Ice, Cloud, and land Elevation Satellite (ICESat) over Arctic sea ice: Retrieval of freeboard

Total freeboard (snow and ice) of the Arctic Ocean sea ice cover is derived using Ice, Cloud, and land Elevation Satellite (ICESat) data from two 35-day periods: one during the fall (October-November) of 2005 and the other during the winter (February-March) of 2006. Three approaches are used to identify near-sea-surface tiepoints. Thin ice or open water samples in new openings, typically within 1-2 cm of the sea surface, are used to assess the sea surface estimates. Results suggest that our retrieval procedures could provide consistent freeboard estimates along 25-km segments with uncertainties of better than 7 cm. Basin-scale composites of sea ice freeboard show a clear delineation of the seasonal ice zone in the fall. Overall, the mean freeboards of multiyear (MY) and first-year (FY) ice are 35 cm and 14 cm in the fall, and 43 cm and 27 cm in the winter. The increases of similar to 9 cm and similar to 12 cm on MY and FY sea ice are associated with the 4 months of ice growth and snow accumulation between data acquisitions. Since changes in snow depth account for >90% of the seasonal increase in freeboard on MY ice, it dominates the seasonal signal. Our freeboard estimates are within 10 cm of those derived from available snow/ice thickness measurements from ice mass balance buoys. Examination of the two residual elevations fields, after the removal of the sea ice freeboard contribution, shows coherent spatial patterns with a standard deviation (S. D.) of similar to 23 cm. Differencing them reduces the variance and gives a near random field with a mean of similar to 2 cm and a standard deviation of similar to 14 cm. While the residual fields seem to be dominated by the static component of unexplained sea surface height and mean dynamic topography (S.D. similar to 23 cm), the difference field reveals the magnitude of the time-varying components as well as noise in the ICESat elevations (S. D. similar to 10 cm).