A Mechanism for Coastal Fog Genesis at Evening Transition

Evening transition from a daytime/convective to a nighttime/stable atmospheric boundary layer often begets fog, in which near-surface visibility reduces below 1 km due to suspended water droplets. Despite the numerous societal impacts of fog (in safety, transportation, defense, and ecology), it remains one of the most poorly predicted meteorological phenomena due to the myriad of atmospheric processes that interact and shape its lifecycle. Here, a new mechanism for fog genesis at evening transition is identified using the comprehensive set of data collected during a local fog event through evening transition on Sable Island, Canada, as part of the Fatima (Fog and Turbulence Interactions in the Marine Atmosphere) campaign. Observations show the appearance of a low-level cloud (LLC) that resulted from the interplay between several stacked boundary layers at the ocean-land discontinuity during the rapid cooling period of evening transition. Top-down turbulent entrainment of the LLC then fills the surface layer with saturated air to form a thick fog layer. While a high-resolution model predicted the appearance of fog in this case study, it did not capture the onset time nor the flow structure, indicating the need for near-surface parametrizations that better capture flow statistics including stratification and turbulent energy transport.