Potential of full-polarimetric P-band SAR in characterizing subsurface soil profile in Arctic tundra

Abstract:

Surface organic layer and soil moisture represent first-order controls on the potential vulnerability of permafrost to thaw. Extensive airborne full polarimetric L-band and P-band SAR (PolSAR) data were collected from NASA ABoVE airborne campaign, which offers opportunities for clarifying heterogeneous tundra soil organic and moisture conditions to better inform Earth system model predictions of northern permafrost regions and their critical role in global climate change. In this study, we investigated the potential of P-band PolSAR data in characterizing subsurface soil moisture and highly organic tundra soil profiles using AirMOSS data collected over Alaska North Slope during pre-ABoVE (2014-2015) and ABoVE (2017) airborne campaigns. The co-polarized ratio (VV/HH) of radar backscatter showed close correlation with the normalized polarization ratio derived from L-band brightness temperature data (R=0.76, p<0.1), which is sensitive to surface wetness in this area. Consistent decreases in the co-polarized ratio with surface drying was observed during active layer thawing. We hypothesized this was due to deeper penetration of the P-band radar pulse and increasing contribution of subsurface scattering to the backscatter signal, with a decrease in the surface dielectric constant and a layered dielectric profile in continuous permafrost. This was supported by the polarimetric target decomposition and model sensitivity analysis. Touzi decomposition using the PolSAR data showed that the P-band backscatter in this area was dominated by surface and subsurface scattering; there was an increase of ~20-30° in the dominant-scattering-type phase in late August (i.e. maximum thaw depth) relative to early June (i.e. early thaw period), mainly attributed to increasing subsurface scattering underlying a drier surface layer. Using the small perturbation method (SPM) and a multi-layer dielectric structure, we showed that changes in the co-polarized ratio were mainly due to the different sensitivity of radar polarizations (HH, VV) to subsurface scattering, with a lower polarized ratio (VV/HH) associated with a drier surface. Our results also showed different characteristic radar signals in areas with different soil organic carbon profiles, likely due to different soil hydrologic and dielectric properties.