A model function for ocean microwave brightness temperatures

Abstract

We derive a relatively simple, yet accurate, relationship between the microwave brightness temperature of the ocean and conventional oceanographic and meteorological parameters. To begin with, the brightness temperature of the ocean and the intervening atmosphere is expressed in terms of integrals for the radiative scattering and emission from the sea surface and the radiative absorption and emission by the atmosphere. We then find closed‐form approximations for these integrals and obtain a simplified model for the radiative transfer and scattering. The model is reduced to a function giving brightness temperature in terms of five variables: sea‐surface temperature, sea‐surface friction velocity, atmospheric columnar water vapor content, atmospheric columnar liquid water content, and surface air temperature« In the derivation of the model, the absorption coefficient for rain clouds and the wind‐induced sea‐surface emissivity are found from SEASAT SMMR observations. The remainder of the derivation is based on wellaccepted microwave theory. The model function is referenced to the five frequencies and 49° incidence angle at which the SEASAT and Nimbus 7 SMMR operate. When compared with the more precise integral formulation, the model function accuracy is 0.2 K at 6.6 GHz and gradually degrades to 2.1 K at 37 GHz, horizontal polarization. These accuracies are an average over an ensemble of winds and atmospheres ranging from a specular surface and clear skies to a 21 m/s wind and a…