Attempts to quantify the effects of sea spray evaporation have many uncertainties to contend with. Some, such as the evolution of the temperature and mass of a single droplet for as long as it remains suspended in the atmosphere, are now quite well known. Others, such as the length of time the droplet actually remains suspended, are more difficult. The processes by which droplets are produced are known, but there remain orders of magnitude of variation between estimates of the rate at which these processes operate at various wind speeds. The issue of feedbacks between droplet evaporation and the atmosphere has some important unanswered questions.
A model of evaporating saline droplets within the atmospheric boundary layer was developed, which includes all relevant processes and feedbacks. The atmospheric part of the model comprises an order 1 1/2 turbulence closure scheme above a similarity theory based surface layer. Thermodynamic transfers to and from spray droplets are described by cloud microphysics equations modified for the effects of salinity. The droplets may additionally be diffused by the turbulence (with a size-dependent diffusion coefficient), and fall due to gravity. Both evaporation and transport is handled in an eulerian sense; that is, droplet concentration is stored as a function of height and radius. This allows the calculation of the effects of droplet evaporation on the atmosphere for a wide range of source strengths and initial radii.
Processes which limit the evaporation of spray are identified and quantified. We show there are several ways in which modification of the near-surface conditions by spray evaporation feeds back into the model response. These include the increase in the direct surface sensible flux and reduction in the surface latent flux due to the cooler, moister air adjacent to the surface, and the reduced evaporation rate of the droplets due to the greater relative humidity of the air around them. It is shown that about 70 % of the droplet mediated latent flux is realised above the droplet evaporation layer. In addition, as the droplet evaporation became larger, the stabilisation of the boundary layer results in a reduction in the stress and total thermodynamic flux. The reduction of droplet transport by the damped turbulence is shown also to be an important negative feedback at high source rates, with a reduction in both the droplet residence time and the ability of the atmosphere to supply warm dry air to evaporate more droplets.
The depth of the droplet evaporation layer was found to be a minimum for large and small droplets, with a surprisingly deep maximum for intermediate sized droplets. This is supported by a simple scaling argument. This suggests that measurements of near-surface fluxes at high wind speeds have been taken within the droplet evaporation layer, which may partially explain the lack of an obvious spray effect