Characterizing fog and the physical mechanisms leading to its formation during precipitation in a coastal area of the northeastern United States
The presence of fog has impacts in areas as varied as public safety and health, economic issues for the transportation industry, health of ecosystems and modification of ambient aerosols. Despite scientific and technological advances, the accurate prediction of fog remains a formidable challenge. The goals of this research consist of identifying and characterizing the various fog regimes in the New York City region and gaining a better understanding of the physical mechanisms influencing the life cycle of fog, with an emphasis on the most common type. This is accomplished through analyses of historical data and observations from sensors deployed at a site in the region, complemented by numerical model simulations. Fog is found to be a superposition of five different types, each exhibiting distinct features in its spatial and temporal variability. Precipitation fog is the most common, with its formation generally taking place as light rain or drizzle falls into saturated low level inversions. Fog formation involves precipitating drops departing from equilibrium and remaining warmer than the ambient air of the inversion. These drops continue evaporating even in a saturated environment, creating supersaturated conditions and activation of cloud condensation nuclei. The existence of non-equilibrium raindrops is theoretically described and investigated with a detailed microphysical model. Simulations are performed and observations taken during a precipitation fog event are analyzed to obtain a quantitative assessment of the key role of rainfall evaporation in the formation of supersaturated conditions and fog.
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