Characteristics of extreme precipitation in southern California and applications to post-fire debris flows, shallow landslides, and water resources
AuthorOakley, Nina S.
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Shallow landslides and post-fire debris flows in southern California can result in loss of life and millions of dollars in damage to property and infrastructure. Persistent drought conditions reduce southern California’s local water resources and have severe socioeconomic impacts. Shallow landslides, post-fire debris flows, and water resources operate on distinct timescales, but in this region share the common theme of extreme precipitation as a driver. I use in-situ observations and atmospheric reanalysis datasets to address the question: What are the characteristics of extreme precipitation associated with mass movements and impacts to water resources in southern California? I explore the spatial and temporal variability of precipitation extremes as well as characterize atmospheric conditions associated with these events. I find that approximately 70% of post-fire debris flow events assessed in the Transverse Ranges of Southern California are associated with atmospheric rivers. Narrow cold frontal rain bands (and other squall lines) are also commonly associated with post-fire debris flow events. I examine 147 hourly precipitation gauges throughout California and find precipitation intensities that have historically triggered shallow landslides (OTPE; over threshold precipitation events) are most frequently observed in south-facing terrain in the Coast Ranges and Transverse Ranges, as well as in the northern Sierra Nevada. Depending on location, 60-90% of OTPE are associated with atmospheric rivers and multiple OTPE may occur within a storm event. Last, I show that the difference between a wet and dry year in the Santa Ynez River Basin is typically two to three >90th percentile precipitation events. While there are often more >90th percentile events in El Niño years, there is considerable variability in the record analyzed. Synoptic to mesoscale conditions producing precipitation extremes are also described in this work. These results provide quantitative documentation of previously qualitative observations such that they can serve as a building block for future research, e.g., how these processes might be influenced by a changing climate. Additionally, my results improve situational awareness of the hazards addressed for weather forecasters, emergency managers, and water resource managers, as well as inform natural hazards-related communication and outreach efforts.