Freeboard (Sea Ice)

Sea-ice freeboard is the height of the ice surface above the local sea level, the quantity altimeters measure to derive ice thickness through hydrostatic balance. Radar altimeters such as CryoSat-2 sense ice freeboard near the snow-ice interface, while laser altimeters such as ICESat-2 sense total (snow plus ice) freeboard, so the two are combined or corrected with a snow-load assumption. Converting freeboard to thickness needs ice and snow densities and snow depth, and small freeboard errors amplify into large thickness errors because ice floats with roughly 90% of its mass submerged. Freeboard retrievals anchor basin-scale ice-thickness records.