Evolution of L-band SAR Response for Soil Freeze/Thaw Monitoring: A Case Study Over Snow-Covered Canadian Mid-latitude Agricultural Region

Abstract. Soil freeze/thaw (F/T) cycles are critical regulators of global hydrological and biogeochemical processes, yet monitoring these subsurface dynamics beneath snow cover remains a significant observational challenge. There is a corresponding need for physically-based retrieval frameworks to support upcoming spaceborne Earth observation missions, such as the NASA-ISRO Synthetic Aperture Radar (NISAR) mission. Despite the importance of these cycles, there remains a critical lack of understanding regarding L-band Synthetic Aperture Radar (SAR) response beneath snow cover, primarily resulting from a reliance on coarse-resolution data and a lack of coincident, season-long ground validation. To address this, we introduce an integrated physical framework that couples high-resolution (1 m) airborne L-band (1.3 GHz) observations with coincident in situ measurements of soil temperature and permittivity. This approach utilizes analysis of backscatter responses, Freeman-Durden polarimetric decomposition, and the Improved Integral Equation Model (I2EM) to physically interpret microwave scattering and characterize subnivean F/T transitions under frozen and thawed conditions. VV-polarized backscatter exhibited the strongest sensitivity to F/T transitions, increasing during thaw and decreasing under frozen soil. Decomposition analysis revealed dominant surface scattering under frozen conditions, increased surface scattering during thaw, and enhanced volume scattering associated with melt–refreeze cycles. The I2EM simulations captured the VV and HV backscatter trends within an acceptable range across most soil stations, while significantly underestimating the HH backscatter. Overall, these results advance process-level understanding of the L-band SAR response to subnivean soil F/T transitions and demonstrate the potential of high-resolution observations for improving retrieval algorithms and calibrating forthcoming global L-band satellite missions.