A Mass-Conserving Bed Solution for the Hubbard Glacier from Airborne Radar Data

Basal topography is a strong control on tidewater glacier behavior, yet the bed of the largest tidewater glacier in North America, the Hubbard Glacier, is poorly constrained. The terminus of Hubbard contains heavily crevassed temperate ice, which makes thickness measurements with ground-based radars difficult. In addition, surface clutter from valley walls complicates airborne measurements further up the glacier. In spite of this, some ice thickness data have been successfully acquired over Hubbard's terminal lobe as a part of NASA's Operation IceBridge (OIB) using three different long-wavelength radar sounders. After surface clutter analysis and picking, these thickness profiles are useful for a first order analysis of bed geometry. However, a map interpolated from the radar tracks would be far more useful for volume estimates and as a basis for ice flow modeling. Interpolation with the commonly used kriging method often causes violation of the principle of mass conservation when used to calculate ice fluxes. Instead, we exploit the principle of mass conservation to interpolate the radar profiles and give a more physically based bed solution. Mass conservation interpolation requires additional constraints beyond the radar data, and we therefore use 1) laser altimetry measurements from the same OIB flights (Larsen, NSIDC, 2018); 2) surface mass balance measurements from nearby Yakutat Glacier (Trüssel et al., J. Glac, 2015) and; 3) GoLive satellite-based velocity measurements (Scambos et al., NSIDC, 2015). The mass conservation solution shows several interesting features, such as a deep trough at the convergence of the Hubbard and Valerie glaciers, as well as grounding below sea level for 15 kilometers up from the terminus.