Glaciology - Geophysics - Radar Theory

Inferring Ice Temperature from Radar

The conductivity of ice is primarily controlled by two properties: its temperature and chemistry. When ice is warm or impurity rich, it has an elevated conductivity, resulting in additional power losses as energy carried by the electromagnetic wave is dissipated as current. For reflection radioglaciology surveys, it is difficult to disentangle power losses during propagation from power losses during reflection, limiting our ability to infer ice and reflector property information from radar data alone. In my research, I am looking into other data sources that can aid in inferring ice conductivity, and therefore temperature. Current work attempts to use surface velocity information (derived from InSAR observations of the ice sheet surface) to constrain thermal gradients in the ice sheets (based on work done during shear and longitudinal strain), to ultimately inform an inverse problem for spatially variable attenuation in radar data.

Diagnosing Stable Bed Properties using Internal Ice Structures

Spatially variable flow rates leave a record in the internal structure of the ice, detectable using radar. As the ice flows over either resistive or lubricated beds, it compresses or stretches, leaving characteristic synformal and antiformal structures in the internal layers of the ice sheet. While challenging to interpret, these structures are some of the few data that directly reflect the frictional resistance of the system, and have the potential to inform our understanding of the basal boundary conditions. One goal of my research is to be able to use measurable features in these structures to invert for the frictional characteristics at the bed. Working toward this objective involves observational studies (collecting and examining radar data over areas of known or expected variability in the bed properties) as well as modeling exercises (cataloging the suite of possible and expected internal structures). The Amundsen Sea sector of West Antarctica is a perfect area to test this method, as the frictional properties of the system are thought to be geologically controlled, and therefore spatially and temporally stable over the glacial cycle.

Evaluating Diurnal Cycling of Glacier Speeds

Improved instrumentation makes it possible to map glacier speeds at high frequencies over large areas. Using a GAMMA Ground-based, portable, radar interferometer (GPRi), we take 2 minute repeat measures of the glacier speed in an effort to evaluate diurnal variations in ice flow during the melt season. These data are calibrated against GPS observations on ice, and provide some of the first 2D maps of glacier flow on a minute-by-minute basis in the North Cascades.

West Antarctic Stability and the Interior Basins

The West Antarctic Ice Sheet (WAIS) rests on a bed almost entirely below sea-level. Its geometry makes it prone to retreat for several reasons:

1) Modern Antarctic mass loss is controlled by warm ocean water, which thins the ice shelves at the margins. Unlike East Antarctica, where the ice sheet would retreat up out of the ocean, into the rocky highlands of the interior, the ocean will have continued access to the margins of WAIS.

2) The flux of ice across the grounding line (the grounded/floating transition) scales with the thickness of the ice. The bed deepens inland of the modern margin, so grounded ice upstream of the current groundingline is, by necessity, thicker than ice at the modern grounding line. Any retreat will increase the flux of ice to the ocean and accelerate retreat.

Now that the initiation of retreat has been observed, it is important that we think carefully about the interior basin geometry and the factors that will control the timing and maximum inland retreat of collapse. I have previously used radar to better constrain the geometric boundary conditions for Marie Byrd Land, and investigated the long term stability and nucleation potential for an ice-cap on a deeper bed than previously estimated.

Antarctic Surface Elevation Antarctic Basal Topography Antarctic Surface Velocity

© Nick Holschuh - February 2019