A study of the influence of evaporating sea spray on the air-sea heat exchange in high-wind condition of a tropical cyclone

The intensity of a tropical cyclone (TC) depends on the air-sea turbulent exchange of sensible and latent heat and momentum in a marine atmospheric boundary layer (MABL). Even though there is significant evidence that ocean spray injected into MABL strongly affects these fluxes, this influence is still not fully understood or properly quantified because of the difficulties associated with a mathematical description of MABL laden with evaporating spray.

In the present work we have investigated the effect of evaporating spume droplets of various sizes on the vertical sensible and latent heat fluxes in a MABL with various vertical profiles of the air temperature and water vapor concentration for several wind speeds. We have employed the recently developed two-temperature mathematical model of non-equilibrium MABL laden with evaporating spray. The governing equations are derived from basic physical principles of conservation of mass, momentum, and thermal energy in a multiphase flow. The model is based on an Eulerian multi-fluid approach that considers spray as a continuous medium interpenetrating and interacting with the gas phase and incorporates the two-equation E-epsilon turbulence closure. The developed theoretical framework consistently accounts for the non-uniform spray distribution and the thermal energy exchange between air and spray droplets, which in general have different temperatures.

We have found that the total heat flux is significantly enhanced due to spray presence even if droplet concentration remains moderate. It reaches maximum for droplets with radii of around 0.5 mm. The effect of such large droplets is felt mostly in a thin layer near the ocean surface where most of them reside. Their evaporation, the associated sensible heat flux and the suppression of turbulence intensity result in steep gradients of the vertical distributions of the moisture and air temperature. Small droplets do not change the total heat flux but rather redistribute the energy between its sensible and latent components. We have demonstrated that vertical profiles of heat fluxes are approximately self-similar for small and moderate spray concentrations i.e. they can be scaled to a reference heat flux profile for a wide range of parameters. This observation can be used for constructing computationally efficient and accurate spray parameterization in global weather prediction models. Our model results have demonstrated that a seemingly anomalous growth rate of the vertical heat flux with the wind speed observed experimentally may be explained by the presence of ocean spray in a MABL.