Surface Height and Sea Ice Freeboard of the Arctic Ocean from ICESat‐2: Characteristics and Early Results

We present the first winter season of surface height and sea ice freeboards of the Arctic Ocean from the new Ice, Cloud, and Land Elevation Satellite (ICESat-2; IS-2) mission. The Advanced Topographic Laser Altimeter System onboard has six photon-counting beams for surface profiling with a 10-kHz pulse rate (interpulse distance similar to 0.7 m) and footprints of similar to 17 m. Geolocated heights assigned to individual photons scattered from the surface allow significant flexibility in the construction of height distributions used in surface finding. For IS-2 sea ice products, a fixed 150-photon aggregate is used to control height precision and obtain better along-track resolution over high reflectance surfaces. Quasi-specular returns in openings as narrow as similar to 27 m, crucial for freeboard calculations, are resolved. The fixed photon aggregate results in unique variable along-track resolutions and nonuniform sampling (17 m x 27 m to 17 m x 200 m for the strong beams) of the surface. The six profiling beams-three pairs separated by 3.3 km with a strong and weak beam in each pair-provide correlated statistics at regional length scales for assessment of beam-to-beam retrieval consistency and accuracy. Analysis shows along-track height precisions of similar to 2 cm and agreement in the monthly freeboard distributions across the strong beams to 1-2 cm. In this paper, we describe briefly the approaches used in surface height and freeboard retrievals from Advanced Topographic Laser Altimeter System photon clouds and detail the key features of these along-track sea ice products, focusing on the first release of data collected over the Arctic Ocean, which spans the period between 14 October 2018-the start of data collection-and the end of March 2019.