The importance of aerosol and cloud microphysics on the properties and lifecycle of wintertime radiation fog in Po Valley

Similar to other types of clouds, the properties and evolution of fog are potentially sensitive to the interaction withaerosols. Assuming constant liquid water content, an increase in the aerosol concentration leads to water vapor competition, resulting in smaller droplet sizes and higher cloud droplet number concentrations. These changes can further influence the microphysical processes of fog, such as droplet sedimentation and aerosol regeneration (aerosol release upon droplet evaporation). However, substantial uncertainties in the representation of these processes pose challenges for accurately simulating fog evolution and for investigating the impact of aerosols on fog characteristics in climate models.

This study employs the large-eddy model MISU-MIT Cloud and Aerosol (MIMICA) to conduct case studies on wintertime radiation fog in the Po Valley, Italy. Observational data from the Fog and Aerosol InteRAction Research Italy (FAIRARI) campaign during the winter of 2021/22 were used as model input and simulation evaluation. Improvements of the parameterisation schemes in MIMICA were implemented, including surface forcing, warm air advection, droplet sedimentation, and aerosol hygroscopic growth. A series of one-at-a-time sensitivity experiments were performed based on the reference case.

Preliminary results indicate that the lifecycle and microphysical properties of wintertime radiation fog (e.g., droplet concentration, droplet size, and liquid water content) are sensitive to the representation of aerosol size distribution. Furthermore, an accurate description of microphysical processes is critical for capturing fog characteristics. For instance, neglecting droplet sedimentation can lead to a significant overestimation (by approximately an order of magnitude) of fog liquid water; the fog layer structure shows a significant response to different predefined parameters of droplet size distribution; aerosol hygroscopic growth plays a pivotal role in reproducing the water vapor budget and fog liquid water. We also find that an accurate representation of the meteorological conditions, such as surface temperature and humidity variations, is key for capturing the timing of fog onset and dissipation.

Financial support from the European Union’s Horizon 2020 research and innovation program (project FORCeS No 821205) and the Swedish Research council (No 2020-04158) is gratefully acknowledged.