ICESat: Thin ice and open water detection for validation of freeboard retrievals

In an examination of ICESat data, Kwok et al. [2004] show that the retrieved elevations from the laser altimeter are sensitive to new openings (containing thin ice or open water) in the sea ice cover as well as to surface relief of old and first-year ice. Uncertainties in the elevation of the sea surface topography however require the availability of surface references for estimation of local freeboard - the height of the air-snow surface above sea level. Here, we use the relative reflectivity signature of thin ice and open water to identify areas suitable as sea level reference. Before being covered by a snow layer, laser returns from thin ice in newly opened areas typically have lower reflectivities in contrast to that of the surrounding ice cover. Returns from open water and perhaps grease ice, however, are frequently saturated and unreliable for surface estimation. These observations are validated by analysis of near-coincident ICESat data and openings in RADARSAT imagery. With these reference levels, and an estimate of the sea surface profile, we show that freeboard can be determined relatively consistently by comparing the estimates at adjacent tracks and crossovers. Results from the three ICESat laser-on periods (Feb-Mar, 2003; Oct-Nov, 2003; Mar-Apr, 2004) will be shown. These datasets allow us to examine the interannual as well as seasonal behavior of the freeboard. In the conversion to ice thickness, the uncertainties in thickness due to snow loading remain. Ref: Kwok, R., H. J. Zwally, and D. Yi. (2004) ICESat observations of Arctic sea ice: A first look., Geophys. Res. Lett., 31, L16401, doi:10.1029/2004GL020309.