Observations of Fog-Aerosol Interactions Over Central Greenland

Supercooled fogs can have an important radiative impact at the surface of the Greenland Ice Sheet, but they are difficult to detect and our understanding of the factors that control their lifetime and radiative properties is limited by a lack of observations. This study demonstrates that spectrally resolved measurements of downwelling longwave radiation can be used to generate retrievals of fog microphysical properties (phase and particle effective radius) when the fog visible optical depth is greater than similar to 0.25. For 12 cases of fog under otherwise clear skies between June and September 2019 at Summit Station in central Greenland, nine cases were mixed-phase. The mean ice particle (optically-equivalent sphere) effective radius was 24.0 +/- 7.8 mu m, and the mean liquid droplet effective radius was 14.0 +/- 2.7 mu m. These results, combined with measurements of aerosol particle number concentrations, provide evidence supporting the hypotheses that (a) low surface aerosol particle number concentrations can limit fog liquid water path, (b) fog can act to increase near-surface aerosol particle number concentrations through enhanced mixing, and (c) multiple fog events in quiescent periods gradually deplete near-surface aerosol particle number concentrations. Plain Language Summary Fogs over the central Greenland Ice Sheet can modify the net radiation that reaches the ice surface. How much a fog influences the net surface radiation is related to the fog lifetime and optical depth. These properties are related to the phase and size distribution of the particles that make up the fog, that in turn depend on the characteristics of the atmospheric aerosol particles on which the fog forms. This study shows that the phase and size distribution of fog particles can be determined from ground-based measurements of downwelling longwave radiation, and explores how fogs interact with the number concentration of atmospheric aerosols measured near the surface during 12 cases of summer-time fog in central Greenland.