Wintertime observations of channeled flow through a prominent gap along the northern California coast during CALJET and PACJET

Experimental observations from coastal and island wind profilers, aircraft, and other sensors deployed during the California Land-falling Jets Experiment (CALJET) of 1997-1998 and the Pacific Land-falling Jets Experiment (PACJET) of 2000-2001 through 2003-2004 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow (~400-500-m-deep), cold air streams flowing westward through California's Petaluma Gap from the Central Valley to the coast during the winter months. The Petaluma Gap, which is the only major airshed outlet from the Central Valley, is ~35-50 km wide and has walls extending, at most, a modest 600-900 m above the valley floor. Based on this geometry, together with winter meteorological conditions typical of the region (e.g., cold air pooled in the Central Valley and approaching extratropical cyclones), this gap is predisposed to generating westward-directed ageostrophic flows driven by along-gap pressure differences. Case studies and a multiwinter (5 y) analysis are presented here to show that flows through the Petaluma Gap significantly impact local distributions of wind, temperature, precipitation, and atmospheric pollutants, and that they contribute to the generation of coastally trapped air streams. These gap flows preferentially occur in pre-cold-frontal conditions, largely because sea-level pressure decreases westward along the gap in a stably stratified atmosphere in advance of approaching cold-frontal pressure troughs. Air streams exiting the Petaluma Gap are only several hundred meters deep and characterized by relatively cold, easterly flow capped by a layer of enhanced static stability and directional vertical wind shear. Despite the shallow character of these gap-flow air streams, they contribute to blocking, which enhances precipitation along the coast and reduces precipitation in the coastal mountains (at least in a mean sense) by acting as an obstacle to incoming moist-neutral oceanic flow. Airborne air-chemistry observations collected offshore by the NOAA P-3 aircraft illustrate the fact that gap-flow events can transport pollutants from inland to the coast.