Fog and low-level stratus in coupled ocean-atmosphere simulations of the northern California Current System upwelling season

Abstract

Fog and low-level stratus during April through September 2009 are examined in a set of coupled ocean-atmosphere numerical simulations of the northern California Current System (CCS). The model configurations differ only in the choice of planetary boundary layer (PBL) parameterization scheme and, in one case, surface flux scheme. The results suggest that fog formation in this region primarily occurs through condensation at the surface induced locally by surface cooling, when moist offshore air is advected over cold upwelled waters and the shallow coastal marine PBL is further stabilized by warm, dry, continental air that extends offshore above the PBL inversion. These results are consistent with some but not all prior hypotheses for fog formation in the CCS region. Fog formation by downward growth of a pre-existing stratus layer is also found in the simulations but dominates only in those simulations with PBL schemes that produce an extensive and evidently unphysical stratus layer at 200-m height, which serves as the source for the downward growth. The stronger fog response in later summer months arises from seasonal warming of offshore SST, which increases the moisture content and temperature of the upstream air mass, while cool coastal SSTs are maintained by upwelling. On synoptic timescales, a similar influence of fog response on upstream conditions is found but controlled instead by changes in wind direction. These results suggest that the critical factors determining the evolution of the coastal fog regime in a warming climate are likely the temperature of upwelling source waters and the offshore flow of continental air. Fog and low-level stratus during April through September 2009 are examined in a set of coupled ocean-atmosphere numerical simulations of the northern California Current System (CCS). The model configurations differ only in the choice of planetary boundary layer (PBL) parameterization scheme and, in one case, surface flux scheme. The results suggest that fog formation in this region primarily occurs through condensation at the surface induced locally by surface cooling, when moist offshore air is advected over cold upwelled waters and the shallow coastal marine PBL is further stabilized by warm, dry, continental air that extends offshore above the PBL inversion. These results are consistent with some but not all prior hypotheses for fog formation in the CCS region. Fog formation by downward growth of a pre-existing stratus layer is also found in the simulations but dominates only in those simulations with PBL schemes that produce an extensive and evidently unphysical stratus layer at 200-m height, which serves as the source for the downward growth. The stronger fog response in later summer months arises from seasonal warming of offshore SST, which increases the moisture content and temperature of the upstream air mass, while cool coastal SSTs are maintained by upwelling. On synoptic timescales, a similar influence of fog response on upstream conditions is found but controlled instead by changes in wind direction. These results suggest that the critical factors determining the evolution of the coastal fog regime in a warming climate are likely the temperature of upwelling source waters and the offshore flow of continental air.