Complex dielectric constant of sea foam at microwave frequencies

Abstract

We present a systematic investigation of the applicability of a group of mixing rules for obtaining the dielectric constant (permittivity) of sea foam (whitecaps) at microwave frequencies, 1.4 to 37 GHz. By demonstrating that the foam scattering is weak at these frequencies, we justify our interest in basic mixing rules, which do not involve explicit scattering computation, namely, the Maxwell Garnett, Polder‐Van Santen, Coherent potential, Looyenga, and Refractive models. The complex dielectric constant of sea foam obtained with these mixing rules is presented and the dependence of foam permittivity on foam void fraction, radiation frequency, sea surface temperature, and salinity is reported. With the exception of the Coherent potential model, all selected mixing rules give reasonable values for the sea foam dielectric constant. To further examine the suitability of a permittivity model for computing the dielectric constant of sea foam, the performance of each mixing rule is evaluated on the basis of three criteria: (1) how well a permittivity model deals with a wide range of void fractions, (2) how a permittivity model behaves approaching the foam‐air and foam‐water boundaries, and (3) how the choice of a permittivity model affects estimates of emissivity and brightness temperature due to foam. The suitability of the basic mixing rules for computing the complex dielectric constant of sea foam at microwave frequencies can be ranked as: (1) Refractive model, (2) Looyenga model, (3) Maxwell Garnett model, and (4) Polder‐Van Santen model.