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

Abstract

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.

Winter Arctic Sea Ice Thickness From ICESat‐2 Freeboards

“…As discussed in Kwok, Markus, Kurtz, et al (2019), it is important to take into account the variable height segment length in any statistical analyses of these data, for example, by weighting the data based on the segment length, thus a mean thickness within a given area can be determined as…”

ICESat‐2 Surface Height and Sea Ice Freeboard Assessed With ATM Lidar Acquisitions From Operation IceBridge

“…IS‐2 employs a multibeam photon counting lidar for profiling the surface, and this mode of altimetry imparts unique characteristics on the retrieved elevations compared to traditional waveform altimetry. In particular, for the IS‐2 sea ice products, a fixed 150‐photon aggregate is used in surface finding to control height precision and for improved along‐track resolution over high reflectance surfaces (Kwok et al, ; Kwok et al, ). Using these fixed‐count aggregates, quasi‐specular returns in openings as narrow as ~27 m, crucial for freeboard calculations, can be resolved.…”

Scientific Challenges and Present Capabilities in Underwater Robotic Vehicle Design and Navigation for Oceanographic Exploration Under-Ice

“…Because only a small fraction of sea ice rises above sea level, and is usually concealed under a layer of snow, sonar measurements of the ice draft provide the most reliable estimate of sea ice thickness. Such observations can help validate satellite altimeter estimates of sea ice thickness [51], particularly at the high spatial resolutions possible with the recently launched ICESAT-2 [52].…”