The role of the synoptic-scale conditions in driving fog and mist during the FATIMA July 2022 Sable Island Field program

In July 2022, the Fog and Turbulence Interactions in the Marine Atmosphere (FATIMA) field program included fog observations within surface-based air temperature inversions on Sable Island (SI). Twenty-six fog events under cyclonic and anticyclonic conditions were identified and one case each of these two conditions were analyzed in detail. A cyclonic case (CY) was characterized by a trough from a deep cyclone over northeast Canada moving eastward, generating southwesterly flow in the SI area, with a high cloud band. The approaching trough lowered the cloud base to 60 m at Sable Island that remained for the entire event. The saturation of the near-surface layer and subsequent fog occurred mainly due to low-level convergence causing vertical motion and mixing. In an anticyclonic case (AC), subsidence was maintaining a low cloud with a divergent low-level flow which was advected in a SW/S direction into the Sable Island area with fog. There were significant differences in thermal and dynamical instability between the CY and AC cases. According to the gradient Richardson number, the marine layer was more unstable during the CY case compared to the AC case with a larger buoyancy/shear term. This was confirmed by the higher turbulence kinetic energy in the CY case compared to the AC case. The liquid water content was much greater in the AC case than the CY case. The kinetic sensible and latent heat fluxes were largest during the mist periods and least during the fog periods in the CY case whereas they were much less in the AC case. Light rain began partway through the first fog and continued until the end of the event for the CY and the AC cases.