Roughness and foam signature on SMOS-MIRAS brightness temperatures: A semi-theoretical approach

The influence of wind speed (WS) on sea surface emission at L-band is revisited using the updated version v6.2 of Soil Moisture and Ocean Salinity (SMOS) brightness temperatures (TB) and an incoherent two-layer foam emissivity model. The influence of the roughness effect due to surface waves is consistent with the one found with an older version of SMOS TB. The two-layer incoherent model of foam emissivity accounts for weak volume scatterings and multiple reflections within a medium with an exponential vertical permittivity profile. The foam emissivity simulated using this model at L band varies from 0.35 to close to 1 with thickness varying from 0.01 cm to 2 cm. The wind induced brightness temperature components deduced from the multi-angular SMOS TB is used to optimize the foam void fraction (defined as the fraction of a unit volume of seawater that is occupied by air) at the air-sea interface, an effective thickness of foam layer and the dependency of foam coverage with wind speed. A new set of parameters for the foam emissivity model and the foam coverage model that can be used for WS up to 22 m s− 1 for the SMOS sea surface salinity retrieval is proposed. Our foam coverage model derived from SMOS data is now in much better agreement with other estimates derived from other sensors, although it predicts slightly lower coverage at all winds speeds, likely due to the longer wavelength of SMOS measurements.