Fog Formation during Gravity Currents Interacting with Coastal Topography

Abstract:

An interesting mixing-fog event was observed during IOP7 of the C-FOG project, where a cold front arriving from the northeast collided with the Downs peninsula in Ferryland, Newfoundland, to produce foggy conditions. An extensive set of field observations indicates that this collision resulted in turbulent mixing of almost saturated ambient air with a nearly saturated cold front, creating conditions for mixing fog. To shed light on the resulting turbulent mixing process, laboratory experiments were conducted to study the interaction of a lock-exchange induced gravity current with a (rectangular) obstacle. Instantaneous velocity and density fields were obtained using particle image velocimetry and planar laser-induced fluorescence. The observations show that the obstacle starts affecting the approaching gravity-current propagation at an upstream distance of 2H and, upon collision, the mixing is occurring over a spatial scale of 0.83H, where H is the depth of the ambient fluid layer. The time for larger-scale turbulent stirring to permeate to the smallest scales of turbulence and activate the condensation nuclei was estimated as 3t^*, where t^*=√(H/g') is the intrinsic time scale of the gravity current and g' the reduced gravity. Extrapolation of results to IOP7 showed a good agreement with observations. (funded by ONR grant # N00014-18-1-2472 entitled C-FOG: Toward Improving Coastal Fog Prediction)