Sensitivity of Multi-frequency Polarimetric SAR Data to Post-Fire Permafrost Changes and Recovery Processes in Arctic Tundra

Abstract— We used full polarimetric L-band and P-band synthetic aperture radar (SAR) data collected from the recent NASA Arctic Boreal Vulnerability Experiment (ABoVE) airborne campaign and Sentinel-1 C-band dual-polarization data to understand the sensitivity of radar backscatter intensity and phase to fire-induced changes in the surface and subsurface soil processes in Arctic tundra underlain by permafrost. The 2007 Anaktuvuk River fire on the Alaska North Slope was used as a case study. At ~10-year post-fire, we observed strong increases (> ~3-4 dB) in the low frequency radar backscatter in severely burned areas during the thaw season, in contrast to limited (< ~0.5 dB) C-band backscatter differences (VV, VH) between burned and unburned areas. However, C-band winter backscatter is generally higher (>1 dB) in burned areas than adjacent unburned areas. Polarimetric decomposition analysis indicated a general trend towards more random surface scattering, and strong increases in double-bounce and volume scattering power at both P- and L-band in the burned areas. The ice-rich yedoma region shows largest backscatter increases in burned areas, and the highest correlation with burn severity and microtopography changes. The above backscatter changes are attributed to increasing surface roughness and microtopography due to ice wedge degradation and thermokarst development, and increasing subsurface scattering due to an overall drier and deeper active layer in burned areas. Among all frequencies, P-band shows consistently larger contrast in backscatter power and phase between burned and unburned areas, which makes it potentially more useful to study fire-permafrost interactions in the Arctic over decadal time scales.