The Emissivity of the Ocean Surface Between 6 and 90 GHz Over a Large Range of Wind Speeds and Earth Incidence Angles

We present a model function for the emissivity of the wind roughened ocean surface for microwave frequencies between 6 and 90 GHz. It is an update, refinement, and extension of model functions we had developed previously. The basis of our analysis are brightness temperature (TB) measurements from the spaceborne microwave radiometer WindSat and the Special Sensor Microwave/Imager, which are collocated with independent measurements of surface wind speeds and directions. This allows the determination of the emissivity model function for Earth incidence angles (EIA) around 55 degrees. We demonstrate that an essential part in the model development is the absolute calibration of the radiometer measurements over the ocean to the computed TB of the radiative transfer model, one of whose components the emissivity model function is. We combine our results with other established measurements for lower EIA and finally obtain a model function which can be used over the whole EIA range between 0 degrees and 65 degrees. Results for both the isotropic, wind direction independent part as well as the four Stokes parameters of the wind direction signal are presented. Special emphasis is made on the behavior at high wind speeds between 20 and 40 m/s by conducting a comparison with data from the step frequency microwave radiometer.