Nioghalvfjerdsfjorden Glacier (N79) is one of the two main outlets for Greenland’s largest ice stream, the Northeast Greenland Ice Stream (NEGIS), and is the more stable of the two, with no calving front retreat expected in the near future. Using a novel elevation reconstruction approach combining digital elevation models (DEMs) and laser altimetry, previously undetected local phenomena are identified complicating this assessment. N79 is found to have a complex network of basal channels that were largely stable between 1978 and 2012. Since then, an along-flow central basal channel has been growing rapidly, likely due to increased runoff and ocean temperatures, and possibly threatening to decouple the glacier’s northwestern and southeastern halves.

A document by Enze Zhang. Click on the document to view its contents.

This paper presents an assessment of the horizon-tal accuracy and precision of the laser altimetry observations collected by NASA’s ICESat-2 mission. We selected the terrain-matching method to determine the position of laser altimeter profiles within a precisely knownn surface, represented by a DEM. We took this classical approach a step further, approx-imated the DEM by planar surfaces and calculated the optimal position of the laser profile by minimizing the square sum of the elevation differences between reference DEMs and ICESat-2 profiles. We found the highly accurate DEMs of the McMurdo Dry Valleys, Antarctica, ideal for this research because of their stable landscape and rugged topography. We computed the 3D shift parameters of 379 different laser altimeter profiles along two reference ground tracks collected within the first two years of the mission. Analyzing these results revealed a total geolocation error (mean + 1 sigma) of 4.93 m for release 3 and 4.66 m for release 4 data. These numbers are the averages of the six beams, expressed as mean + 1 sigma and lie well within the mission requirement of 6.5 m.

ICESat-2, the first photon counting satellite, maps the earth’s surface with unprecedented details and accuracy. However, the resulting photon clouds, distributed as the ATL-03 geocoded photon product, are vast, unstructured, and noisy datasets. The high density and four-dimensional nature of the photon dataset (location and time), coupled with different responses over different surfaces (e.g., ice, forest cover, water), pose a unique and challenging problem regarding surface detection’s overall objective of intelligently reducing the data volume. Multiscale models uncover hidden structures in data due to their ability to analyze the underlying processes at multiple scales. Besides the traditional wisdom of using multiple scales for improving local and global approximations, in this work, we show their application as an intelligent sampling mechanism for redundant and noisy datasets. Our proposed approach’s fundamental idea is the generation of data dependent and multiscale basis functions and corresponding representative sparse representations, which retain points essential for minimizing the error of reconstruction. Thereby, points associated with rapid spatial change are chosen, while those that are easily reconstructed using the smooth basis functions are discarded. As the final output, the algorithm provides an efficient sparse representation of the data that captures all relevant features for modeling and prediction with quantified uncertainty. Our presentation includes a detailed description of the algorithm and theory as applied to process the ATL-03 geolocated photon product of the ICESat-2 mission. We will demonstrate the efficiency of our approach by examples of different ice sheet surfaces, including heavily crevassed glaciers, that pose a challenge for currently used change detection methods.

The rapid acceleration of Greenland Ice Sheet mass loss over, particularly the last two decades, is well documented.However, limits in early remote sensing restricted the details with which we could examine local changes on an ice-sheet-wide scale, particularly in areas of slow motion, along shear margins and complex coastal terrain. We explore the localcharacter of rapid contemporary change marine-terminating glaciers using satellite-derived ice sheet surface velocities,glacier terminus advance/retreat history, and surface elevation-change data from the 1980s to the present. Widespread glacierterminus retreat is a strong and more consistent climate response indicator than velocity change, but local changes in velocityare critical indicators of rapid ice sheet reconfiguration. Ice thickness changes related to changing ice dynamics often providethe first evidence of processes that initiate outlet glacier retreats and mass loss, such as the development of sub-ice shelfcavities and subglacial hydrology changes. Reconfiguration is observed locally as narrowing zones of fast-flow, ice flowrerouting, shear margin migration, and likely glacier outlet abandonment. These patterns are apparent in all ice sheet sectorsand observable from space-borne instruments. The rapid reconfiguration now well underway in Greenland has wide-rangingimplications, including expected changes in subglacial hydrology, ice discharge, freshwater flux to the ocean, and transport ofnutrients and sediment. Lacking detailed observations of earlier deglaciations and current limits on ice-sheet modelcapabilities, the expanding details of these combined observational records may provide a valuable analog for studying pastice sheet dynamics and projecting future ice loss.