Whitecap Fraction From Satellite Measurements: Algorithm Description

Many air-sea interaction processes are quantified in terms of whitecap fraction W because whitecaps are the most visible and direct way of observing breaking waves with air entrainment in the open ocean. Photographs or video images of the sea state collected from towers, ships, and aircrafts have been used over the years to obtain W. Satellite remote sensing of W is a recent development that allows long-term, consistent observations of whitecapping on a global scale. The method for estimating W uses the variations of ocean surface emissivity at microwave frequencies (6 to 37GHz) due to presence of sea foam on a rough sea surface. Satellite-borne microwave radiometers detect these variations at the ocean surface as changes of the brightness temperature T-B at the top of the atmosphere. We present the physical and parameterized models, as well as the input data, necessary to calculate W from WindSat T-B observations with our W (T-B) algorithm. We describe the implementation versions through which the W (T-B) algorithm has developed. We show that satellite-based W data vary with the frequency and polarization of the T-B observations. The wind speed dependence of the W retrievals at horizontal and vertical polarizations shows different behavior and compares differently with in situ W data and existing wind speed parameterizations W(U-10). We discuss the applicability of our methodology to other radiometric data. We indicate possible modifications and tuning of the models in the W (T-B) algorithm that can help to further improve the accuracy of the satellite W retrievals. Plain Language Summary When out in the open sea, we all fancy the bright, fleeting sea foam capping the waves. In addition to their beauty, these whitecaps also play a critical role in the interaction between the ocean and the atmosphere. Oceanic whitecaps form when waves break and entrain air in the water. The entrained air breaks up into bubbles, which then rise to the surface clustering into patches of sea foam. Whitecaps on the surface and bubbles in the water enhance the exchange of gases and particles across the air-sea interface. Accurate modeling of these exchange processes is necessary for weather prediction and climate studies. Whitecaps are traditionally measured from photographs of the ocean surface. However, these measurements are difficult and only on a local scale. We developed a method to measure whitecaps from satellites. This method allows observations of oceanic whitecaps, and the processes associated with them, on a global scale. In this paper, we describe the models and the data, which we use to measure whitecaps from satellites. We show how these satellite measurements compare to the traditional photographic measurements of whitecaps. We discuss how this methodology can be improved and applied to sensors on different satellites.