Using regional climate models to examine how future high-elevation precipitation may decrease due to changes in storm frequency and intensity

Fog over the California coastline

The people, industries, and ecosystems of the Western U.S. depend on its precipitation, making understanding its projections under climate change scenarios essential. In the West, topography has a big influence on where and how much precipitation lands. Global climate models do not well-capture the diversity of the region’s complex terrain and, because of this, struggle with its precipitation projections as well. These factors motivate climate change examinations with tools like regional climate models that better-represent the terrain and its effect on precipitation processes.

In a new study to be published in Climate Dynamics, NOAA and CIRES researchers with the Physical Sciences Lab and collaborators at NCAR examine projected end-of-21st-century cool season precipitation changes in regional climate models (NA-CORDEX) over the western United States. Comparable to the projections from global climate models that drive their large-scale conditions, the NA-CORDEX models project increased precipitation across much of the western U.S. However, unlike the driving global climate models, more than half of the NA-CORDEX model runs project decreased cool season precipitation across the Sierra Nevada range. This study examines the reasons for these precipitation changes through the lens of changes in atmospheric river-like coastal integrated water vapor transport (IVT) events.

Rather than examining the change in all IVT-events, the paper separates the most extreme IVT-events from more moderate events. It turns out that the projected changes of these two classes of IVT-events differ. End-of-21st-century extreme IVT-events increase in frequency and their total western U.S. precipitation generally increases. In contrast, moderate IVT-events decrease in frequency, and their total precipitation decreases across higher elevations. Taken together, the precipitation changes from moderate and extreme IVT-events closely resemble those for the entire cool season, including the decreases over the mountains.

Although many studies have investigated climate projections for atmospheric rivers, few studies examine how they change across a range of intensities. This study suggests that the impact of climate change on these events may not be uniform for moderate versus extreme events, which has important implications for their precipitation impacts, such as droughts and floods.

Hughes, M. (PSL), D. Swales (PSL/CIRES), J. D. Scott (PSL/CIRES), M. A. Alexander (PSL), K. M. Mahoney (PSL), R. McCrary (NCAR), R. Cifelli (PSL) and M. Bukovsky (NCAR) (2022): Changes in extreme Integrated water Vapor Transport on the U.S. west coast in NA-CORDEX, and relationship to mountain and inland precipitation. Clim. Dyn.,

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