The Challenge

California’s water infrastructure is set up for flood control, with conveying streamflow to the ocean as efficiently as possible as its primary aim. Capturing and using more stormwater is one key way that Los Angeles County can decrease its dependency on water supplies sourced from hundreds of miles away. In adapting the state’s infrastructure for stormwater capture, storage and use, it is essential to plan carefully for the precipitation extremes of the future, which will become more intense and frequent due to climate change. 

Atmospheric rivers—long corridors of water vapor traveling from the Pacific Ocean to California—are responsible for producing heavy precipitation and determining the state’s flood risk. Given this context, it is critical to understand how atmospheric river events will change in a warming world. In this project, researchers quantify projected changes in future precipitation driven by extreme atmospheric rivers in California by combining global climate model (GCM) with regional modeling.  

The Solution

Global Climate Models (GCMs) have the potential to successfully capture large-scale atmospheric dynamics associated with atmospheric rivers and offer invaluable insights regarding large-scale variability. However, they are not appropriate tools to quantify local impacts or capture fine-scale physical processes that drive extreme precipitation events. Thus, researchers implemented a modeling framework that combines GCM and regional modeling approaches. Leveraging the complementary strengths of a GCM and a high-resolution regional model, they examined changes in extreme atmospheric river events affecting California’s Sierra Nevada mountains under future warming scenarios. The Sierra Nevadas provide water for approximately 23 million Californians, from the Bay Area to Southern California. 

Results

• A substantial (10% to 40%) increase in total accumulated precipitation is observed across the southern, central and northern Sierra Nevada. Much larger relative increases in event total precipitation are observed in drier Sierra Nevada valleys and mountain lee-side areas, with increases greater than 80% in the Owens Valley. This larger relative increase may be due to weakened rain shadow effects in a warmer climate. 
• More spatially uniform increases in hourly maximum precipitation intensity are observed. Average increases in hourly maxima are 27% over the Sierra Nevada and 32% over the non-Sierra portion. This large increase is especially notable because short-duration bursts of intense precipitation pose a much greater risk of flash flooding and other hazards (e.g. debris flows, mudslides) than equally large accumulations over a longer period of time.  
• A majority of the simulated increase in precipitation associated with extreme atmospheric rivers stems from increases in atmospheric water vapor (up to 85%, when averaged over the Sierra Nevada watersheds). A smaller but still positive contribution (up to 15%) is from intensified larger-scale wind strength, especially across central and southern California. 

Next Steps

The magnitude of the above projected changes in atmospheric river-related extreme precipitation has substantial implications for future policy development, water and flood management practices in California.  The projected increase in event total precipitation implies increased runoff and inflow into California’s mainstem river systems while the projected warming during these events would most likely increase runoff potential even further. These findings motivate additional work in research – to explore potential consequences.  

Additional Outcomes to Date

Additional funding sources include the U.S. Department of Energy’s Office of Science, the UCLA Institute of the Environment and Sustainability, the Center for Climate and Weather Extremes at the National Center for Atmospheric research and The Nature Conservancy of California.  

Publications and Reports

Huang, X., Stevenson, S., & Hall, A. D. (2020). Future Warming and Intensification of Precipitation Extremes: A ‘Double Whammy’ Leading to Increasing Flood Risk in California. Geophysical Research Letters. doi: 10.1029/2020GL088679 

Huang, X., Swain, D. L., & Hall, A. D. (2020). Future Precipitation Increase from Very High-Resolution Ensemble Downscaling of Extreme Atmospheric River Storms in California. Science Advances, 6(29), eaba1323. doi:10.1126/sciadv.aba1323  

Huang, X., Swain, D. L., Walton, D. B., Stevenson, S., & Hall, A. D. (2020). Simulating and Evaluating Atmospheric River Induced Precipitation Extremes Along the U.S. Pacific Coast: Case studies from 1980‐2017. Journal of Geophysical Research Atmospheres, 125(4), n/a. doi:10.1029/2019jd031554