old bumph

Sea ice modelling

The sea ice and polar oceanography modelling group focuses on the development of improved understanding and theoretical representation of physical processes that play a pivotal role in the balances of heat, mass, and momentum in the polar seas. The approach is to use small-scale and/or highly idealised process studies, calibrated and tested with a wide range of observations, to generate understanding of the controlling processes. Typically this work involves both paper-and-pen fundamental studies and numerical modelling. Example processes are the role of melt ponds in controlling the summer melt of Arctic sea ice, the role of anisotropy in controlling the momentum and mass budgets of sea ice, and the role of frazil ice in controlling the mass balance of sea ice and ice shelves. These process studies allow us to develop simplified representations, known as parameterisations, of the relevant processes that can be incorporated into the sea ice component of climate models. We use a stand-alone (forced) climate sea ice model to allow us to investigate the impact of newly-developed physics on regional and pan-Arctic simulations of the sea ice cover.


Ice sheet and glacier dynamics

Satellite observations of the Earth’s ice sheets, ice caps and glaciers are immediately applicable to a wide range of cryospheric problems. CPOM is using these to determine directly the mass imbalance of the Antarctic and Greenland Ice Sheets and smaller ice caps and glaciers. A more accurate determination is required to understand 20th and 21st sea level rise, and to unravel the interactions of Holocene ice distribution, post-glacial rebound, sea level rise and Earth rotation. In combination with ground observations, CPOM is using satellite-derived surface velocities as boundary conditions in numerical models of ice dynamics to attack a range of problems. These include the role of ice streams (which are presently absent from models of entire ice sheet evolution) in basin dynamics and near-future sea level evolution, the thermal evolution of the Antarctic ice sheet bed, and related high latitude environmental problems, such as the issue of ancient microbial life in the subglacial Lake Vostok.

The southern Greenland Ice Sheet and Arctic sub-polar ice masses will respond rapidly to climate warming, providing a net contribution to sea level rise due to increased ablation rates at the surface. Increased surface melting will modify snow consolidation, affect melt water run off, and change the scattering and absorption characteristics, altering the spectrally-averaged albedo of the snow. Surface melting also has dynamic implications: as melt water percolates through cracks, it freezes, narrowing the cracks, increasing the thermal inertia of the ice cover, and releasing latent heat which contributes significantly to the thermally-activated, slow flow of ice. Correlations between surface and basal drainage, and surging behaviour are likely to strengthen. These processes introduce new parameterisation problems into the modelling of ice flow which will be addressed through consideration of the snow cover water fraction, and the reaction of the ice sheet to melt water percolation and redistribution of sensible and latent heat. CPOM is developing modelling approaches to these processes, using the specific contexts of the ice caps of the Russian, Norwegian and Canadian Arctic which, because of their relatively small thickness and higher accumulation, have more rapid response times than the great ice sheets.

Our ice sheet modelling expertise is also applicable to modelling the climatically-forced behaviour of former ice sheets, in conjunction with palaeoceanographers (to whom we supply time-dependent meltwater- and iceberg-flux data) and Quaternary scientists (whose chronologically-controlled data on ice-sheet extent will be used to test our model reconstructions). Satellite radar interferometry is fundamental to the provision of wide-area ice sheet surface velocity. The interferometric coverage of ESA’s satellites has provided a wealth of information on the spatial and temporal fluctuation of the Earth s ice sheets and the substantial data volumes require an international pooling of effort and coordinated access to the ESA archive. [/read]


Earth’s ice mass fluxes

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 is also using 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.  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 has a central role in the production of higher-level products from the mission. Duncan Wingham coordinated 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. These provided 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 provided an exceptional concentration of experimental platforms in the Arctic Ocean. CPOM is ensuring the wider NERC and European community takes advantage of this opportunity, through, for example, a co-ordinated NERC Thematic Programme.