Heat Fluxes, Whitecaps, and Sea Spray

The exchange of moisture and sensible heat between the atmosphere and the earth’s surface is important in driving weather, climate, and a variety of factors of relevance to human activities (e.g., clouds, atmospheric optical properties, ocean mixed-layer dynamics) addressed by oceanography and meteorology. This exchange is often dominated by molecular and turbulent diffusive processes in the atmospheric surface layer (a region in contact with the surface on the order of 10 m thick where the height dependence of the fluxes is negligible). The exchange is quite dependent on the nature of the surface. Over land the roughness of the surface, the subsurface moisture and the transfer properties of the plant canopy complicate this exchange process. Ice and snow also have physical properties that require special treatment. Historically, the transfer processes over the ocean and over more solid surfaces have been interpreted and parameterized in terms of Monin-Obukhov similarity theory. The ocean has two unique properties that have greatly simplified this approach: the surface humidity is assumed to be in equilibrium with the saturation vapor pressure at the ocean surface temperature and the roughness is considered to be determined by the mean wind speed or friction velocity. This second assumption is now being recast in terms of the surface wave spectrum (e.g., Chapters 5 and 8). However, the formation of whitecaps over the ocean strongly modifies the exchange process (see Slinn et al., 1978; Hasse, 1980; Coantic, 1980) for virtually all constituents (i.e., heat, moisture, trace gases, and particles). This modification is due to the release of gases and particles by the bursting of whitecap bubbles and, in the case of sensible and latent heat fluxes, the evaporation of the seawater spray droplets produced by the bursting bubbles.