A method to automatically determine sea level for referencing snow freeboards and computing sea ice thicknesses from NASA IceBridge airborne LIDAR

Abstract

The NASA IceBridge flights have obtained critical observations for Earth's polar ice since ICESat stopped collecting data in 2009. This study develops an automatic method in processing IceBridge Airborne Topographic Mapper (ATM) altimeter L1B data (one elevation per 3–4m horizontally) to derive a local sea level height for referencing snow freeboards and then computing sea ice thicknesses. Four 30-km L1B profiles (A, B, C and D) flown on October 21, 2009 over the Bellingshausen Sea in Antarctica are selected. The local sea level reference is first obtained by visual examination of ATM L1B heights over leads or thin ice identified on images simultaneously acquired from the Digital Mapping System camera (called manual selection). This sea level reference is then used as ground truth to validate sea level heights derived by automatic calculations using five thresholds of 2%, 1%, 0.5%, 0.2% and 0.1% of the lowest L1B data. The L1B_0.2% method gives a similar sea level height as from the L1B manual selection, by mean (absolute) difference of −0.01 (0.06) m. The sea level heights demonstrate a near linear gradient of 0.01m/km to 0.03m/km within each ~30-km L1B profile along the flight track from section A to D. The resulting mean snow freeboards are 0.59m, 0.67m, 0.53m, and 0.60m on sections A, B, C and D, respectively. Three empirical equations and the buoyancy equation (with zero ice freeboard assumption) all give similar statistics in ice thickness estimation with mean ice thicknesses of 1.91m for section A, 2.16m for B, 1.76m for C, and 1.94m for D. The sea level over leads cannot be accurately resolved from the ATM L2 data (~60m x 80m horizontal averaging). However, by using a sea level reference obtained from the L1B data, the ATM L2 data can achieve reasonable ice thickness estimates with mean absolute difference of only 0.10m compared to thickness derived from the L1B data.