The WISE 2000 and 2001 field experiments in support of the SMOS mission: sea surface L-band brightness temperature observations and their application to sea surface salinity retrieval

Soil Moisture and Ocean Salinity (SMOS) is an Earth Explorer Opportunity Mission from the European Space Agency with a launch date in 2007. Its goal is to produce global maps of soil moisture and ocean salinity variables for climatic studies using a new dual-polarization L-band (1400-1427 MHz) radiometer Microwave Imaging Radiometer by Aperture Synthesis (MIRAS). SMOS will have multiangular observation capability and can be optionally operated in full-polarimetric mode. At this frequency the sensitivity of the brightness temperature (T_B) to the sea surface salinity (SSS) is low: 0.5 K/psu for a sea surface temperature (SST) of 20°C, decreasing to 0.25 K/psu for a SST of 0°C. Since other variables than SSS influence the T_B signal (sea surface temperature, surface roughness and foam), the accuracy of the SSS measurement will degrade unless these effects are properly accounted for. The main objective of the ESA-sponsored Wind and Salinity Experiment (WISE) field experiments has been the improvement of our understanding of the sea state effects on T_B at different incidence angles and polarizations. This understanding will help to develop and improve sea surface emissivity models to be used in the SMOS SSS retrieval algorithms. This paper summarizes the main results of the WISE field experiments on sea surface emissivity at L-band and its application to a performance study of multiangular sea surface salinity retrieval algorithms. The processing of the data reveals a sensitivity of T_B to wind speed extrapolated at nadir of ∼0.23-0.25 K/(m/s), increasing at horizontal (H) polarization up to ∼0.5 K/(m/s), and decreasing at vertical (V) polarization down to ∼-0.2 K/(m/s) at 65° incidence angle. The sensitivity of T_B to significant wave height extrapolated to nadir is ∼1 K/m, increasing at H-polarization up to ∼1.5 K/m, and decreasing at V-polarization down to -0.5 K/m at 65°. A modulation of the instantaneous brightness temperature T_B(t) is found to be correlated with the measured sea surface slope spectra. Peaks in T_B(t) are due to foam, which has allowed estimates of the foam brightness temperature and, taking into account the fractional foam coverage, - the foam impact on the sea surface brightness temperature. It is suspected that a small azimuthal modulation ∼0.2-0.3 K exists for low to moderate wind speeds. However, much larger values (4-5 K peak-to-peak) were registered during a strong storm, which could be due to increased foam. These sensitivities are satisfactorily compared to numerical models, and multiangular T_B data have been successfully used to retrieve sea surface salinity.