Summary
Sea ice freeboard distributions are detennined from ICESat measurements of mean surface elevations with a range precision of -2 cm over 70 m footprints spaced by 170 m along track. Freeboard (snow cover plus sea ice) distributions are constructed from elevation measurements within :i: 50 km of each measurement point, from which the lowest 1 % of the points are selected as the ocean reference level (i.e., open water or very thin ice). Reiterative mapping of the ocean reference level, and averaging over different time periods, is used to reduce the effect of errors in the initial estimate of the geoid. Sea-ice thickness is derived from the freeboard measurements using estimates of snow and sea-ice densities, combined with snow-depth estimated from passive microwave data from AMSR-E. Along track sea-ice freeboard and thickness values are then mapped onto 50 x 50 km grids. Since 2003, measurements have been made for about 33-day periods three times a year (February/March, May/June, and September/October). The distributions show the seasonal growth/decay cycle, regional variations, and interannual variations of the sea-ice pack.