuations have been achieved through a detailed understanding of the scattering properties of sea ice and instrument response to echoes from the ice and ocean. Development of processing algorithms of the new ENVISAT, ICESAT and CryoSat altimeters, and cross-calibration of these different systems, is required to ensure a consistent time-series. It also requires a close examination of changes in electromagnetic scattering due to firn densification or instruments polarisation. For sea ice, continued, detailed comparison with submarine and surface observations will establish biases in the satellite measurements arising from, e.g., snowfall and ridging.
CryoSat is the first ESA Earth Science mission selected by open, scientific competition. DJW is its Lead Investigator (LI), and CPOM s altimeter-related activities will give it special emphasis. The importance of CryoSat is that the new radar altimeter is optimised for the measurement of sea ice floe thickness, and to provide complete coverage of the Earth s ice sheets, ice caps and glaciers. CPOM will have a central role in the production of higher-level products from the mission. DJW is coordinating the CryoSat validation field campaigns in 2003, 2004, 2005 and 2006 on the Greenland Ice Sheet and Arctic Ocean deploying aircraft, ice breakers, helicopters and in-situ surface observations. Directly, these will provide detailed physical and electromagnetic data used to accurately validate sea- and land-ice data products (and to reduce errors in the historical time-series arising from the ERS and ENVISAT time-series). In addition, this activity will provide an exceptional concentration of experimental platforms in the Arctic Ocean. CPOM intends to ensure the wider NERC and European community takes advantage of this opportunity, through, for example, a co-ordinated NERC Thematic Programme.
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