ICESat over Arctic sea ice: Interpretation of altimetric and reflectivity profiles

We provide an assessment of the ICESat altimeter for studying the Arctic Ocean and examine the magnitude of the large- and small-scale expressions of geophysical processes embedded in the elevation profiles. This analysis includes data from six surveys. At the large scale the response of the ice-covered ocean to atmospheric loading is near ideal (i.e., approximately -1 cm/hPa). After removal of the inverted barometer effects and best available geoid the elevation signal is still dominated by unresolved geoid residuals (similar to 0.4 m) that can be seen in the similarity of the remaining spatial patterns. Seasonal differences in elevations over multiyear ice are consistent with snow depth climatology; the broad differential spatial patterns are indicative of interannual differences in multiyear ice coverage associated with advection. Patterns in the derived surface roughness fields correspond to the seasonal and perennial ice zones seen in QuikSCAT data. At the small scale, near-coincident RADARSAT imagery provides a spatial context for understanding the signature of the observed elevations, waveforms, and reflectivity, in particular, those associated with thin ice, open water, multiyear ice, and ridges. The precision of the elevation estimates measured over relatively flat sea ice, identified in synthetic aperture radar (SAR) imagery, is similar to 2 cm. The unambiguous identification of ridged areas in coupled ICESat/RADARSAT analysis could be used to enhance the utility of SAR imagery for examining ridge distributions. Over a 20 day period we monitored the evolution of the reflectivity of a newly opened lead. The steep increase in reflectivity due to snow coverage suggests that dips in ICESat reflectivity are likely areas of thin ice and could serve as a basis for selection of tie points for use as sea level reference. Identification of these tie points is crucial for accurate estimation of sea ice freeboard.