Abstract. Supraglacial lakes and melt ponds occur in the ablation zones of Antarctica and Greenland during the summer months. Detection of lake extent, depth, and temporal evolution is important for understanding glacier dynamics. Previous remote sensing observations of lake depth are limited to estimates from passive satellite imagery, which has inherent uncertainties, and there is little ground truth available. In this study, we use laser altimetry data from the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) over the Antarctic and Greenland ablation zones and the Airborne Topographic Mapper (ATM) for Hiawatha Glacier (Greenland) to demonstrate retrievals of supraglacial lake depth. Using an algorithm to separate lake surfaces and beds, we present case studies for 12 supraglacial lakes with the ATM lidar and 12 lakes with ICESat-2. Both lidars reliably detect bottom returns for lake beds as deep as 7 m. Lake bed uncertainties for these retrievals are 0.05–0.20 m for ATM and 0.12–0.80 m for ICESat-2, with the highest uncertainties observed for lakes deeper than 4 m. The bimodal nature of lake returns means that high-confidence photons are often insufficient to fully profile lakes, so lower confidence and buffer photons are required to view the lake bed. Despite challenges in automation, the altimeter results are promising, and we expect them to serve as a benchmark for future studies of surface meltwater depths.
A first look at data from NASA’s laser altimeter mission ICESat-2 reveals very high resolution 3-D profiles of ice on land and sea, forests, and shallow bodies of water.
In July and August 2014, NASA conducted an airborne lidar campaign based out of Fort Wainwright,Fairbanks, Alaska, in support of Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) algorithm development. The survey targeted Alaskan glaciers and icefields and sea ice in the Arctic Ocean during the summer melt season. Ultimately, the mission, MABEL Alaska 2014, including checkout and transit flights, conducted 11 science flights, for a total of over 50 flight hours over glaciers, icefields, and sea ice.
The State's geology is presented on eight full-color map sheets, one for each of the major islands. These map sheets, the illustrative meat of the publication, can be downloaded in pdf format, ready to print. Map scale is 1:100,000 for most of the islands, so that each map is about 27 inches by 36 inches. The Island of Hawai`i, largest of the islands, is depicted at a smaller scale, 1:250,000, so that it, too, can be shown on 36-inch-wide paper. The new publication isn't limited strictly to its map depictions. Twenty years have passed since David Clague and Brent Dalrymple published a comprehensive report that summarized the geology of all the islands, and it has been even longer since the last edition of Gordon Macdonald's book, Islands in the Sea, was revised. Therefore the new statewide geologic map includes an 83-page explanatory pamphlet that revisits many of the concepts that have evolved in our geologic understanding of the eight main islands. The pamphlet includes simplified page-size geologic maps for each island, summaries of all the radiometric ages that have been gathered since about 1960, generalized depictions of geochemical analyses for each volcano's eruptive stages, and discussion of some outstanding topics that remain controversial or deserving of additional research. The pamphlet also contains a complete description of map units, which enumerates the characteristics for each of the state's many stratigraphic formations shown on the map sheets. Since the late 1980s, the audience for geologic maps has grown as desktop computers and map-based software have become increasingly powerful. Those who prefer the convenience and access offered by Geographic Information Systems (GIS) can also feast on this publication. An electronic database, suitable for most GIS software applications, is available for downloading. The GIS database is in an Earth projection widely employed throughout the State of Hawai`i, using the North American datum of 1983 and the Universal Transverse Mercator system projection to zone 4. 'This digital statewide map allows engineers, consultants, and scientists from many different fields to take advantage of the geologic database,' said John Sinton, a geology professor at the University of Hawai`i, whose new mapping of the Wai`anae Range (West O`ahu) appears on the map. Indeed, when a testing version was first made available, most requests came from biologists, archaeologists, and soil scientists interested in applying the map's GIS database to their ongoing investigations. Another area newly depicted on the map, in addition to the Wai`anae Range, is Haleakala volcano, East Maui. So too for the active lava flows of Kilauea volcano, Island of Hawai`i, where the landscape has continued to evolve in the ten years since publication of the Big Island's revised geologic map. For the other islands, much of the map is compiled from mapping published in the 1930-1960s. This reliance stems partly from shortage of funding to undertake entirely new mapping but is warranted by the exemplary mapping of those early experts. The boundaries of all map units are digitized to show correctly on modern topographic maps.
Elevation-change-rate estimates from the two missions have been corrected for firn-air content changes. For grounded ice, they have been corrected for changes in the elevation of the bedrock under the ice sheet, and for floating ice, the hydrostatic relation has been used to convert elevation changes to thickness changes. We provide a set of grids that contains fully corrected but unsmoothed gridded data, from which our calculations of drainage-by-drainage mass change were derived, and a second set of smoothed grids that are intended for display purposes only. Each set of grids provides ice-sheet surface height and thickness change rate in meters of ice per year (m/yr).
Full details of the processing and analysis of these data are provided in Smith et al., (2020, Science). Two sets of files are provided in geotif format: one unfiltered set, which are suitable for mass-balance integrations, and a filtered set that have been smoothed for display, which match figures 2 and 3 in Smith et. al, 2020.
