Variability in satellite-derived whitecap coverage

Wave breaking and whitecaps have a significant influence on air-sea gas exchange and control the production of sea-spray aerosol (SSA) at the ocean surface. The global average whitecap fraction W, has been cited to be of the order of a few percent. Estimates of W traditionally come from in-situ measurement through ship- or platform-based photography of the ocean surface. Wind speed is the driving force behind wave breaking and whitecapping, and being readily measurable, is often used to parameterise W. Several studies have shown that other factors - including SST, sea state, and atmospheric stability - can significantly influence whitecap coverage. However, the limited spatial and temporal coverage of in-situ datasets can restrict a thorough investigation into variability in W.

Anguelova and Webster [2006] demonstrated a method of estimating whitecap coverage from routine satellite measurements. At microwave frequencies, whitecaps appear as regions of enhanced surface emissivity, thus affording the possibility of satellite-retrieval of W through passive radiometric measurements of the ocean surface. The initial retrieval algorithm has since been improved in several respects; notably, the retrieval now makes use of WindSat measurements, and the models for the emissivities are now more physically sound. With satellite retrieval of other important environmental factors such as sea surface temperature, and information about wave conditions from model reanalysis, a global database of W and accompanying measurements has been assembled. The database can be used to quantify the relative importance of these variables, to assess if they should be included in more accurate parameterisation of whitecap coverage.

Here, we discuss the general behaviour of the satellite-derived W estimates, and compare to estimates from wind speed parameterisations formulated from in-situ data. The influence of several variables on whitecap coverage estimates is investigated once the dominant wind speed signal has been removed. We also discuss possible implementation of the satellite retrieval for studies of both SSA production and gas exchange.

Anguelova, M. D., and F. Webster, 2006: Whitecap coverage from satellite measurements: A first step toward modelling the variability of oceanic whitecaps. J. Geophys. Res., 111, C03017.