Conference Agenda

The snow water equivalent (SWE) describes the amount of water stored in a snow pack. The retrieval of SWE with SAR methods has mostly been explored for snow over land. This study seeks to explore SAR-based SWE accumulation retrieval on glaciers.

Differential Interferometric SAR (D-InSAR) combines temporally separated acquisitions that ideally have no spatial baseline. A common D-InSAR assumption is that scatterers stay constant in between acquisitions. This assumption is already debatable for snow over land, but requires even more consideration on glaciers. When it holds, the D-InSAR phase is solely a propagation effect from SWE change and an established inversion method for snow over land exists. The objective of this study is to better understand what, besides the SWE-induced propagation effect, needs to be accounted for in SWE change retrieval with D-InSAR over glaciers.

For this, polarimetric SAR (PolSAR) plays an important role. The copolar phase difference (CPD), which is the phase difference between HH and VV polarizations, can be related to the snow height. The polarimetric scattering angle α describes scattering mechanisms independent of line-of-sight rotation. Possible values are 0° ≤ α ≤ 90° where angles close to zero tend to relate to surface scattering, while angles closer to 90° correspond to dihedral scattering events. If scatterers do not change in between acquisitions, even if there was snowfall, α should remain constant, while the new snow should give a change in CPD. Further, changes in scattering mechanisms and snow accumulation should manifest differently across frequencies in α and CPD, where in an ideal scenario, the CPD should scale linearly with wavelength, while scattering mechanisms are generally frequency dependent.

The data used in this study stems from two fully polarimetric, airborne SAR campaigns of the Aletsch Glacier in Switzerland in 2022 and 2024 conducted by DLR and partners. Both campaigns entail short time series with DLR’s F-SAR sensor in X-, C-, and L-band over the course of multiple weeks. Snow accumulation and density were measured at several locations during the campaigns and corner reflectors were placed as reference targets. In 2024, snow anisotropy measurements were carried out by SLF. In the campaigns, the weather conditions differed greatly, with warm temperatures in 2022 and cooler, more stable conditions with more fresh snow in 2024. As a first assessment of the constant-scatterer assumption, the scattering angle α was analyzed for the 2022 and 2024 Aletsch campaigns.

In the polarimetric analysis of the 2022 data changes in α are observed, fitting a melting event that took place between two acquisitions, resulting in more surface scattering. After a snowfall event, α displays more volume scattering. In contrast, the 2024 acquisitions barely differ in scattering mechanisms. Constant cold temperatures and new snow relate to a very stable α. The contrast of these two campaigns shows that while the assumption of constant scatterers does hold for 2024, it would be incorrect for 2022. Therefore, α is an important parameter in deciding where D-InSAR SWE retrieval is reasonable.

Further research should help understand what assumptions common to snow over land hold up for snow over glaciers, and how polarimetric SAR can help to better understand influences on the D-InSAR phase. This will facilitate the model development towards D-InSAR SWE change retrieval over glaciers.