Assessing evaporative cooling from seawater spraying in the marine boundary layer

Abstract. Increasing frequency and intensity of extreme heat events motivate improved understanding of processes that may modify near-surface thermal conditions. This study investigates seawater spraying as a method for direct atmospheric cooling. Using large-eddy simulations (LES), we quantify plume dynamics, cooling response, and impacts on heat stress across a range of atmospheric conditions. Simulated spray rates of 50 L s^-1 produce near-surface temperature reductions of approximately 0.5–1 °C within the first few kilometers. Although theoretical analysis suggests that evaporative cooling potential can exceed -20 °C under hot and dry conditions, atmospheric mixing and advection substantially reduce this potential under realistic conditions. Cooling is strongest near the source due to evaporatively driven downdrafts, but decays rapidly downstream. We compare deployment from wind turbines with ship-based platforms. A key finding is that injection height governs plume structure and cooling response. Ship-based spraying produces strong, localized near-surface cooling, whereas wind turbine deployment redistributes cooling more effectively within the lower boundary layer. The turbine wake modifies local mixing, but vertical redistribution is mainly governed by evaporatively driven buoyancy perturbations. Extending the analysis to human thermal comfort reveals that temperature reductions alone do not determine effectiveness. Changes in Heat Index are non-linear and depend on background humidity, with benefits near the source that may diminish or even reverse downstream. Overall, seawater spraying can provide measurable cooling, but its performance is highly sensitive to background atmospheric conditions.