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1 NCEO Centre for Polar Observation and Modelling, University College London, UK.*swl@cpom.ucl.ac.uk
2 Alfre Wegener Institute, Bremerhaven, Germany.
3 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada.
4 ESA/ESTEC, Noordwijk, Netherlands
Abstract
Arctic sea ice has undergone major changes in recent years but there remains much uncertainty about its ultimate fate, in particular the timing of an ice-free Arctic in summer. Although satellite measurements of ice extent are well-established, wide-area measurements of sea ice thickness are key to understanding the fate of Arctic sea ice cover in the future. Satellite altimeters can provide direct measurements of sea ice freeboard from which sea ice thickness can be calculated. We present the first data on sea ice thickness from Cryosat-2, validated by in situ and aircraft data. We describe the primary uncertainties for sea ice thickness retrieval, and outline strategies for addressing these in the future.
C41A-0374
Fig. 1: CryoSat-2 was launched on the 8 th of April 2010.
Method
CryoSat L0 data was processed to L1B by UCL using the most up to date version of the L1B processor. The L1B data is then converted into sea ice freeboard using
UCL's own software, based on a modified version of the software used for Envisat freeboard determination. Returns from leads are identified by their high pulse peakiness (>18) and low stack standard deviation (<4) (figure 2). The pulse peakiness is computed from the 128 echo samples and the stack standard deviation is available in the L1B data. Returns from floes and open water are identified by their low pulse peakiness (<9) and high stack standard deviation (>4).
The flow and open water returns are then separated by looking at the sea ice concentration, with valid floes needing a value of 70% or higher and open water needing a value of 0%. The satellite to surface range is first computed from the window delay reported in the L1B data, and corrected for atmospheric and tidal effects using corrections supplied in the L1B data. A final 'retracking correction' is the applied to compensate for the fact that the echo may not be exactly in the centre of the range window. For specular echoes from leads, a Gaussian rise to the echo followed by a negative exponential tail is fitted to the echo shape, with the tracking point being positioned at the peak of the Gaussian. For diffuse echoes from the floes and open water, the tracking point is positioned at the point where the rise reaches 95% of the computed OCOG threshold. The surface height above a reference ellipsoid is then computed for both floes and leads by subtracting the fully corrected range from the satellite altitude reported in the L1B data. The final freeboard calculation involves removing a mean sea surface model form both lead and floe elevations, then interpolating the sea surface height under the floes using linear regression on the lead heights with a full width of 200km. For the interpolated sea surface height to be valid, at least one input lead must occur on either side of the floe location. The interpolated sea surface height under each floe is then deducted from the floe height to leave freeboard. Freeboard is subsequently converted to ice freeboard using values of snow loading and ice/water densities from climatology.
Fig. 2: Classification of CryoSat-2 echoes overlaid on Envisat
ASAR imagery. Low (blue) values represent returns from floes whilst high (yellow/red) returns represent returns from leads.
Fig. 4: CryoSat-2
Snow+Ice thickness compared with the Polar
-5 EM measurements in
April 2011
Conclusions
The comparisons of CryoSat-2 freeboard and thickness measurements show good agreement with in-situ and aircraft measurements. The causes of the observed differences remain to be understood. For example both the in-situ and aircraft measurements are likely to be biased low in the presence of thick rough sea ice. Further issues for example related to the use of a snow depth climatology and penetration of the radar into the snow/ice surface also require further investigation once the full set of ground and aircraft data from the April 2011 CryoVex campaign are available .