A Lagrangian turbulent transport model of evolving sea-spray droplets over water waves

Sea-spray contributes to the momentum, mass, heat, and moisture transfer between the ocean and the atmosphere. Although the direction of these fluxes is generally known, their magnitudes remain unclear. Sea-spray is predominantly formed through two mechanisms. The first mechanism, resulting in film or jet droplets, is the ejection of water droplets into the air from bursting bubbles at the surface. The magnitude of their radii is on the order of O(1-10) mm. The second mechanism, resulting in spume droplets, is the forceful separation of water particles at the top of waves caused by sufficiently strong winds. The magnitude of spume droplet radii are on the order of O(10-1000) mm. Because of their relative size, spume droplets have the potential to transfer considerably more mass, momentum, heat, and moisture than jet or film droplets. While the potential is greater, intuition suggests that the resident time of spume droplets will be shorter due to their faster settling velocities and vastly different ejection mechanism. Therefore, it seems necessary to gain a greater understanding of both the microphysics and the transport of sea-spray droplets, spume droplets in particular. Numerous studies (e.g. Rouault et al. 1991, Andreas 1992, Edson and Fairall 1994, Andreas 1995, Andreas et al. 1995, Edson et al. 1996, Mestayer et al. 1996, Makin 1998, Andreas and Emanuel 2001, Van Eijk et al. 2001, Meirink 2002) have appropriately investigated the microphysics and transport of sea-spray droplets. There has, however, been a lack of exclusively Lagrangian models, which simultaneously solve the microphysical evolution of the droplets during their transport through the marine boundary layer. Our Spray Lagrangian Turbulent Transport and Evolution (SpLaTTE) model is an attempt to fill the aforementioned void.

The SpLaTTE model is a Monte-Carlos type simulation, which follows individual droplets from ejection into the air until they reenter the ocean or attain a quasi-equilibrium state. The model includes a realistic surface wave spectrum, forming the bottom boundary. While suspended in the air, the droplet traverses an atmospheric boundary layer that includes a viscous sublayer, a wave boundary layer (WBL), and a stratified log layer. In addition, the droplet is subjected to turbulent velocities and turbulent scalars following the Kolmogorov-Obukhov-Corrsin theory. Furthermore, the droplet's velocity and evolution are solved using the complete, linear, unsteady equation of motion and the complete microphysical equations (Pruppacher and Klett.1978; Andreas, 2005) respectively. While the model is computationally expensive, it provides new insight into the properties of the droplets that reenter the oceanic surface via the viscous sublayer and WBL. Our results suggest that typical simplifications in previous models lead to an overestimation of the heat and moisture fluxes due to sea-spray.

Preliminary results, seen in the figure below, show that a sea-spray droplet's temperature will move rapidly toward the surface temperature as it falls back into the ocean. Most models assume that the sensible heat flux of the droplet occurs instantaneously. Additionally, they approximate the latent heat flux based on results from full microphysical runs under simplified, constant conditions. Thus, solving the full microphysical equations simultaneously with the transport model does provide new insight regarding the properties of the sea-spray droplets that reenter the ocean. Although the prime focus of this study is the nature of sea-spray upon reentry into the ocean, other interesting results will be discussed.