SCATTERING MECHANISMS IN THE ICE SHEET’S PERCOLATION ZONES INFERRED FROM SMOS INCIDENCE DIAGRAMS

P. Zeiger^1,2, G. Picard^3,2, M. Leduc-Leballeur⁴, B. Vandecrux⁵

¹Centre National d’Etudes Spatiales (CNES), Toulouse, France, ²Institut des Géosciences de l’Environnement (IGE), Grenoble, France, ³Université Grenoble Alpes, Grenoble, France, ⁴IFAC, National Research Council, Firenze, Italy, ⁵Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark

The Soil Moisture and Ocean Salinity (SMOS) mission conducts L-band passive microwave measurements since 2010 with an incidence angle ranging from nadir to ~65°. Spatial and temporal variations of SMOS observations have been extensively used over the ice sheets to map subsurface melt and refreeze and internal temperature profiles. These applications have been developed using measurements restricted to a small incidence angle bin of 5° (e.g. 50 – 55°), to limit the complexity and sources of uncertainties arising from the use of full incidence diagrams. Here, we explore the incidence signatures of SMOS over characteristical ablation, percolation and dry snow areas using radiative transfer modeling and observations from the Level 3 brightness temperature (L3TB) and the new SMOS enhanced resolution dataset at 40° and 52.5° incidence angle. The shape of SMOS incidence diagrams is closely related to scattering mechanisms in the firn, which is reflected in the ratio of 40° over 52.5° brightness temperature (TB). The “incidence ratio” shows strong spatial variations at both H and V polarizations with minima located in the percolation zone of the Greenland ice sheet (GIS) and at specific locations of Antarctic ice shelves. Simulations of L-band TB using the Snow Microwave Radiative Transfer (SMRT) model show that neither the superposition of semi-infinite layers with alternated densities nor artificially-created isotropic scattering are able to reproduce these minima. Only anisotropic scattering created by disk-type ice features in the firn explains well the observed incidence signatures in the percolation zones. The spatial and temporal variability of this signal is informative about the evolution of scattering mechanism in ice sheet’s firn.