A Mechanism for Coastal Fog Genesis at Evening Transition

An intriguing fog generation mechanism was observed in the Grand Banks area of the North Atlantic Ocean during a field campaign conducted during July 1-31, 2022, designed to study ‘Fog and Turbulence Interactions in the Marine Atmosphere (Fatima)’. The observational platforms included an instrumented islet (Sable Island), research vessel (R/V Atlantic Condor), wave gliders and an oil platform (Hibernia). On July 21, fog appeared locally on Sable Island during the evening transition. A low-level cloud first appeared at a height of $\sim$30-50 m, followed by cloud ‘puffs’ descending to the ground in what appears to be a top-down entrainment event. At this time, the lower atmospheric boundary layer was rich in structure due to an ocean-land discontinuity presented by the islet, with a marine convective boundary layer (MCBL) from the southwest advected over a convective internal boundary layer (IBL) over Sable Island, wedging a stable interface (inversion layer IL) in between. Initial stability of the IL quantified by a bulk Richardson number was weak, allowing turbulent eddies from IBL and MCBL to penetrate past the IL freely, but IL stability progressively became stronger and separated the two layers dynamically, allowing only local interfacial mixing at the IL. Such mixing across the near-saturated and gradually (but differentially) cooling IBL and MCBL appeared to cause local saturation, entrainment at the edges being the cause of descending ‘puffs’ that intermittently reduced the visibility first, then lead to fog (visibility $<$ 1 km) that persisted for many hours. While forecasting with mesoscale numerical models and climatological analyses predicted fog for the evening of July 21, there was a mismatch of the time of genesis and pertinent meteorological parameters, calling for further tuning of small-scale near-surface model physics.