Sea Ice Elevation From ICESat/GLAS Laser Altimetry

Arctic sea ice is currently in a state of change. Such changes in sea ice thickness and extent can be used to assess polar climate variability. Observational records suggest that over the past few decades a substantial decline in sea ice extent together with rapid thinning have occurred. Continuous monitoring of sea ice thickness has however proved to be a more difficult task than measuring ice extent and no data sets cover the entire Arctic. Recent research shows that radar altimetry from the ERS satellites can successfully be used to measure sea ice freeboard, from which ice thickness can be estimated. However the exploitation of this data is limited to some extent due to the large radar footprint (10km), and limited latitudinal coverage (to 81.5° ). In addition all satellite altimeter estimates suffer inaccuracies due to uncertainties in snow loading on sea ice. The Geoscience Laser Altimeter System (GLAS) extends satellite altimetric coverage of sea ice to 86° latitude for the first time. In addition, the much smaller laser footprint (70m) provides improved resolution of sea ice and ocean topography. On the other hand, laser altimetry data is more susceptible to atmospheric effects such as cloud cover and forward scattering of laser pulses. Using data from the first month of operations, we examine the feasibility of deriving sea ice elevation from GLAS altimetry data. We investigate the influence of instrument errors and environmental effects (e.g. pulse broadening from clouds) on the accuracy of GLAS elevation measurements. We compare GLAS elevations over sea ice to a state-of-the-art Arctic geoid model and to a mean sea surface derived from ERS radar altimetry data. We analyse the characteristics of laser waveforms and compare these with available in situ data. Finally, we compare GLAS sea ice elevations with sea ice freeboards and thickness derived from ERS data. By combining these two datasets, the potential exists to obtain estimates of snow loading and hence more precise estimates of sea ice thickness.