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

Data are presented from a survey by airborne scanning laser profilometer and an AUV‐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 a 300 × 300 m area. Data were separated into level and deformed ice fractions using the surface roughness of the sonar data. The relation ( R = d / f ) between draft, d , and surface elevation, f , was then examined. Correlation between top and bottom surfaces was essentially zero at full resolution, requiring averaging over patches of at least 11 m diameter to constrain the relation largely because of the significant error (∼15 cm) of the laser instrument. Level ice points were concentrated in two core regions, corresponding to level FY ice and refrozen leads, with variations in R attributed primarily to positive snow thickness variability. Deformed ice displayed a more diffuse “cloud,” with draft having a more important role in determining R because of wider deformed features underwater. Averaging over footprints similar to satellite altimeters showed the mean surface 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” characteristic reported 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 isostasy suggests 70 m as an isostatic length scale for level ice. The isostatic scale for deformed ice appears to be longer than accessible with these data (>300 m).