Impact of new permittivity measurements on sea surface emissivity modeling in microwaves

As part of a measuring program dedicated to the analysis of the dielectric properties of seawater in the frequency range 3-89 GHz, a new dielectric permittivity model based on the standard Debye theory has been developed for remote sensing applications over the ocean below 40 GHz, together with polynomial interpolations at the millimeter frequencies 85.5 and 89 GHz. The aim of this paper is to test the relevance of these new dielectric measurements through statistical comparisons of radiative transfer predictions with satellite and airborne radiometric data between 18 and 89 GHz. A radiometric sensitivity analysis to the permittivity measurement errors is proposed, which yields a sea surface brightness temperature accuracy of at least 0.5 K below 20 GHz, 1 K at 24 GHz, and 1.5 K at 37 and 89 GHz. At frequencies less than 40 GHz, superiority of the revised Debye model is pointed out over the most commonly used model of Klein and Swift [1977]. At millimeter frequencies the new permittivity expressions deviate significantly from the standard Debye predictions, especially at low temperature, suggesting the influence of a second "high-frequency" Debye relaxation. Our comparisons with radiometric data at 89 GHz and in the channel 85.5V of the special sensor microwave imager tend to support this hypothesis. The results emphasize the importance of an adequate modeling of the complex permittivity of seawater as input to the surface emissivity models, at any frequency of the microwave spectrum, and augur interesting outputs in both in-flight calibration and interpretation of satellite data.