TIME CONSTANTS FOR THE EVOLUTION OF SEA SPRAY DROPLETS

Sea spray droplets start with the same temperature as the ocean surface from which they form. In high-latitude, polar-low conditions, they therefore cool and evaporate in a relatively cold wind and may alter the air-sea exchange of heat and moisture. This paper presents equations that model the thermal and size (moisture) evolution of a spray droplet from the time it forms until it reaches equilibrium with its environment. The model does well when tested against some of the scanty data available on the evolution of saline droplets. We parameterize the thermal and size evolution of spray droplets with the time constants tau-T and tau-r, which are, respectively, the times required for a droplet to come to within e-1 of its equilibrium temperature and within e-1 of its equilibrium radius. tau-r is always about three orders of magnitude larger than tau-T; the thermal exchange is thus complete before the moisture exchange even starts. Consequently, the ambient humidity has little effect on the thermal exchange rate, and the initial droplet temperature has negligible effect on the moisture exchange rate. We also parameterize the gravitational settling of droplets and their potential for turbulent suspension with the time scales tau-f and tau-w, respectively. Comparing the four time scales, we see that spray droplets with initial radii less than 10-mu-m reach both thermal and size equilibrium with the ambient air. Droplets with initial radii greater than 300-mu-m, on the other hand, fall back into the sea before exchanging appreciable heat or moisture; they thus have little impact on air-sea exchange. In the mid-range, droplets with initial radii between 10 and 300-mu-m, the physics is more complex. Even after comparing tau-T and tau-r with tau-f and tau-w, we still cannot say unequivocally which process is fastest because of the rudimentary nature of the tau-w estimates. Future work must thus focus on the generation and turbulent transport of droplets of this size if we are to understand how sea spray affects air-sea exchange.