Why do satellite-based estimates of whitecap fraction depend on the probing frequency and how to use this for air-sea interaction studies?

Estimates of whitecap fraction, W, from radiometric satellite-based observations of ocean thermal emission could be used to study and parameterize W over a wide range of meteorological and oceanographic conditions. These radiometric measurements complement the existing database of W values extracted from photographs and video images of the sea state. However, the values of W obtained from radiometric data at different probing frequencies (e.g., from 6 GHz to 37 GHz for the WindSat sensor) differ. This frequency dependence of W is analogous to extracting different W values from photographs by setting different intensity thresholds that separate sea foam from the surrounding water.

The reason behind this frequency dependence of radiometric measurements of whitecap fraction is that the skin depths of sea foam at various microwave frequencies differ. Coupled with the foam layer thickness, this frequency dependence leads to different sensitivity to whitecaps in different stages of their lifetime. As a result all WindSat frequencies react to foam thicker than 1 cm, but as the frequency decreases from 37 to 6 GHz, its sensitivity to thinner foam layers decreases. The lower limit of detectable foam thickness for 37 GHz is around 1 mm; for 10 GHz it is 4 mm; and for 6 GHz it is around 1 cm.

Since thick foam is associated with the active wave breaking (stage A whitecaps), while thin foam characterizes decaying foam patches (stage B whitecaps), low frequencies (e.g., 6 GHz) will detect predominantly active whitecaps. Meanwhile higher frequencies (e.g., 37 GHz) will detect even the smallest presence of sea foam on the surface. The frequency sensitivity to whitecaps in different lifetime stages implies that radiometric estimates of whitecap coverage at various frequencies can be used to study different processes, e.g., turbulent mixing, gas exchange, spray production with different rates, and sea surface albedo.

We will explain the physics of the frequency dependence of whitecap fraction from radiometric data; present its variations caused by different conditions; and identify the suitability of different frequencies for studies of different air-sea interaction processes.