Abstract
Foam fraction can be retrieved from space-based microwave radiometric data at frequencies from 1 to 37 GHz. The retrievals require modeling of ocean surface emissivity fully covered with sea foam. To model foam emissivity well, knowledge of foam properties, both mechanical and dielectric, is necessary because these control the radiative processes in foam. We present a physical description of foam dielectric properties obtained from the foam dielectric constant including foam skin depth; foam impedance; wavelength variations in foam thickness, roughness of foam layer interfaces with air and seawater; and foam scattering parameters such as size parameter, and refraction index. Using these, we analyze the scattering, absorption, reflection and transmission in foam and gain insights into why volume scattering in foam is weak; why the main absorption losses are confined to the wet portion of the foam; how the foam impedance matching provides the transmission of electromagnetic radiation in foam and maximizes the absorption; and what is the potential for surface scattering at the foam layers boundaries. We put all these elements together and offer a conceptual understanding for the high, black-body-like emissivity of foam floating on the sea surface. We also consider possible scattering regimes in foam.
Highlights
Oceanic whitecaps are a conspicuous expression of wave breaking with air entrainment
One physical quantity that is used to evaluate these air-sea interaction processes is the whitecap fraction, defined as the fraction of the ocean surface covered by foam
Whitecap fraction can be evaluated with a number of parameterizations developed for open ocean [8] and coastal zone [9] using photographic measurements [10,11,12] in the visible portion of the electromagnetic (EM) spectrum
Summary
Oceanic whitecaps are a conspicuous expression of wave breaking with air entrainment. Basis for the dielectric properties of a medium is its relative dielectric constant (permittivity) Permittivity determines such intrinsic characteristics of the medium as skin (and penetration) depth, impedance, refractive index, etc. These quantities control the attenuation due to absorption and scattering and the transmission and reflection of the EM radiation in the medium. We consider the foam properties at microwave frequencies, which control fundamental radiative processes such as reflection, scattering, and transmission in vertically structured foam layers. The parameters used for this consideration are foam impedance η, size parameter x, and refractive index m (Section 3) Analysis of these dielectric properties helps to identify the unique traits, which the fundamental radiative processes acquire when microwave radiation interacts with sea foam (Section 4). The information, findings, and generalizations in this series of papers form the physical basis on which we have built our foam emissivity model (briefly described in [16] and to be detailed in a forthcoming paper)
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