Geophysical and atmospheric controls on Ku-, X- and C-band backscatter evolution from a saline snow cover on first-year sea ice from late-winter to pre-early melt

This study investigates changes in Ku-, X- and C-band microwave backscatter, co-polarized ratio and dual-frequency ratios, to changes in in-situ measured snow thermophysical properties, driven by surface radiation measurements, during a 7-day transition from late winter to pre-early melt on a highly saline snow cover on smooth first-year ice. A surface-based Ku-, X- and C-band microwave scatterometer system is used near coincident with in-situ thermophysical snow measurements. A frequency-dependent penetration depth model is utilized to simulate diverse variations in Ku-, X- and C-band penetration during fluctuating snow thermophysical and atmospheric conditions. These results helped improve the interpretation of observed changes in Ku-, X- and C-band backscatter. Overall, dielectric loss associated with presence of brine throughout the snow volume, is observed to be the governing factor affecting microwave penetration. Our results indicate significant differences in observed microwave backscatter and derived co-polarized ratio and dual-frequency ratios, for all three frequencies during the transition from cold late winter phase to warm pre-early melt onset phase. C-band demonstrates the greatest increase in backscatter and co-pol ratios, followed by X- and Ku-bands. The dual-frequency ratio combinations clearly demonstrate their ability to separate sensitivities in frequencies by characterizing changes in backscatter during the 7-day transition. Our results indicate strong direct influence of downwelling shortwave and longwave radiation on snow brine volume and dielectric properties, which drives changes to Ku-, X- and C-band microwave backscatter and its associated co-polarized ratio and dual-frequency ratios. Our results demonstrate the ability of an observational multi-frequency active microwave approach to illustrate frequency-, polarization- and incidence angle-dependent changes in microwave backscatter from snow covers on first-year ice, at varying atmospheric and snow thermophysical conditions.