Absorption and scattering by active and residual sea foam at millimeter-wave frequencies

Abstract:

We quantify the attenuation of electromagnetic radiation at millimeter-wave frequencies (mmW) by foam layers of active breaking waves and patches of residual foam. We use generalized formulation of the classical Mie scattering theory that is applicable to clusters of closely-packed bubbles. We obtain the attenuation of active and residual whitecaps by investigating, respectively, 3D and 2D foam structures formed by monodisperse bubbles. We need foam attenuation to investigate the contribution of foam scattering and how it affects the foam absorptivity, thus emissivity.

Foam emissivity is the basis of passive remote sensing of whitecap fraction W from ocean surface brightness temperature T_B. Radiometric W measurements can provide observations of air-sea fluxes of momentum, heat, and mass. Reliable estimates of air-sea fluxes is necessary for accurate representation of the ocean-atmosphere coupling in weather, wave, and climate models. Microwave radiometers (1 to 40 GHz) retrieve W at low spatial resolution, which is acceptable for open ocean where the conditions are relatively homogeneous over expansive areas. However, it is a limiting factor when approaching land or ice. Therefore, radiometric surface observations at higher resolution are necessary to monitor the dynamic and complex environments in coastal zones and polar regions.

To this end, we assess the feasibility of using the surface signal at mmW frequencies (40 to 200 GHz) to detect whitecaps. We need to investigate three aspects to achieve this goal, namely: (1) Evaluate the sensitivity of foam-covered surface signal to the atmospheric signal; (2) Evaluate the performance of existing models in representing the rough sea surface; (3) Develop a model for the emissivity of sea foam at mmW frequencies that accounts for both absorption and scattering. The work presented here focuses on the third aspect, i.e., foam scattering. We will describe our method, present results, and discuss the dependence of foam scattering on radiometric frequency and dimensions of different foam structures.