Droplet sedimentation regulates liquid water in marine advection fog: Large-eddy simulations with interactive sectional microphysics

Abstract. Marine advection fog strongly affects visibility and maritime operations, yet numerical models frequently overestimate its liquid water content. Although fog-top radiative cooling is known to promote condensation, how size-dependent gravitational droplet sedimentation redistributes and removes the resulting liquid water remains poorly quantified. Here, we use the University of California, Los Angeles Large-Eddy Simulation model coupled with the sectional aerosol–cloud microphysics module from a Lagrangian perspective to quantify this process during a typical advection-fog event over the northwestern Pacific. Longwave radiative cooling near the fog top promotes condensational growth and the formation of large droplets, generating intermittent sedimentation signals that propagate downward through the fog layer. Lead–lag correlations show that sedimentation initiated near the fog top reaches the fog base after approximately 1 h. The liquid water path budget identifies fog-top radiative cooling as the dominant source (about 30 g m^-2 h^-1) and gravitational sedimentation as the largest microphysical sink (approximately 20 g m^-2 h^-1), offsetting nearly two-thirds of the radiatively driven production. Droplets with radii larger than 10 μm dominate the sedimentation flux. Sensitivity experiments confirm the robustness of this mechanism: removing sedimentation causes excessive liquid-water accumulation and substantially deepens the fog layer, while suppressed collision–coalescence or enhanced aerosol loading weakens sedimentation by limiting large-droplet formation. These results identify droplet sedimentation as a vertically coupled control on liquid-water redistribution and fog-layer evolution, implying that marine-fog parameterizations need to represent sedimentation and large-droplet microphysics explicitly.