Understanding the origin of atmospheric rivers in the Southwest US

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Episode recorded November 3, 2022
Episode released on August 24, 2023

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Mike Dettinger is a Visiting Research Hydrologist at Scripps Institution of Oceanography in the Center for Western Weather and Water Extremes. 

Highlights | Transcript

• Western US in extreme drought, mostly since 2000 through most of 2022. Warmer temperatures and thirstier atmosphere make recent droughts much more severe than earlier droughts in the 1980s and 1990s, even though precipitation deficits are similar. 
• California drought extended from 2012 – 2016. 2016 had normal precipitation but insufficient to end the drought. Drought ended essentially over a weekend in January 2017 with extreme precipitation, eventually resulting in flooding, erosion of the Oroville Dam spillway, and evacuation of 180,000 people downstream. 
• Three discoveries in late 1990s resulted in atmospheric rivers (ARs) being identified:
  1. Analysis of weather model-based forecasts globally by Zhu and Newell (1998) showed narrow bands of intense water vapor transport, termed ARs.
  2. Global satellite coverage of integrated water vapor by satellite microwave sensors (Special Sensor Microwave/Imager, SSM/I) at daily time scales and 25 km grid scale allowed mapping of ARs continually and globally.
  3. NOAA used scientific aircraft to assess storm conditions over the North Pacific in winter 1998 when a very wet West Coast winter was predicted due to a developing strong El Nino. These aircraft were like hurricane hunters but in this case found that Pacific ARs are the cause of wettest storms. They now operationally fly over ARs and drop measurement devices down through them. They identified long narrow and moist low level jets like fire hoses at about 1.5 km above sea surface (Ralph et al., 2004).
• Atmospheric rivers account for ~90% of the water vapor transport from the tropics to the poles while only occupying ~10% of the area, like big fire hoses in the sky, ~ 6 – 8 present at all times in winter and ~ 4 in summer globally, progressing from W to E around the globe. Meandering rivers in the sky ~1000 km wide with lengths > 2 times the width. 
• ARs strongly linked to 70 – 80% of floods along West Coast from Canada to San Francisco. 
• Average AR transports more than 10 times the flow at mouth of Mississippi and strong AR closer to 20 times flow. 
• ARs hit the mountains in the West Coast and are lifted up with orographic effect, condensing and resulting in large rainfall or snowfall. 
• In the past, some ARs on West Coast were known from major “Pineapple Express” storms that extended from Hawaii, just one type of AR. 
• ARs account for 30% to 50% of total precipitation along the West Coast, despite occurring there only about one to three’s worth of days per year. 
• ARs end many droughts as described in Dettinger paper “Atmospheric Rivers as Drought Busters on the US West Coast“, 30 – 50% of droughts in California up to 70 -80% of droughts in Pacific Northwest. All major droughts in California ended with strong ARs, 1976-1977; 1987 – 1992, etc. 
• Climate extremes (droughts and floods) are challenging for water resource managers.
• For example, the Yuba and Feather river basins, looking at data beginning in 1980s, indicate that when strong ARs were forecasted 3 days ahead 86% of time they showed up, 5 days ahead 80% of time they happened, 7 days out about even odds, 50% of time floods happened. 
• Forecast skill of AR driven floods sufficient to allow reservoir operators to base their releases on forecasts.
• For example, Russian River: Working with US Army Corps of Engineers and Sonoma Water Agency. The Corps is already (now) operating the Lake Mendocino reservoir based on modern forecasts, with study results indicating that these operations can allow about 20% more water to be salvaged from ARs than did strict adherence to older non-forecast informed curve rules. 
• Forecast Informed Reservoir Operations (FIRO) allows them to retain flood water if they don’t forecast any ARs forming over the Pacific, takes 4 – 7 days for ARs to show up, so have long lead forecasts of “safe” times. 
• Summary: identification of ARs in 1998 from modeling, satellites, and aircraft measurements. Understanding their pdominant role in flooding and drought busting on the West Coast. Forecast skill parlays to managing reservoirs better using FIRO and storing water for longer with 7-day AR forecasts.