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

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 employing five different planetary boundary layer (PBL) parameterization schemes. Fog occurs frequently during June through September in the coastal zone, where warm, moist, offshore air is advected over upwelled waters with cool SST. Fog formation primarily occurs either at the surface from condensation induced locally by surface cooling, or by downward growth of a shallow, persistent stratus layer, which also occurs preferentially over the cooler coastal waters, with the dominant formation mechanism depending on PBL scheme. Coastal fog formation depends also on stabilization of the inversion above the shallow coastal marine PBL by warm, dry, continental air that extends offshore above the PBL inversion. The stronger fog response in later summer months is attributed to the increasing offshore SST. On synoptic timescales, a similar influence of fog response on upstream conditions is found, but controlled instead by changes in wind direction and advection pathway.