In high winds, breaking waves and whitecaps disrupt the ocean surface; spray proliferates. Because these spray droplets start with the same temperature and salinity as the surface water, they effectively increase the ocean's surface area and may thereby enhance the exchange of any constituent normally transferred across the air-sea interface. My interest here is in how spray affects the air-sea exchange of momentum and sensible and latent heat.

I will present a bulk turbulent flux algorithm that accounts for both the interfacial and spray routes by which sensible and latent heat cross the air-sea interface. The algorithm is appropriate for 10-meter wind speeds up to at least 30 m/s. To model the interfacial fluxes, the algorithm uses the COARE bulk flux algorithm (Fairall et al., 1996) with some high-wind-speed modifications. The spray component of the algorithm results from tuning Andreas's (1992) theoretical spray model with heat flux data from HEXOS, the experiment to study Humidity Exchange over the Sea (DeCosmo et al., 1996).

When spray droplets are formed, they accelerate quickly to the local air speed. This process extracts momentum from the wind. When the spray droplets ultimately crash back into the ocean, they transfer this momentum to the sea surface and therefore also, potentially, enhance the surface stress. The algorithm also models this exchange process. Although the spray momentum flux is small for wind speeds less than 30 m/s, it increases as the fourth power of the friction velocity, while the usual wind-driven, interfacial stress increases only as the square of the friction velocity. Consequently, as the wind speed approaches 60 m/s, in hurricanes for example, the interfacial and spray momentum fluxes become comparable.

References

Andreas, E. L, 1992: Sea spray and the turbulent air-sea heat fluxes. J. Geophys. Res., 97, 11,429-11,441.

DeCosmo, J., K. B. Katsaros, S. D. Smith, R. J. Anderson, W. A. Oost, K. Bumke, and H. Chadwick, 1996: Air-sea exchange of water vapor and sensible heat: The Humidity Exchange over the Sea (HEXOS) results. J. Geophys. Res., 101, 12,001-12,016.

Fairall, C. W., E. F. Bradley, D. P. Rogers, J. B. Edson, and G. S. Young, 1996: Bulk parameterization of air-sea fluxes for Tropical Ocean-Global Atmosphere Coupled-Ocean Atmosphere Response Experiment. J. Geophys. Res., 101, 3747-3764.