Abstract
Aerosols influence fog properties such as visibility and lifetime by affecting fog droplet number concentrations (Nd). Numerical weather prediction (NWP) models often represent aerosol–fog interactions using highly simplified approaches. Incorporating prognostic size-resolved aerosol microphysics from climate models could allow them to simulate Nd and aerosol–fog interactions without incurring excessive computational expense. However, microphysics code designed for coarse spatial resolution may struggle with sub-kilometer-scale grid spacings. Here, we test the ability of the UK Met Office Unified Model to simulate aerosol and fog properties during case studies from the ParisFog field campaign in 2011. We examine the sensitivity of fog properties to variations in Nd caused by modifications to simulated aerosol activation. Our model, with a 500 m horizontal resolution and interactive aerosol and cloud microphysics, significantly underpredicts Nd, although it only slightly underestimates the cloud condensation nuclei concentration. With an updated version of the Abdul-Razzak and Ghan (2000) activation scheme, we produce Nd that are more consistent with those predicted by a cloud parcel model under fog-like conditions. We activate droplets only by adiabatic cooling. We incorporate more realistic hygroscopicities for sulfate and organic aerosols and explore the sensitivity of simulated Nd to unresolved updrafts. We find that both Nd and simulated fog liquid water content are very sensitive to the updated activation scheme but remain less affected by the update to hygroscopicities. Our improvements offer insights into the physical processes regulating Nd in stable conditions, potentially laying foundations for improved operational fog forecasts that incorporate interactive aerosol simulations or aerosol climatologies.
| Original language | English |
|---|---|
| Pages (from-to) | 11129-11156 |
| Number of pages | 28 |
| Journal | Atmospheric Chemistry and Physics |
| Volume | 25 |
| Issue number | 18 |
| DOIs | |
| State | Published - Sep 24 2025 |
Funding
We thank the scientists responsible for the ParisFog field campaign. Model simulations are material produced using Met Office software. The computational resources on Air Force Weather HPC11 are provided by the Oak Ridge Leadership Computing Facility (OLCF) Director’s Discretion Project NWP501 and ATM112. The OLCF at Oak Ridge National Laboratory (ORNL) is supported by the Office of Science of the US Department of Energy under contract no. DE-AC05-00OR22725. This work also used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant ACI-1548562. Specifically, it used the Bridges-2 system, which is supported by the NSF Award ACI-1928147, at the Pittsburgh Supercomputing Center (PSC). This work also used Bridges-2 at the PSC through allocation atm200005p from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296. This research was supported by the US Air Force Life Cycle Management Center (LCMC) collaboration with Oak Ridge National Laboratory (ORNL).