Penetration of interferometric radar signals in Antarctic snow

• Main Authors: Rott, Helmut, Scheiblauer, Stefan, Wuite, Jan, Krieger, Lukas, Floricioiu, Dana, Rizzoli, Paola, Libert, Ludivine, Nagler, Thomas
• Format: Text
• Language: English
• Published: 2021
• Subjects: Antarctic; West Antarctica; Ellsworth Mountains; Union Glacier; Antarc*; Antarctica
• Online Access: https://doi.org/10.5194/tc-2020-380; https://tc.copernicus.org/preprints/tc-2020-380/

Synthetic aperture radar interferometry (InSAR) is an efficient technique for mapping the surface elevation and its temporal change over glaciers and ice sheets. However, due to the penetration of the SAR signal into snow and ice the apparent elevation in uncorrected InSAR digital elevation models (DEMs) is displaced versus the actual surface. We studied relations between interferometric radar signals and physical snow properties and tested procedures for correcting the elevation bias. The work is based on satellite and in-situ data over Union Glacier in the Ellsworth Mountains, West Antarctica, including interferometric data of the TanDEM-X mission, topographic data from optical satellite sensors and field measurements on snow structure and stratigraphy undertaken in December 2016. The study area comprises ice-free surfaces, bare ice, dry snow and firn with a variety of structural features related to local differences in wind exposure and snow accumulation. Time series of laser measurements of NASA’s Ice, Cloud and land Elevation Satellite (ICESat) and ICESat-2 show steady state surface topography. For area-wide elevation reference we use the Reference Elevation Model of Antarctica (REMA). The different elevation data are vertically co-registered on a blue ice area and an ice-free slope, surfaces not affected by radar signal penetration. The backscatter simulations with a multi-layer radiative transfer model show large variations for scattering of individual snow layers due to different size and structure of the scattering elements. The average depth-dependent backscatter contributions can be approximated by an exponential function. We obtain estimates of the elevation bias by inverting the interferometric volume correlation coefficient (coherence) applying a uniform volume model for describing the vertical loss function. Whereas the mean values of the computed elevation bias and the elevation difference between the TDM DEMs and the REMA show good agreement, a trend towards overestimation of penetration is evident for heavily wind-exposed areas and towards underestimation for areas with higher accumulation rates. The angular gradients of the backscatter intensity show also distinct differences between these two domains. This behaviour can be attributed to the anisotropy of the snow/firn volume structure showing differences in the size and shape of the scattering elements and in stratification related to snow accumulation and wind-driven erosion and deposition.