Formation, evolution, and dissipation of coastal sea fog

Evolution of sea fog has been investigated using three-dimensional Mesoscale Model 5 (MM5) simulations. The study focused on widespread fog-cloud layers advected along the California coastal waters during 14-16 April 1999. According to analysis of the simulated trajectories, the intensity of air mass modification during this advection significantly depended on whether there were clouds along the trajectories and whether the modification took place over the land or ocean. The air mass, with its trajectory endpoint in the area where the fog was observed and simulated, gradually cooled despite the gradual increase in sea-surface temperature along the trajectory. Modelling results identified cloud-top cooling as a major determinant of marine-layer cooling and turbulence generation along the trajectories. Scale analysis showed that the radiative cooling term in the thermodynamic equation overpowered surface sensible and latent heat fluxes, and entrainment terms in cases of the transformation of marine clouds along the trajectories. Transformation of air masses along the trajectories without clouds and associated cloud-top cooling led to fog-free conditions at the endpoints of the trajectories over the ocean. The final impact on cloud-fog transition was determined by the interaction of synoptic and boundary-layer processes. Dissipation of sea fog was a consequence of a complex interplay between advection, synoptic evolution, and development of local circulations. Movement of the high-pressure system over land induced weakening of the along-shore advection and synoptic-pressure gradients, and allowed development of offshore flows that facilitated fog dissipation.