Modeling sea fog on the U.S. California coastduring a hot spell event

Abstract

The occurrence of sea fog along the U.S. Pacific Coast in summer is frequently associated with the movement of a high pressure system from the eastern Pacific to the land. Subsequently there is strong heating of the land over several days or more and development of »hot spells« and offshore flows in the coastal region preceding sea fog formation. This study focuses on modeling the formation and evolution of sea fog in response to interaction between the warm and dry offshore flows and the cool and moist marine atmospheric boundary layer. Simulation results support a conceptual model of fog formation and evolution based on physical processes initiated by offshore flows that efficiently lower the marine inversion near the sea surface. In spite of the warm and dry advection, fog formed in the shallow, near-surface marine layer capped by a strong temperature inversion of 10 °C or more and a hot-air layer above the inversion. Prior to sea fog formation, negative surface heat flux initiates cooling and condensation, while the surface moisture flux contributes to increased humidity and turbulence within the surface layer. The dryness of the hot-air layer overlying the shallow and moist marine layer triggers enhanced radiative cooling at the marine layer top and facilitates the marine layer’s saturation. The thin cloud forms, rapidly propagates downward, and transforms into fog. As soon as the fog is formed, longwave radiative cooling at its top generates turbulent mixing and the growth of the fog as a mixed layer. Due to the fog-top radiative cooling, the fog layer is initially colder than the underlying surface. In the later stage of the fog evolution, continuous mixing of the cool and moist near-surface air with the dry and warm layer above the inversion during the fog growth generally curtails turbulence. This process elevates the lifting condensation level and can lead to sea fog dissipation or generation of stratus.