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,) to the sea surface salinity (SSS) is low: 0.5 K/psu for a sea surface temperature (SST) of 20 degreesC, decreasing to 0.25 K/psu for a SST of 0 degreesC. Since other variables than SSS influence the TB 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 TB 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 TB to wind speed extrapolated at nadir of similar to0.23-0.25 K/(m/s), increasing at horizonal (H) polarization up to similar to0.5 K/(m/s), and decreasing at vertical (V) polarization down to similar to-0.2 K/(m/s) at 65degrees incidence angle. The sensitivity of TB to significant wave height extrapolated to nadir is similar to1 K/m, increasing at H-polarization up to similar to1.5 K/m, and decreasing at V-polarization down to -0.5 K/m at 65degrees. 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 similar to0.2-0.3 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.