Abstract. Between 1993 and 2019, NASA and NSF sponsored 26 separate airborne campaigns that surveyed the thickness and radiostratigraphy of the Greenland Ice Sheet using successive generations of coherent VHF radar sounders developed and operated by The University of Kansas. Most of the ice-sheet’s internal VHF radiostratigraphy is composed of isochronal reflections that record its integrated response to past centennial-to-multi-millennial-scale climatic and dynamic events. We previously generated the first comprehensive dated radiostratigraphy of the Greenland Ice Sheet using the first 20 of these campaigns (1993–2013) and investigated its value for constraining the ice sheet’s history and modern boundary conditions. Here we describe the second major version of this radiostratigraphic dataset using all 26 campaigns, which includes substantial improvements in survey coverage and was mostly acquired with higher-fidelity systems. We incorporated several lessons learned from our previous efforts for improved quality control and accelerated tracing, including an automatic test for stratigraphic conformability, a cutoff length for semi-automatic tracing propagation, a thickness-normalized reprojection for radargrams, and automatic inter-segment reflection matching. We reviewed and augmented the 1993–2013 radiostratigraphy and applied an existing independently developed method for predicting radiostratigraphy to the previously untraced campaigns (2014–2019) to accelerate their semi-automatic tracing. The result is a more robust radiostratigraphy of the ice sheet that can validate the sensitivity of ice-sheet models to past major climate changes and constrain long-term boundary conditions (e.g., accumulation rate). Based on these results, we make several recommendations for how radiostratigraphy may be traced more efficiently and reliably in the future. This dataset is freely available at https://doi.org/10.5281/zenodo.14531734 (MacGregor et al., 2024). It includes all traced reflections at the spatial resolution of the radargrams and grids (5 km horizontal resolution) of the depths of isochrones between 3–115 ka and ages between 10–80 % of the ice thickness; associated codes are available at https://doi.org/10.5281/zenodo.14183061 (MacGregor, 2024a).
The location of snow dunes over the course of the ice‐growth season 2007/08 was mapped on level landfast first‐year sea ice near Barrow, Alaska. Landfast ice formed in mid‐December and exhibited essentially homogeneous snow depths of 4–6 cm in mid‐January; by early February distinct snow dunes were observed. Despite additional snowfall and wind redistribution throughout the season, the location of the dunes was fixed by March, and these locations were highly correlated with the distribution of meltwater ponds at the beginning of June. Our observations, including ground‐based light detection and ranging system (lidar) measurements, show that melt ponds initially form in the interstices between snow dunes, and that the outline of the melt ponds is controlled by snow depth contours. The resulting preferential surface ablation of ponded ice creates the surface topography that later determines the melt pond evolution.
Abstract We present results from a comprehensive field study carried out near Barrow, Alaska, USA, designed to characterize local- to intermediate-scale sea-ice processes in the Arctic coastal zone of central importance to the annual cycle and evolution of the coastal sea ice. Included in this are the behavior of the snow cover of the ice and adjacent tundra and lake system; concurrent studies of mass balance of the sea ice and lake ice; interaction of shortwave radiation with the shore-fast ice and the adjacent land surfaces; evolution of the area coverage and distribution of the various surface types; and the resulting regional albedo values. Maximum snow depths decreased during 2000–02 from 0.38 m to 0.26 m. Ice-melt rates in 2001 were 0.05 and 0.028md –1 at the top and bottom of the sea ice respectively, two to three times larger than observations from the central Arctic. Detailed surface results combined with aircraft photography were used to calculate regional albedos for the late spring and early summer of 2001. Values ranged from 0.8 for all cold snow-covered surfaces to approximately 0.4 for melting sea ice and lake ice vs 0.18 for bare tundra. Regional and surface-based values of cumulative shortwave radiation entering the ice were consistent, indicating that albedo sampling on a scale of 200m can provide a useful representation for regional sea-ice albedo.
Abstract 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 hydrology. Unanticipated discoveries included a reliable method for constraining the thickness within difficult‐to‐sound incised troughs beneath ice sheets, the extent of the firn aquifer within the Greenland Ice Sheet, the vulnerability of many Greenland and Antarctic outlet glaciers to ocean‐driven melting at their grounding zones, and the dominance of surface‐melt‐driven mass loss of Alaskan glaciers. For sea ice, OIB significantly advanced our understanding of spatiotemporal variability in sea ice freeboard and its snow cover, especially through combined analysis of fine‐resolution altimetry, visible imagery, and snow radar measurements of the overlying snow thickness. Such analyses led to the unanticipated discovery of an interdecadal decrease in snow thickness on Arctic sea ice and numerous opportunities to validate sea ice freeboards from satellite radar altimetry. While many of its data sets have yet to be fully explored, OIB’s scientific legacy has already demonstrated the value of sustained investment in reliable airborne platforms, airborne instrument development, interagency and international collaboration, and open and rapid data access to advance our understanding of Earth’s remote polar regions and their role in the Earth system.
