The Importance of Aerosol and Droplet Microphysics for the Properties and Life Cycle of Radiation Fog in the Po Valley

Abstract. Employing high-resolution Large Eddy Simulation (LES) coupled with interactive aerosol and cloud microphysics schemes, this study investigates the influence of aerosol and droplet microphysics on the life cycle and properties of wintertime radiation fog in the Po Valley, Italy. For the simulated case, the results show that the main drivers of radiation fog onset and dissipation are nocturnal longwave cooling and surface warming, respectively. Increasing aerosol loading increases droplet number concentration, liquid water content, and fog optical thickness, which reduces droplet sedimentation rates and prolongs fog duration by up to 54 minutes. Overall, the microphysical influence of aerosols and droplets weakens under heavily polluted conditions. We also show that non-activated hydrated aerosols have a limited influence on total liquid water content and fog-layer mixing. However, they critically affect visibility and fog duration prediction, underscoring the importance of explicitly incorporating hydrated particles in fog forecasting and accurately representing aerosol composition. Additional sensitivity experiments reveal that the prescribed droplet spectral shape parameters significantly influence fog characteristics. Parameter settings that represent a broad droplet size spectrum overestimate the number of large droplets compared to observations, which increases mean droplet sedimentation rates and decreases mean liquid water content by up to 104 % and 78 %, respectively, compared to the settings that best represent the observed spectrum.