Is a more physical representation of aerosol chemistry needed for fog forecasting?

With the changing climate, the study of fog formation is essential due to the impact of the complexity of natural and anthropogenic aerosols. The evolution of the droplet size distribution in the presence of different aerosol species remains poorly understood. To make progress towards reducing the uncertainty of fog forecasts, the Eulerian-Lagrangian particle-based small-scale model for the diffusional growth of droplets is used to better understand the droplet activation and growth. The small-scale model simulations are performed using observed data from the Winter Fog Experiment study over Indira Gandhi International Airport, New Delhi. The microphysical properties, such as droplet number concentrations (Nd) and liquid water content (LWC), important for fog simulation, are evaluated to gain more insights. The small-scale simulations have shown the droplet microphysical properties at different evolutionary stages. The Nd and effective radius reff$$ {r}_{\mathrm{eff}} $$ change with variations in LWC for different aerosol chemistries (i.e., organics, mix, and inorganic). The calculated visibility at small scale is also shown with the variation of Nd and LWC. This study compared visibility from an existing parametrization with parcel-direct numerical simulation calculation. The hygroscopicity kappa$$ \kappa $$, which is highly related to the activation of aerosols to condensation nuclei, is taken into account to demonstrate the contribution of aerosol chemistry to fog droplet formation. The results highlight that hygroscopicity is essential in the numerical model for fog and visibility prediction as the microphysical properties of fog are regulated by aerosol species. Composition of aerosol particles can impact on droplet size distribution and the formation of fog visibility. Progress towards reducing the uncertainty of fog forecasts, the Eulerian-Lagrangian particle-based small-scale model is used to better understand the droplets activation. Fog visibility parametrizations are evaluated based on the small-scale model simulations using observed data from the Winter Fog Experiment (WiFEX) study. image