An Examination of an Inland-Penetrating Atmospheric River Flood Event under Potential Future Thermodynamic Conditions

Abstract

Atmospheric rivers (ARs) are well-known producers of precipitation along the U.S. West Coast. Depending on their intensity, orientation, and location of landfall, some ARs penetrate inland and cause heavy rainfall and flooding hundreds of miles from the coast. Climate change is projected to potentially alter a variety of AR characteristics and impacts. This study examines potential future changes in moisture transport and precipitation intensity, type, and distribution for a high-impact landfalling AR event in the U.S. Pacific Northwest using an ensemble of high-resolution numerical simulations produced under projected future thermodynamic changes. Results indicate increased total precipitation in all future simulations, although there is considerable model spread in both domain-averaged and localized inland precipitation totals. Notable precipitation enhancements across inland locations such as Idaho’s Sawtooth Mountain Range are present in four out of six future simulations. The most marked inland precipitation increases are shown to occur by way of stronger and deeper moisture transport that more effectively crosses Oregon’s Coastal and Cascade mountain ranges, essentially “spilling over” into the Snake River Valley and fueling orographic precipitation in the Sawtooth Mountains. Moisture transport enhancements are shown to have both thermodynamic and dynamic contributions, with both enhanced absolute environmental moisture and localized lower- and midlevel dynamics contributing to amplified inland moisture penetration. Precipitation that fell as snow in the present-day simulation becomes rain in the future simulations for many mid- and high-elevation locations, suggesting potential for enhanced flood risk for these regions in future climate instances of similar events.

Responses and impacts of atmospheric rivers to climate change

“…Theory further suggests that an increase in watervapour transport above crest level, as might be expected given higher atmospheric-moisture content and a deepening troposphere, will increase the frequency with which ARs penetrate inland across significant topographic barriers 100 . Consequently, inland regions such as the western USA are anticipated to have a greater availability of atmospheric moisture.…”

Future precipitation increase from very high resolution ensemble downscaling of extreme atmospheric river storms in California

“…Forcing WRF with CESM-LENS boundary conditions (henceforth, "LENS-WRF"), therefore, joins the relative strengths of each modeling tool. We emphasize that these methods differ substantially from the "pseudo global warming" approach used in many other downscaling studies (47), which applies the same mean GCM climate change signals to all individual meteorological events, irrespective of possible differences in those signals from mean-state changes during extreme events.…”

North American extreme precipitation events and related large-scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

“…Using convection permitting simulations at 4 km resolution over the CONUS, Prein et al (2017) found that MCS frequency more than triples in North America, with a 15-40% increase in maximum precipitation rates and larger storm size resulting in up to 80% increases in total MCS precipitation volume within a 40 km radius around the storm center. Convection-permitting simulations have also been used to explore how individual AR events might change in a warmer climate, with the resolution important to resolving both the meteorology and the interaction with terrain (Dominguez et al 2018;Mahoney et al 2018). These high resolution studies have demonstrated the significant value provided by convection permitting modeling in providing insights about extreme precipitation changes and flood risk in the future.…”