Dominant Factors Controlling the Hydrometeorology of Northern California: Landfalling Atmospheric Rivers and Sierra Barrier Jets

The hydrometeorology in the northern half of California's Central Valley (CV) and the adjacent Sierra Nevada and Mt. Shasta-Trinity Alps region has received considerable attention over the last decade through ongoing field campaigns associated with the National Oceanic and Atmospheric Administration's Hydrometeorological Testbed program and the California Energy Commission's CalWater project. The motivation for this attention has been driven largely by water-supply and hydroelectric issues and flood-control concerns affecting a majority of the state's ~38 million residents, since much of the region's snowmelt and rainfall collect in expansive reservoirs behind large dams. This precipitation can be heavy in the mountains due to orographic enhancement during the landfall of winter storms. In addition to providing valuable water resources across California, the heavy precipitation subjects vulnerable population centers to the threat of major flooding, including the state capital of Sacramento, which is recognized as one of the most vulnerable cities in the United States to the ravages of catastrophic flooding.

Two atmospheric phenomena significantly modulate the distribution of precipitation, high-altitude snowpack, and runoff in the mountains surrounding California's northern CV: transient atmospheric rivers (ARs) and terrain-locked Sierra barrier jets (SBJs). This presentation will highlight key AR and SBJ influences on orographic precipitation distributions and intensities across northern California, both from a case-study perspective and by employing a multi-case compositing approach. The analysis is driven by wind-profiler and global-positioning-system (GPS) observations in tandem with soil moisture probes, stream gauges, and a 6-km-resolution regional reanalysis dataset. Key results are as follows: Inland-directed ARs override a shallow ~1-km-deep, Sierra-parallel SBJ located above the windward slope of the Sierra. The SBJ increases in altitude up the Sierra's windward slope and poleward toward the north end of the CV, but it does not reach the westernmost CV. Above the developing SBJ, strengthening southwesterly flow descends temporally in response to the landfalling AR. The moistening SBJ reaches maximum intensity during the strongest AR flow aloft, at which time the core of the AR-parallel vapor transport slopes over the SBJ and intersects the Sierra. The inland penetration of the AR through the San Francisco Bay gap in the coastal mountains contributes to SBJ moistening and deepening. The SBJ subsequently weakens with the initial cold-frontal passage aloft. A statistical analysis of orographic forcing reveals that both the AR and SBJ are crucial factors in determining the amount and spatial distribution of precipitation in the northern Sierra Nevada and in the Shasta-Trinity region at the northern terminus of the CV. As the AR and SBJ flow ascends the steep and tall terrain of the northern Sierra and Shasta-Trinity region, respectively, the precipitation becomes enhanced. This research has expanded our understanding of orographic precipitation enhancement from coastal California to its interior.

An open question remains regarding the transport of water vapor near the northern terminus of the CV. Namely, a portion of the AR-modulated SBJ flow may evacuate through a prominent gap in the terrain between Mt. Lassen and Mt. Shasta, near the town of Burney. From conservation of mass principles, the magnitude of this exiting gap flow may be inversely correlated with the strength of the flow and orographic precipitation enhancement impacting the Trinity/Mt. Shasta region to the northwest. A recent back-trajectory analysis based on the 150 wettest 24-h periods in southwestern Idaho shows a highly preferred pathway through the Burney Gap region. Hence, the SBJ in the northern CV, and the adjacent Burney Gap, may allow AR water vapor to penetrate inland and modulate heavy precipitation events far downwind. An ideal opportunity exists to monitor and better understand the water vapor pathways in this topographically complex region during the out-years of the CalWater2 field campaign, where the northern end of the CV can be more fully instrumented with wind profilers, GPS receivers, and other complementary instrumentation.