Extreme daily precipitation in the Northern Sierra Precipitation 8-Station index: The combined impact of landfalling atmospheric rivers and the Sierra barrier jet

Abstract:

The Upper Sacramento River area is vital to California’s water supply, and is susceptible to major floods. Recent studies indicate that orographic precipitation in this complex terrain involves both inland penetrating atmospheric rivers (ARs) and the Sierra barrier jet (SBJ). The southerly SBJ induces orographic precipitation along south-facing slopes in the Shasta region, whereas landfalling ARs ascend up and over the statically stable SBJ and induce orographic precipitation along west-facing upper slopes in the Northern Sierra Nevada. This paper explores the hypothesis that extreme daily precipitation here is controlled by the presence of both a landfalling AR and a SBJ.

Three 10-year-long (2000–2011) observational datasets are used. ARs are identified from the Neiman et al. (2008) AR catalog that uses an SSM/I satellite-based AR-detection method from Ralph et al. (2004), whereas SBJ conditions are determined from Chico, CA wind profiler data using the method from Neiman et al. (2010). Extreme daily precipitation is identified from the average of 8 rain gauges spanning the region known as the “Northern Sierra 8-Station Index.” The “index” is used by water managers to assess water supply. Extreme events are defined as the 50 largest daily precipitation totals in the index for the 10-year period (the top ~1.37%).

These dates in the 8-station index are compared with the catalogs of landfalling ARs and SBJs. In summary, 46 of 50 (92%) extreme daily precipitation events are associated with landfalling ARs on either the day before or the day of precipitation, whereas 45 of 50 (90%) extreme daily precipitation events are associated with SBJ conditions. 38 of 50 (76%) extreme daily precipitation events are associated with both a landfalling AR and an SBJ. The 10 days with the largest daily precipitation in the index were all associated with both a landfalling AR and an SBJ. Thus, extreme daily precipitation in Northern California is strongly controlled by the presence of both a landfalling AR and a SBJ.