Whitecap Fraction of Actively Breaking Waves from Satellite Observations

As a surface expression of breaking waves, the oceanic whitecaps are manifestation of various air-sea interaction processes. Whitecap fraction observed from satellites-borne microwave radiometers provide the total whitecap fraction W, including foam generated during active wave breaking and residual foam left behind by these breaking waves. The total fraction is important for predictions of sea spray aerosol production and heat exchange. However, the active portion W_A is necessary for evaluating processes such as turbulent mixing, gas exchange, ocean ambient noise, and spray-mediated intensification of tropical storms. It is thus highly pertinent to be able to separate W_A from W. One possible way to make such a separation is to develop a scaling factor R = W_A /W, which when applied to satellite-derived data for W will provide data for W_A on a global scale. Theoretical basis for our approach in deriving such a scaling factor is the Phillips concept which connects W_A to the energy dissipation rate of breaking waves.

We pursue our goal to extract W_A from W in two major steps. We first work on a regional scale by obtaining energy dissipation rates and W_A from buoy measurements of directional wave spectra. Matching W_A from five buoys in time and space to W from WindSat overpasses, we have obtained regional scaling factors R. Factors R are expected to vary over the globe because different meteorological and environmental conditions foster or suppress the extents of W_A and W differently. For example, W_A is mostly affected by wind stress and atmospheric stability, while surfactants and sea surface temperature influence the residual whitecaps and thus the extent of W. As a result, Rs at low and high latitudes will follow the differences between W_A and W. It is, therefore, necessary to have R values over wide range of conditions in order to expand the regional Rs on a global scale. However, the data available from five buoys do not suffice to represent well the natural variations of W_A, W, and R. This necessitates the second step in our approach.

In the second step, we obtain energy dissipation rates and W_A from wave model and match them with the values we have for the five buoys. Comparisons of energy dissipation and W_A from buoys and wave model help to tune parameters in both the wave model and Philips-concept calculations. Results from both buoys and wave model are constrained with available data and parameterizations. Important in this tuning and comparing is to understand the variations introduced in the results by the assumptions and input parameter choices made for both wave model and Philips-concept calculations. Such understanding will give a sound basis for extending the regional scaling factors R on a global scale. We will present details of our approach, latest results, and will discuss the possibilities that databases of W and W_A from satellite observations give for estimates of surface fluxes.