Glaciers on the Brink: New Alaskan Ice thickness Constraints from Operation IceBridge Airborne Radar Sounding

Over the past decade, NASA Operation IceBridge flights in Alaska have targeted glacier surface elevation change using scanning lidar. Radar sounding was added to provide measurements of ice thickness and subglacial bed topography since Alaskan glaciers are changing rapidly and contributing to global sea level rise, yet little information has been available to constrain bed geometry for the major glaciers and icefields.

The radars employed are long wavelength due to extensive crevassing and the warm, wet ice typical of Alaska. The latest evolution of these sounders is a chirped, 2 kW radar operating at a center frequency of 2.5 MHz with a 2.5 MHz bandwidth, or 5 MHz with a 5 MHz bandwidth, using a towed, resistively loaded dipole antenna. Interpretation of data is challenging due to prevalent off-nadir surface reflections from valley walls and other topography present in this region. We therefore employ clutter simulations for all data using a geometric optics, facet-based simulator and a digital terrain model. This analysis has also been applied radar sounding data previously acquired to provide a consistent and accurate overall view.

As a result of this extended effort, a number of Alaska's icefields have been sounded including the Harding, Yakutat, Juneau, and portions of the Bagley. The terminal lobes and lower trunks of many large glaciers have also been sounded, including Malaspina, Bering, Hubbard, and Kennicott. In most cases, clutter obscures bed returns higher in the trunks. In all cases, however, overdeepenings and/or troughs that reach well below sea level have been observed, indicating that the recent expansion of proglacial lakes at most these major glaciers will continue and could lead to accelerated retreat as has already taken place for Columbia Glacier.