Updated Whitecap Database from WindSat Observations

Oceanic whitecaps, formed by wave breaking with air entrainment, are the major source of sea spray, the dominant natural aerosols in remote marine air. Sea spray aerosols (SSA) contribute to radiative and chemical processes affecting climate. SSAs affect global climate directly by contributing to the planetary albedo; indirectly by acting as cloud condensation nuclei (CCN) thus altering cloud albedo; and chemically by removing methane and surface ozone. Under certain conditions, SSAs alter the cooling effect of the sulfate aerosols by removing of natural and anthropogenic sulfates from the atmosphere and by preventing the activation of sulfate aerosols as CCN. Whitecaps and SSAs also contribute to the turbulent surface fluxes of heat (sensible and latent), thus affecting storm intensification, and gases, including carbon dioxide (CO₂). Whitecaps also affect retrieval algorithms of wind speed, ocean color, and salinity. All these air-sea interaction (ASI) processes are quantified with surface fluxes of energy, heat and mass. The surface fluxes are usually parameterized in terms of whitecap fraction W.

Improved accuracy of the ocean-atmosphere coupling in weather prediction and climate models requires improved representation of surface fluxes. Parameterizations of turbulent surface fluxes typically use the dependence of whitecap fraction W on wind speed U. However, field measurements do not constrain well the turbulent fluxes predicted using the relationship W(U) alone. Other variables, besides the wind, affect the formation of whitecaps and surface fluxes. The traditional in situ photographic measurements of W are not sufficient to build a database necessary to investigate and model the geophysical variability of W and surface fluxes. Satellite remote sensing observations of W can help to improve parameterizations of ASI processes by introducing additional variables and quantifying the natural variability of W. Moreover, remote sensing of W can provide global, long-term monitoring of surface fluxes.

The Naval Research Laboratory has developed a method of estimating W from satellite-based passive radiometric data. The algorithm relies on changes 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. Early versions of the algorithm proved the utility of the satellite W estimates to build a whitecap database suitable for studying whitecap variability and ASI processes. New version of the algorithm now provides improved W estimates, including improved modeling and higher spatial resolution. In contrast to the early algorithm versions, the new satellite W estimates use both the WindSat T_B observations and the WindSat retrievals—namely, wind speed and direction U and f, sea surface temperature (SST) T, water vapors V, and cloud liquid water L—as input variables to the algorithm geophysical model.

We obtained estimates of W with the new version of the algorithm for year 2014. We compiled a new, improved whitecap database, suitable for studying W variability and surface fluxes, using the satellite-based W data along with additional meteorological and oceanographic data. We matched the W data in time and space with surface data for wind vector, SST, and air temperature from numerical weather prediction models. We used the 6-h re-analyses of NAVGEM (the Navy Global Environmental Model) and ECMWF (European Centre for Medium-range Weather Forecasting). We added oceanographic variables, such as significant wave height and peak wave period, from the NOAA/NCEP wave hindcasts done with the WAVEWATCH III model. We will report results on whitecap fraction spatial and temporal features over the globe and will describe the new whitecap database.