Consistent Simulations of Aerosol-Fog Interactions across Scales with the Met Office Unified Model

Numerical weather prediction models often struggle to forecast fog, partly due to the difficulty in simulating fine-scale turbulence and surface-atmosphere exchange, and partly because aerosol effects on fog are often over-simplified in the models. Accurate modeling of both aerosols and fine-scale turbulence is needed to predict cloud droplet number concentration (CDNC), which impacts visibility and fog lifecycle. Consequently, fog is typically modeled using high-resolution simulations. Here we attempt to build consistent high-resolution and coarse-resolution simulations of fog with the UK Met Office Unified Model (UM), focusing on accurate simulations of CDNC.

To promote consistency in CDNC across model resolutions, modifications were made to the aerosol activation scheme in the UM. This was done by incorporating turbulent kinetic energy (TKE) into the activation updraft velocity, promoting aerosol activation into cloud droplets. To validate modeled CDNC values, we ran simulations during the coastal fog (C-FOG) field campaign, conducted in the Northwest Atlantic during the fall months of 2018. As expected, for coarser resolutions (e.g., dx=30 km), where sub-grid processes are not well resolved, the TKE becomes increasingly important.

Alongside improving surface CDNC in the UM for visibility prediction, it is important to accurately model droplet concentrations at the fog top, which are relevant to cloud albedo and climate. In doing so, simulations can also be validated against satellite retrievals with much greater spatial and temporal coverage. Satellite products from MODIS can be used to retrieve cloud-top CDNC and liquid water path from measured radiance. Initially, modeled droplet concentrations were severely underestimated compared to satellite observations. This can be attributed to uncertainties in both model and satellite algorithms alike. To better understand uncertainties in the satellite retrievals, we explored alternative ways to determine droplet concentrations in fog from satellite data and surface measurements.