Mapping Arctic Sea-Ice Freeboard-Height Distributions and Ice Thicknesses With ICESat

The state of the sea ice pack can be well described by distributions of sea-ice thickness obtained over spatial scales such as 50 km by 50 km. Information that can be derived from thickness distributions includes the mean sea-ice thickness and the ratio of thicker multiyear ice to thinner ice types. We use the capability of ICESat to measure the mean surface elevation over 70 m footprints spaced at 170 m with a range precision of less than 3 cm to detect the freeboard height (i.e., snow cover plus sea ice above sea level). Probability-density functions (PDF) of freeboard heights are constructed using data within 25 km of each measurement point along track. A single or double dual-sigma gaussian is fitted to each PDF to derive an ocean reference level for the PDF. The dual-sigma guassian accounts for the typical asymmetry of the distributions and the double gaussian accounts for the bi-modal distributions observed in areas of first-year and multiyear ice mixtures. The methodology is based on the likelihood of the existence of at least 1 to 2 percent open water and/or thin ice within each 50 km segment and/or the inclusion of sufficient information in the PDF to allow extrapolation to the reference level. Sea-ice thickness is calculated using snow-cover climatology and estimates of densities. Maps of sea-ice thickness show similarities to those from satellite radar altimetry. The spatial distributions for the winters (February/March) of 2003 and 2004 show significant interannual variations. In 2004, thicker ice is more compacted in its more usual distribution near the Canadian Arctic than it was in 2003, with a larger area of thinner ice in the Beaufort and Chukchi Seas where the summer ice cover has been rapidly decreasing. In 2003, the multiyear ice pack extends farther southward in the direction of 45 degrees East than normal.