Impact of Surface-Volume Scattering Interaction on C-Band Radar Depolarization Signal in Snow-Covered Regions

The depolarization ratio (PR), defined as the ratio of C-band HV to VV backscattering coefficients, is sensitive to snow depth, making it a useful metric for estimating large-scale snow depth distribution using Sentinel-1 data. However, its application in certain regions has led to significant errors, indicating an incomplete understanding of the underlying physical mechanisms. In an experiment conducted in the Altay Mountains, China, we observed markedly different depolarization ratios in adjacent areas with similar snow depths, where the underlying surface conditions differed. Directly using the depolarization ratio for snow depth retrieval in such cases leads to significantly different estimates, which are not supported by in situ observations. In this study, we demonstrate that this inconsistency is likely caused by the generation of cross-polarization backscattering due to the coupling of bistatic rough surface scattering and snow volume scattering. In particular, a rough surface may produce high cross-polarization scattering under certain incident and scattering angles, which interacts with the snow volume through bistatic scattering. An iterative solution to the vector radiative transfer equation is used to properly account for the coupling between polarimetric bistatic rough surface scattering and snow volume scattering. The simulation results show that after this improvement, the depolarization ratio under rough surface conditions increased, and its potential to match real-world observations was significantly enhanced. It is further demonstrated that simply subtracting the depolarization ratio under snow-free conditions cannot fully cancel the influence of the rough surface, as the remaining depolarization ratio still contains terms related to snow-surface interaction. Therefore, further correction of the rough surface effect is necessary to improve the accuracy of C-band depolarization ratio-based snow depth retrieval.