Radiometric observations of whitecaps at high spatial resolution

Accurate representation of the ocean-atmosphere coupling in weather, wave, and climate models requires reliable estimates of air-sea surface fluxes of momentum, heat and mass. Fractional whitecap coverage W usually quantifies the enhancement of the surface fluxes due to wave breaking. Satellite-based passive remote sensing of W from ocean surface brightness temperature T_B observes surface fluxes at low spatial resolution using microwave radiometers operating at frequencies of 6 to 37 GHz. Such a resolution 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 are assessing the feasibility of using the surface signal at millimeter-wave (mmW) frequencies (89 to 150 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 use generalized formulation of the classical Mie scattering theory that is applicable to clusters of closely-packed bubbles. We obtain the attenuation of different foam structures—such as foam streaks, floating foam patches, and thick foam layers—formed by monodisperse bubbles. We use foam attenuation to investigate the contribution of foam scattering and how it affects the foam absorptivity, thus emissivity. We will describe our method, present results, and discuss the dependence of foam scattering on radiometric frequency and dimensions of the foam structures.