Refinement of a semi-empirical model for the microwave emissivity of the sea surface as a function of wind speed

Abstract

In 1979, Wilheit introduced a sea surface emissivity model for microwave frequencies. This model is used in a radiative transfer model (RTM) to obtain simulated brightness temperatures for various atmospheric conditions. The brightness temperatures are used to obtain algorithms for atmospheric variables. These algorithms can then be used to retrieve atmospheric variables from the microwave measurements-Therefore, it is important to get the surface emissivity right. Several changes are made to Wilheit's sea surface emissivity model. The first change to the model is to the model's treatment of multiple reflections. Multiple reflections are now treated as if the radiation is reflected back into the view path of the microwave sensor. This change lowered the computed emissivity of the sea surface-, which is more representative of observations without sea foam. The second change is made to the sea surface roughness parameter. An increase in roughness is needed at frequencies above 16.6 GHz and a decrease below 16.6 GHz. The roughness is increased to 132% of the Cox and Munk roughness at 37 GHz and 30% of the roughness at 6.6 GHz. The last change to the model is in the treatment of sea foam. The foam effect is now a smooth transition of increasing foam as the wind speed increases-instead of being switched on at 7 m/s. These changes yield an improved sea surface model that is within 2% of the sea surface emissivity given by the Wentz sea surface emissivity functions.