Passive Remote Sensing of Oceanic Whitecaps: Algorithm Description

Many air-sea interaction processes are quantified in terms of whitecap fraction W because oceanic whitecaps are the most visible and direct way of observing breaking of wind waves in the open ocean. Breaking waves enhance the surface fluxes of momentum, heat, and mass. These surface fluxes realized ocean-atmosphere coupling. Therefore, their accuracy affects models used for weather forecast, storm intensification prediction, and climate change studies.

Whitecap fraction has been traditionally measured from photographs or video images collected from towers, ships, and aircrafts. Satellite-based passive remote sensing of whitecap fraction is a recent development that allows long term, consistent observations of whitecapping on a global scale. The method estimating W relies on variations of ocean surface emissivity at microwave frequencies (6 to 37 GHz) due to presence of sea foam on a rough sea surface. Satellite-born microwave radiometers can detect these variations at the ocean surface as changes of the brightness temperature T_B at the top of the atmosphere.

Our first algorithm providing satellite-based W data uses passive microwave observations from WindSat at low spatial resolution, early version of the WindSat geophysical model function (GMF) for surface emissivity and the atmosphere, and input variables from various sources. We has recently updated this algorithm with new input data and the latest versions of the WindSat GMF to produce W data at higher spatial resolution. The updated algorithm yields improved W data as they show consistency at different microwave frequencies and expected threshold behavior of whitecap formation at all frequencies. In this presentation, we describe the algorithm W(T_B) obtaining whitecap fraction W from satellite-based data of brightness temperature T_B, show new results for W, and compare them to W data from our initial algorithm implementation.