Numerical simulation of the dynamics of sea spray over the waves

SeaCluse is a code describing and quantifying the nonlinear interactions between spray droplet concentrations and the scalar fields of water vapor concentration and temperature in the marine atmospheric surface layer as a function of the basic micrometeorological parameters. It is currently developed to simulate most of the dynamics of the evaporating sea spray droplets, their transformations, and their influence on the structure of the marine lower atmosphere. It includes two parts, a "preprocessor" computing the air flow structure and droplet trajectories over the waves in the absence of turbulence and evaporation, and a "main program" computing along the vertical the horizontally averaged budgets of droplet and water vapor concentrations, and sensible heat, including the dynamic and thermodynamic air-droplet interactions. This paper presents the first simulations, without evaporation, which reveal several characteristic features of the spray droplet dynamics over the waves. The mean air flow induced by the wave motion generates an efficient transport of many droplets up to the wave crest level or slightly higher, where they accumulate. In contrast, turbulence appears to be a rather inefficient process to elevate further the droplets that have nonnegligible weight and inertia (and an efficient process for very small droplets). The residence times in the air Tfly of droplets larger than 90 μm in radius do not increase much, compared to their values in still air or over a flat surface, and they increase with increasing wind speed much less rapidly than the wave height H0:Tfly ∼ U0.3 while H0 ∼ U2. Above the wave crest, the droplet concentrations are several orders of magnitude smaller than between wave troughs and crests. The precision of the turbulent diffusivity model for droplets therefore appears crucial to correctly infer the spray surface source function from concentration measurements in the atmosphere