Field and Numerical Investigation of Moist Thermodynamical Structure of Marine Fog

A series of studies on the interplay between atmospheric moist thermodynamics (e.g., related to fog and clouds) and stratified turbulence is described. The first study presents ship-based measurements of fog off St John’s, Newfoundland, on 13 September 2018 during the Coastal Fog (C-FOG) field campaign. The measurements included cloud-particle spectra, cloud-base height and aerosol backscatter, radiation, turbulence, visibility, and sea surface temperature. Fog occurred in two episodes during the passage of an eastward-moving synoptic low-pressure system, characterized by multiple inversions with capping subsidence inversion, and one well-mixed fog layer capped by a subsidence inversion. Low wind speeds and stable stratification maintained weak surface-layer turbulence during fog. Counter-gradient heat fluxes observed are attributed to turbulence, entrainment, and stratification that overwhelmed the air–sea temperature difference influence. While synoptic-scale dynamics preconditioned the area for fog formation, the final step of fog appearance was nuanced by stratification–turbulence interactions, local advective processes, and microphysics.

Inspired by recent field campaigns, the interaction between stratification, thermodynamics and turbulence, in particular, the emergence of layered stratification in marine surface layer was investigated. Large Eddy Simulation with newly added moist thermodynamics and phase changes was applied to idealized case with various stratification and forcing magnitudes. Spatial correlation between density interfaces and cloud layers was discovered, with cloud underneath the interfaces. Scaling analysis shows that, at quasi-stationary state, the normalized mean mixed-layer height linearly increases with a length-scale based on the root-mean-square velocity and buoyancy frequency. The proportionality constant differs due to phase change, and the mean vertical mixed-layer thickness decreases with phase change, due mainly to the increase of the local buoyancy frequency with heat release. The mixing efficiency converges to 0.2 for cases with phase change and to 0.25 for cases without. The normalized turbulent eddy diffusivity linearly increases with the square of a Froude number.