C-FOG Observations: Mechanisms of Coastal Fog Genesis

Fog is defined as suspended water droplets and ice crystals (size ~ 1-50 microns) in the atmospheric boundary layer (ABL) that impair visibility to <1 km. A myriad of classifications are used for fog, but the commonly used types are radiation fog, advection/movement fog, and mixing fog. The C-FOG project concerns fog in coastal areas (or coastal fog), where the life cycle of fog is controlled by collusion of terrestrial, oceanic and atmospheric processes. While diurnal heating/cooling cycle is damped in the marine ABL, and hence fog tends to be advection and mixing fog, terrestrial influence causes coastal fog to have additional complexity. The C-FOG field campaign was designed to delve into such intricacies, in particular toward improving fog predictions via better fog physics. The campaign was conducted during September-October 2018, in Nova Scotia and Newfoundland as well as over the contiguous coastal ocean aboard a research vessel. An array of in situ, path-integrating, remote-sensing and profiling instruments was deployed, allowing measurements across a swath of space-time scales. While fog appearances were less frequent than anticipated, the exhaustive dataset collected undergirds detailed studies of processes (life cycle), dynamics (two-phase flow and turbulence), microphysics (droplet characterization, nucleation) and thermodynamics (radiative mechanisms, phase changes) as well as modeling. This presentation deals with two cases of mixing fog, from premonitory to evolution stages. In the first, interactions between a marine cold front and land are considered, and the second concerns the genesis of fog due to lowering of stratus clouds swayed by surface layer processes and cloud-top instability.

This research is by the Office of Naval Research Award # N00014-18-1-2472 entitled: Toward Improving Coastal Fog Prediction (C-FOG).