Ocean surface roughness spectrum and microwave brightness temperature

After a long period of planning and development, global remote sensing of sea surface salinity (SSS) from space is becoming a reality with the launch of SMOS (Soil Moisture and Ocean Salinity) satellite by the European Space Agency (ESA) in November 2009. A similar mission (AQUARIUS/SAC-D), jointly sponsored by the United States National Aeronautics and Space Administration (NASA) and the Argentina Comisión Nacional de Actividades Espaciales (CONAE), is scheduled for launch later this year. Soil moisture and ocean salinity are important factors influencing the global water cycle and represent key parameters in ocean circulation and climate research.

The microwave L band centered on a frequency of 1.413 GHz and with a bandwidth of 25 MHz is selected for soil moisture and ocean salinity retrievals because this frequency band is protected for astronomical observations. For this band, it has been determined that the ocean surface roughness is the leading geophysical error source for sea surface salinity retrieval. We will present brightness temperature computations using four different models of the ocean surface roughness spectrum. The computed results are compared to field data sets reported in the literature. The qualitative agreement between computations and measurements is reasonably good considering the large scatter in the field data. Depending on the choice of the ocean surface roughness spectrum, the calculated rate of change of brightness temperature with respect to wind speed can vary by a factor of about two.