Feedback of Sea-spray on Air-Sea Heat Fluxes and the Influence of Droplet Lifetime

Accurately predicting surface fluxes, especially heat and moisture fluxes, at the marine boundary layer (MBL) is essential for numerical weather prediction. Sea-spray generated in large quantities from the ocean surface under strong winds exchanges heat and water vapor with the turbulent airflow, which potentially modifies the sensible and latent heat fluxes between the ocean and the MBL. To understand the physical mechanisms governing this interaction, we utilize direct numerical simulations to study canonical wall-bounded turbulence with two-way coupled Lagrangian droplets. We focus on how spray influences the heat fluxes as well as temperature and humidity at the interface. Results confirm that the heat flux modification is related to multiple important droplet timescales, each determined by droplet size: the droplet residence time and its thermodynamic timescales. When residence time is larger than the thermodynamic timescales, the modification of the total heat flux is attenuated, which is due to cancellations between sensible and latent spray-mediated heat fluxes via spray evaporation. This cancellation indicates the necessity of treating different time scale regimes differently when estimating the spray sensible and latent heat fluxes, especially considering that this effect is not taken into account in most spray models. Moreover, spray droplets with undergoing cancellation have different effects on the surrounding turbulent humidity and temperature fields. In the current study, we further examine the dependence of the modification of heat fluxes on turbulent scales, evaluate droplet statistics to explain the physical mechanisms of spray feedback effects, and outline necessary considerations for parameterizing the spray feedback to the air-sea heat fluxes.