Abstract
California is regularly impacted by floods and droughts, primarily as a result of too many or too few atmospheric rivers (ARs). This study analyzes a two-decade-long hourly precipitation dataset from 176 California weather stations and a 3-hourly AR chronology to report variations in rainfall events across California and their association with ARs. On average, 10-40 and 60-120 hours of rainfall in southern and northern California, respectively, are responsible for more than half of annual rainfall accumulations. Approximately 10-30% of annual precipitation at locations across the state is from only one large storm. On average, northern California receives 25-45 rainfall events annually (40-50% of which are AR-related). These events typically have longer durations and higher event-precipitation totals than those in southern California. Northern California also receives more AR landfalls with longer durations and stronger Integrated Vapor Transport (IVT). On average, ARs contribute 79%, 76%, and 68% of extreme-rainfall accumulations (i.e., top 5% events annually) in the north coast, northern Sierra, and Transverse Ranges of southern California, respectively. The San Francisco Bay Area terrain gap in the California Coast Range allows more AR water vapor to reach inland over the Delta and Sacramento Valley, and thus, influences precipitation in the Delta’s catchment. This is particularly important for extreme precipitation in the northern Sierra Nevada, including river basins above Oroville Dam and Shasta Dam. This study highlights differences between rainfall and AR characteristics in coastal versus inland northern California, differences that largely determine the regional geography of flood risks and water-reliability. These analyses support water resource, flood, levee, wetland, and ecosystem management within the catchment of the San Francisco estuary system by describing regional characteristics of ARs and their influence on rainfall on an hourly timescale.
Highlights
California’s precipitation is vital to its people, agriculture, and ecosystems—and dictates its frequent flooding and droughts
Rainfall events in northern California generate a median of 10–22 mm rainfall per event, where higher values of event-totals are associated with events along the north coast and some stations in the northern Sierra and northern Central Valley
Southern California rainfall events have lower median event-totals (10–14 mm per event) than those in northern California (10–22 mm per event), with the exception of some parts of the Transverse Ranges, which receive a median of 20 mm rain per event (Figure 1B)
Summary
California’s precipitation is vital to its people, agriculture, and ecosystems—and dictates its frequent flooding and (when lacking) droughts. An important example of this is the role of the gap in coastal terrain near the San Francisco Bay, which recent studies have found allows greater water vapor transport in ARs to penetrate inland into the Central Valley and enhance precipitation in the Sierras (Neiman et al 2013; White et al 2015). These studies are important in California, where future increases in heavy precipitation and horizontal water vapor transport are projected in a warming climate (Dettinger 2011; Lavers et al 2013; Warner et al 2014; Dettinger 2016; Hagos et al 2016; Polade et al 2017; Espinoza et al 2018). Precipitation extremes are central to California’s water resources, floods, and ecosystems, and the more precisely we understand their details, the better we will be able to anticipate and manage the state’s resources and hazards
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