Abstract
Radar altimeters are important tools to monitor the volume of the ice sheets. The penetration of radar waves in the snowpack is a major source of uncertainty to retrieve surface elevation. To correct this effect, a better understanding of the sensitivity of the radar waveforms to snow properties is needed. Here, we present an extension of the Snow Model Radiative Transfer (SMRT) to compute radar waveforms and conduct a series of simulations on the Antarctic ice sheet. SMRT is driven by snow and surface roughness properties measured over a large latitudinal range during two field campaigns on the Antarctic Plateau. These measurements show that the snowpack is rougher, denser, less stratified, warmer, and has smaller snow grains near the coast than on the central Plateau. These simulations are compared to satellite observations in the Ka, Ku, and S bands. SMRT reproduces the observed waveforms well. For all sites and all sensors, the main contribution comes from the surface echo. The echo from snow grains (volume scattering) represents up to 40% of the amplitude of the total waveform power in the Ka band, and less at the lower frequencies. The highest amplitude is observed on the central Plateau due to the combination of higher reflection from the surface, higher scattering by snow grains in the Ka and Ku bands, and higher inter-layer reflections in the S band. In the Ka band, the wave penetrates in the snowpack less deeply on the central Plateau than near the coast because of the strong scattering caused by the larger snow grains. The opposite is observed in the S band, the wave penetrates deeper on the central Plateau because of the lower absorption due to the lower snow temperatures. The elevation bias caused by wave penetration into the snowpack show a constant bias of 10 cm for all sites in the Ka band, and a bias of 11 cm, and 21 cm in the Ku band for sites close to the coast and the central Plateau, respectively. Now that SMRT is performing waveform simulations, further work will address how the snowpack properties affect the parameters retrieved by more advanced retracking algorithms such as ICE-2 for different snow cover surfaces.
Highlights
Radar altimeters are active sensors emitting microwave pulses and measuring the backscattered energy as a function of time, providing the so-called waveform
Snow Model Radiative Transfer (SMRT) simulations agree with L08 simulations within 1.7% and 1.4% Root Mean Square (RMS) in the Ku and S bands, respectively
This study extended an existing snow radiative transfer model (SMRT) for altimetry applications over snow-covered surfaces
Summary
Radar altimeters are active sensors emitting microwave pulses and measuring the backscattered energy as a function of time, providing the so-called waveform. Additional processes specific to the ice sheets such as the penetration of the wave into the snowpack for instance, limit the accuracy of the inferred elevation (Partington et al, 1989, Legresy and Remy, 1997, Remy and Parouty, 2009). The volume echo depends on the wave penetration and the mechanisms of scattering and absorption in the snowpack (Remy et al, 2012). 40 The idea is to exploit the fact that the penetration depth of the wave is larger at lower frequencies, which in turn makes it possible to untangle the respective contributions from the surface and from inside the snowpack and correct the inferred surface elevation error due to the volume echo Guerreiro et al (2016) and Guerreiro et al (2017) showed that combining altimeter observations at several frequencies (Ka and Ku bands) helps to characterize the volume echo. 40 The idea is to exploit the fact that the penetration depth of the wave is larger at lower frequencies, which in turn makes it possible to untangle the respective contributions from the surface and from inside the snowpack and correct the inferred surface elevation error due to the volume echo
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