The Advanced Topographic Laser Altimeter System on Ice, Cloud and land Elevation Satellite 2 (ICESat-2) offers a new remote sensing capability to measure complex sea ice surface topography. We demonstrate the retrieval of six sea ice parameters from ICESat-2/Advanced Topographic Laser Altimeter System data: surface roughness, ridge height, ridge frequency, melt pond depth, floe size distribution, and lead frequency. Our results establish that these properties can be observed in high fidelity, across broad geographic regions and ice conditions. We resolve features as narrow as 7 m and achieve a vertical height precision of 0.01 m, representing a significant advance in resolution over previous satellite altimeters. ICESat-2 employs a year-round observation strategy spanning all seasons, across both the Arctic and Southern Oceans. Because of its higher resolution, coupled with the spatial and temporal extent of data acquisition, ICESat-2 observations may be used to investigate time-varying, dynamic, and thermodynamic sea ice processes.

The National Aeronautics and Space Administration (NASA)'s Operation IceBridge (OIB) was a 13-year (2009-2021) airborne mission to survey land and sea ice across the Arctic, Antarctic, and Alaska. Here, we review OIB's goals, instruments, campaigns, key scientific results, and implications for future investigations of the cryosphere. OIB's primary goal was to use airborne laser altimetry to bridge the gap in fine-resolution elevation measurements of ice from space between the conclusion of NASA's Ice, Cloud, and land Elevation Satellite (ICESat;2003-2009 and its follow-on, ICESat-2 (launched 2018). Additional scientific requirements were intended to contextualize observed elevation changes using a multisensor suite of radar sounders, gravimeters, magnetometers, and cameras. Using 15 different aircraft, OIB conducted 968 science flights, of which 42% were repeat surveys of land ice, 42% were surveys of previously unmapped terrain across the Greenland and Antarctic ice sheets, Arctic ice caps, and Alaskan glaciers, and 16% were surveys of sea ice. The combination of an expansive instrument suite and breadth of surveys enabled numerous fundamental advances in our understanding of the Earth's cryosphere. For land ice, OIB dramatically improved knowledge of interannual outlet-glacier variability, ice-sheet, and outlet-glacier thicknesses, snowfall rates on ice sheets, fjord and sub-ice-shelf bathymetry, and ice-sheet MACGREGOR ET AL.

High-resolution observations of melt ponds (MPs) across the Arctic are lacking, yet essential for understanding the sea ice energy budget and under-ice ecology. We present a pixel-based classification scheme to identify undeformed and deformed ice, open water, and light, medium, and dark MPs in images of sea ice undergoing melt. The scheme was applied to 0.1-m resolution Operation IceBridge Digital Mapping System imagery covering an area of~4,000 km 2. Observations of both the unconsolidated, marginal ice zone of the Beaufort/Chukchi Seas (B/C Seas) and the consolidated, multiyear ice of the central Arctic (CA) were obtained. Sea ice concentration (SIC), melt pond fraction (MPF), and pond color fraction (PCF) were derived on a per-image basis. SIC averaged 69% in the B/C Seas and 90% in the CA. We find that both MPF and PCF are dependent on the ice type on which ponds form. MPF averaged 25% in the B/C Seas, where dark ponds dominated and had a PCF of 60%, compared to a PCF of 9% and 31%, for medium and light ponds, respectively. MPF averaged 14% in the CA, where the PCF of light ponds was 68%, compared with 16% for both medium and dark ponds. As the multiyear ice of the Arctic Ocean is replaced by a younger, more seasonal ice cover, our results suggest that MPF will increase, and MP color will darken. This would enhance the ice albedo feedback, exacerbating that already due to the multidecadal decline in summer ice extent. Plain Language Summary Detailed observations of summer melt features on Arctic sea ice are limited, yet essential for modeling and understanding summer sea ice processes. An algorithm was developed to classify ice, open water, and melt ponds in high-resolution NASA Operation IceBridge Digital Mapping System (DMS) imagery. More than 17,000 images over~4,000 km 2 were analyzed. Sea ice concentration (SIC), melt pond fraction (MPF), and pond color fraction (PCF) were derived from the classified images. We compared results from two regions with distinct ice conditions: the unconsolidated marginal ice zone in the Beaufort and Chukchi (B/C) Seas, which consisted of predominantly first year ice, and the consolidated, predominantly multiyear ice pack of the central Arctic (CA) Ocean. In the B/C Seas, we found that the MPF is greater, SIC is lower, and ponds are darker than in the CA region. As the percentage of first year ice in the Arctic increases, our results suggest that the Arctic-wide MPF will increase, and pond color will darken. This will contribute to the positive ice-albedo feedback mechanism and has implications for modeling sea ice albedo.

The Advanced Topographic Laser Altimeter System on Ice, Cloud and land Elevation Satellite 2 (ICESat-2) offers a new remote sensing capability to measure complex sea ice surface topography. We demonstrate the retrieval of six sea ice parameters from ICESat-2/Advanced Topographic Laser Altimeter System data: surface roughness, ridge height, ridge frequency, melt pond depth, floe size distribution, and lead frequency. Our results establish that these properties can be observed in high fidelity, across broad geographic regions and ice conditions. We resolve features as narrow as 7 m and achieve a vertical height precision of 0.01 m, representing a significant advance in resolution over previous satellite altimeters. ICESat-2 employs a year-round observation strategy spanning all seasons, across both the Arctic and Southern Oceans. Because of its higher resolution, coupled with the spatial and temporal extent of data acquisition, ICESat-2 observations may be used to investigate time-varying, dynamic, and thermodynamic sea ice processes.

No abstract