What can polarimetric SAR data tell us about snow accumulation on polar ice sheets?

Stefanie.Linow [ at ] awi.de


Climate processes in the polar regions affect global sea level variations, and also impact ocean circulation patterns by changing the freshwater flux into the oceans. Those processes need to be carefully monitored and the assessment of the mass balance of the polar ice sheets is crucial in this context. Here, microwave remote sensing instruments can provide observations in regions which are otherwise difficult to access. The mass balance of the polar ice sheets can be described in terms of mass gain by snow accumulation and mass loss by melting and iceberg calving. The effect of melt processes and the export of ice mass beyond the grounding line of the ice shelves can be observed using passive microwave radiometers, scatterometers, imaging radar, and laser and radar altimeters. Also for gathering information on snow accumulation rates, different sensors are in use. Several snow accumulation rates retrieval methods exist, but are still afflicted by large uncertainties. Satellite data from sensors with large spatial coverage and high temporal sampling rate (such as passive microwave radiometers and scatterometers) are used to map snow accumulation on continent-wide scale, and SAR data have been applied on regional and local scales. In the case of SAR data, the backscattering intensity depends on the snow grain size and the thickness of the annual firn* layers, but also on firn density, temperature and wind regime. As large grain sizes and thin annual layers are characteristic for low accumulation, and small grains deposited in thicker layers are typical for high accumulation, radar is used to indirectly measure accumulation rates. Radar backscattering characteristics of firn depend on size, shape, orientation, volume fraction, and absorption loss of the scattering elements, and on the absorption loss of the background medium. Due to the high scatterer density, coherent effects between the scatterers also influence backscattering characteristics. The radar backscattering coefficient additionally exhibits an azimuthal anisotropy which is related to wave-like undulations of the ice surface on scales of decimetres to tens of metres (sastrugi). Since the surface scattering contribution of dry snow and firn is very low, it is assumed that also the surfaces from former years, which are buried under the most recent snow layers, contribute to the radar scattering. The complexity of the radar scattering processes not only requires a number of environmental parameters, such as temperature and wind, to be considered in snow accumulation retrieval approaches, but also calls for an increase in the number of directly observed variables. The objective of our investigation is to analyse whether multi-frequency and multi-polarization SAR data improve the retrieval of accumulation rates and provide additional insight into the interaction between radar waves and firn. We will study polarimetric SAR data from different sensors (Terrasar-X, Radarsat-2) in comparison to conventional SAR systems (Envisat ASAR and possibly Sentinel-1). These data were acquired over the Kottas Traverse in Dronning Maud Land, Antarctica. For this site, a large number of in-situ measurements of snow accumulation are available for validation purposes. We will focus on the differences of the azimuthal anisotropy observed at C- and X-band, taking into account the scene headings and radar penetration depths. We will also examine to which extent the phase difference and correlation between the HH- and VV-channel might give information about snow and ice conditions that is complementary to the radar intensity patterns. * Firn is an intermediate stage between snow and glacial ice, with densities ranging from 400 - 830 kg/m³.

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ESA POLinSAR 2015 - 1st BIOMASS Science Workshop, 26 Jan 2015 - 30 Jan 2015, Frascati, Italy.
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Linow, S. , Dierking, W. and Rack, W. (2015): What can polarimetric SAR data tell us about snow accumulation on polar ice sheets? , ESA POLinSAR 2015 - 1st BIOMASS Science Workshop, Frascati, Italy, 26 January 2015 - 30 January 2015 .

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