Complex dielectric constant of sea foam at microwave frequencies

Abstract

[1] 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) PolderVan Santen model.

A Numerical Study on Physical Characterizations of Microwave Scattering and Emission From Ocean Foam Layer

“…The bubbles on the top level of the foam layer are polyhedral with lower water fractions, whereas the bubbles on the bottom of the foam layer tend to be small with higher water fractions and almost buried in seawater [5,6,9]. Fig.…”

“…Raizer [8] developed a composite microwave model for estimating the emissivity of sea foam, by incorporating the vertical profile of an effective permittivity, which was associated with air-water stratifications. Anguelova and Gaiser [9,10] determined the effective permittivity of sea foam layers by comparing five kinds of mixing rules based on the effective medium theory; the importance of multiple interactions between air-foam and foamseawater interfaces was noted in their studies. To achieve more robust applications of microwave remote sensing of an ocean surface, more comprehensive physical characterizations of microwave scattering and emission from a foam-covered ocean surface are required, such as the study of surface roughness driven by wind over the ocean surface, interactions of the foam layer with the sea surface, and their effects on the directional wave spectrum of short gravity and capillary waves [11].…”

Dielectric and Radiative Properties of Sea Foam at Microwave Frequencies: Conceptual Understanding of Foam Emissivity

“…Whitecaps in their decaying phase are thinner and dimmer and are referred to as residual whitecaps. Anguelova [24] gives an extended definition of the sea foam.…”

“…To describe oceanic whitecaps under various conditions and in different lifetime stages, the full range of void fractions f a (from 0 to 1) needs to be considered [24]. It was shown that various functional forms could represent the shape of the void fraction profile in the foam depth [25].…”

“…Anguelova [24] analyzed the available information to determine suitable formula to predict the complex permittivity of sea foam ε f . Anguelova and Gaiser [25] focused on the skin depth of foam layers with a vertical void fraction profile.…”