Analysis of a coastal marine fog episode during C-FOG field campaign

Abstract:

A warm sea fog (cold air advection over warmer water) event observed on 13 September 2018 aboard the research vessel Hugh R. Sharp during the C-FOG field campaign is analysed for formation, evolution and dissipation (i.e., lifecycle) of fog. This episode occurred off the coast of St. John’s, Newfoundland, Canada. An array of measurement platforms with observing capabilities over a wide range of space-time scales -- thousands of kilometres/days (satellites and reanalysis) to metres/hours (soundings, ceilometer) to centimetres/seconds (anemometers, microphysical sensors and radiometers) -- were used. The local climatology is typically conducive for cold sea fog, promoted by southerly warm moist air advected over colder Labrador water. While such conditions existed but without fog, an eastward moving low-pressure system caused switching of local conditions to colder air flowing over warmer water. This shift was associated with a cold front, mixing of near-saturated colder and warmer air at the front, and appearance of mixing fog spread over multiple horizontal layers separated by inversions. The visibility (strength of fog) was directly related to the liquid water content and droplet number concentration, and to some extent to the mean volume diameter and effective radius. The air–sea temperature difference was found to play a secondary role in this warm sea fog event since the effects of associated positive heat flux, though significant, was dwarfed by the surface stable stratification induced by frontal activity that suppressed turbulence and entrainment at the inversion layer. Intensification of turbulence and associated enhancement of entrainment that caused upward erosion of inversion layers due to increased flow speed appear to have caused eventual dissipation of fog. This study emphasizes the importance of synoptic condition in preconditioning of air masses for fog genesis as well as the importance of smaller scale processes (turbulence, entrainment) in the lifecycle of fog.