A Multiwinter Analysis of Channeled Flow through a Prominent Gap along the Northern California Coast during CALJET and PACJET

Abstract

Experimental observations from coastal and island wind profilers, aircraft, and other sensors deployed during the California Land-falling Jets Experiment of 1997/98 and the Pacific Land-falling Jets Experiment of 2000/01-2003/04 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow (ϳ400-500 m deep), cold airstreams 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 air shed outlet from the Central Valley, is ϳ35-50 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. Two case studies and a five-winter composite analysis of 62 gap-flow cases are presented here to show that flows through the Petaluma Gap significantly impact local distributions of wind, temperature, precipitation, and atmospheric pollutants. 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. Airstreams 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. 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, and that they can contribute to coastally blocked airstreams. The strongest gap-flow cases occur when comparatively deep midtropospheric troughs approach the coast, while the weak cases are tied to anticyclonic conditions aloft. Low-level cold-frontal pressure troughs approaching the coast are stronger and possess a greater along-gap pressure gradient for the strong gap-flow cases. These synoptic characteristics are dynamically consistent with coastal wind profiler observations of stronger low-level gap flow and winds aloft, and greater rainfall, during the strong gap-flow events. However, gap flow, on average, inhibits rainfall at the coast.

An overview of barrier winds off southeastern Greenland during the Greenland Flow Distortion experiment

“…Some similarities can be found between the Greenland barrier flow and the hybrid barrier jets by the west coast of the USA (Neiman et al, 2006;Olson et al, 2007), i.e. offshore maximum wind speed and cold jet core, but there are also fundamental differences.…”

Representation of the Sierra Barrier Jet in 11 years of a high‐resolution dynamical reanalysis downscaling compared with long‐term wind profiler observations

“…Although NARR's representation of the climatological SBJ at CCO is fairly realistic, plan views and cross sections reveal that its representation of topography introduce suspicious‐looking factors: large grid point‐to‐grid point discrepancies with seemingly isolated maximum wind speeds, a complete lack of coastal barrier jet (we expect a coastal barrier jet during SBJ cases [ Neiman et al , 2006]), and substantial differences in magnitude and direction of water vapor transport. These factors severely limit NARR's applicability for an investigation of SBJ dynamics, and require higher resolution, as provided by the 6 km resolution WRF‐RD.…”

Transport of pollution to a remote coastal site during gap flow from California's interior: impacts on aerosol composition, clouds, and radiative balance

“…Also during the winter months, conditions known as channeled gap flow can transport air masses from much further inland to the remote coast. These episodic periods result when very low near-surface buoyancy and an onshore-directed gap-parallel pressure gradient co-occur in one of several prominent gaps in the coastal mountain ranges (Overland and Walter Jr., 1981;Neiman et al, 2006;Loescher et al, 2006;Colle et al, 2006). One such prominent gap is located near the town of Petaluma in Sonoma County, CA, and can act to channel air from the north San Francisco Bay Area (SFBA), the Sacramento River Delta, and California's Central Valley (CV) to coastal northern California (see schematic in Fig.…”