Simulating Aerosol Activation in Stratus Lowering Fog Observed During C-FOG

Aerosol particles play a crucial role in determining droplet concentrations in warm clouds and fog. To reach critical supersaturation in stratus or cumulus clouds, parcels adiabatically cool as they rise in updrafts. However, fog has much weaker vertical motion, and relies heavily on radiative cooling processes to reach critical supersaturation. Often in climate models, radiative cooling does not inform aerosol activation, and fog droplet concentrations are generated by adiabatic cooling using a minimum updraft speed, for example 0.1 m/s. This approach systematically overestimates aerosol activation from adiabatic cooling (if the minimum is set too high, as it often is) and neglects spatial or temporal variability in radiative cooling rates.

We are investigating the potential importance of radiative cooling in aerosol activation in coastal fog using the UM regional high-resolution model. Through separation of adiabatic (updraft) and non-adiabatic (radiative cooling) supersaturation sources for coastal fog, we aim to improve the representation of aerosol activation in the UK Met Office Unified Model (UM).

To compare model performance to measured data, we are simulating case studies from C-FOG, a 2018 field campaign that took place in Newfoundland and Nova Scotia during September and early October.[1] This campaign measured three distinct coastal fog events. Atmospheric measurements such as aerosol/fog droplet size distribution and liquid water content are complemented by in-situ sensing data. Satellite products from GOES and MODIS can be used to retrieve droplet number concentration and liquid water path over a larger geospatial area. Our initial simulation runs indicate that simulated droplet concentrations are overestimated by a factor of 2-3, suggesting that further improvements to the aerosol activation scheme are needed before activation by radiative cooling can be introduced. We outline several possible explanations for the overestimates inspired by complementary work studying urban radiation fog.

[1] Fernando et al., Bulletin of the American Meteorological Society. 2021. DOI: 10.1175/BAMS-D-19-0070.1