Dual frequency airborne SAR experiment on snow water equivalent retrieval in Alpine terrain

Abstract. In March 2021 a field experiment was conducted in the high-Alpine test site Woergetal of the Austrian Alps, exploring the feasibility and performance of snow water equivalent (SWE) and snow depth retrievals by means of C- and L-band SAR data. Multiple repeat-pass acquisitions were acquired along two flight tracks with an airborne SAR system. The SAR acquisitions spanned two snowfall events of different intensity. In situ data on physical snow properties were collected throughout the campaign. Main objective was the development and evaluation of procedures for SWE monitoring by measuring the interferometric path delay in snow, with focus on the future Radar Observation System for Europe at L-band (ROSE-L), the Sentinel-1 Next Generation mission, and the proposed geosynchronous C-band SAR mission Hydroterra. The response of the VH/VV backscatter ratio and the co-polarized phase differences (CPD) to snow accumulation was also studied. These two parameters do not show a reproducible relation with snow depth and SWE of the snowfall events. CPD measured after snowfall shows an increase with the incidence angle that can be attributed to radar wave propagation through a refrozen crust formed before the snowfall. For the interferometric SWE retrievals three input data configurations were applied: (1) C- and L-band repeat-pass SAR data at full spatial resolution; (2) differential wavenumber (Delta-k) interferograms obtained by split-bandwidth processing; (3) simulated geosynchronous C-band SAR data reflecting the properties of the Hydroterra mission. Coherence, interferometric phase and computed snow accumulation (ΔSWE) images and their properties are documented for these configurations. L-band case 1 shows high coherence and good retrieval performance for both snowfall events. C-band case 1 and case 3 show high coherence and good performance for the low intensity snowfall but low coherence and 2π phase ambiguity prohibit the application for the intense snowfall. This deficiency can be overcome by the Delta-k approach which delivers suitable results for both frequencies. Options for complementary use of the different approaches are discussed.