The relation between Arctic sea ice surface elevation and draft: A case study using coincident AUV sonar and airborne scanninglaser

© G.Redvers/Tara Expeditions

Tara Arctic

Martin J. DOBLE, Henriette SKOURUP, Peter WADHAMS, and Cathleen A. GEIGER

Journal of Geophysical Research, Volume 116, November 2011.

Abstract

Data are presented from a survey by airborne scanning laser profilometer and anAUV‐mounted, upward looking swath sonar in the spring Beaufort Sea. The air‐snow(surface elevation) and water‐ice (draft) surfaces were mapped at 1 × 1 m resolution over a300 × 300 m area. Data were separated into level and deformed ice fractions using thesurface roughness of the sonar data. The relation (R = d/f ) between draft, d, and surfaceelevation, f, was then examined. Correlation between top and bottom surfaces wasessentially zero at full resolution, requiring averaging over patches of at least 11 mdiameter to constrain the relation largely because of the significant error (∼15 cm) of thelaser instrument. Level ice points were concentrated in two core regions, correspondingto level FY ice and refrozen leads, with variations in R attributed primarily to positivesnow thickness variability. Deformed ice displayed a more diffuse “cloud,” with drafthaving a more important role in determining R because of wider deformed featuresunderwater. Averaging over footprints similar to satellite altimeters showed the meansurface elevation (typical of ICESat) to be stable with averaging scale, with R = 3.4 (level)and R = 4.2 (deformed). The “minimum elevation within a footprint” characteristicreported for CryoSat was less stable, significantly overestimating R for level ice (R > 5)and deformed ice (R > 6). The mean draft difference between measurements and isostasysuggests 70 m as an isostatic length scale for level ice. The isostatic scale for deformed iceappears to be longer than accessible with these data (>300 m).