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

• Published in: Journal of geophysical research. Oceans 2019-10, Vol.124 (10), p.6942-6959
• Main Authors: Kwok, R., Markus, T., Kurtz, N. T., Petty, A. A., Neumann, T. A., Farrell, S. L., Cunningham, G. F., Hancock, D. W., Ivanoff, A., Wimert, J. T.
• Format: Article
• Language: English
• Subjects: Aggregates; Altimeters; altimetry; Antarctic; Arctic; Arctic sea ice; Climate change; Clouds; Data collection; Elevation; Floating ice; Freeboard; Geophysics; Glaciation; Global warming; Height; Ice; Ice cover; Ice environments; Ice sheets; Ice thickness; Laser altimeters; Laser beams; Lasers; Lidar; Logistics; Ocean circulation; Ocean currents; Oceans; Photon beams; Photons; Polar environments; Polar regions; Products; Profiling; Pulse rate; Reflectance; Regional analysis; Satellites; Sea ice; Sea ice thickness; Sea level; Sea surface; Statistical analysis; Statistical methods; surface height; Thickness; Topography; Uniqueness; Water circulation
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2019JC015486

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 ~0.7 m) and footprints of ~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 ~27 m, crucial for freeboard calculations, are resolved. The fixed photon aggregate results in unique variable along‐track resolutions and nonuniform sampling (17 m × 27 m to 17 m × 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 ~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. Plain Language Summary NASA's Ice, Cloud, and Land Elevation Satellite (ICESat‐2) was launched in September of 2018. This mission is specifically designed to measure accurate surface heights for understanding changes in ice sheets, sea ice, ocean circulation, and vegetation biomass. For sea ice, the topic of focus here, the instrument onboard is tasked to measure freeboard—the vertical height of the floating ice above the sea surface—which will be used to estimate the thickness of the Arctic and Southern Ocean ice covers. Sea ice thickness is not only an important indicator of how the polar oceans are responding to a warming climate but also useful for forecasting of future changes and for supporting logistics and operations in the polar regions. I