End-of-Century Changes in Orographic Precipitation with the Intermediate Complexity Atmospheric Research Model over the Western United States

Journal Article · Thu May 01 00:00:00 EDT 2025 · Journal of Hydrometeorology

DOI:https://doi.org/10.1175/JHM-D-24-0071.1· OSTI ID:2565985

Currier, William Ryan ^[1]; McCrary, Rachel ^[2]; Abel, Mimi Rose ^[1]; Eidhammer, Trude ^[2]; Kruyt, Bert ^[2]; Smith, Abigail ^[2]; Enzminger, Thomas ^[2]; Mahoney, Kelly ^[1]; Cifelli, Rob ^[1]; Gutmann, Ethan D. ^[2]

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1. NOAA/Physical Sciences Laboratory, Boulder, Colorado
2. National Science Foundation National Center for Atmospheric Research, Research Applications Laboratory, Boulder, Colorado

Abstract

Downscaled precipitation projections were created using the Intermediate Complexity Atmospheric Research (ICAR) model over the western United States to increase the physical realism in orographic precipitation changes. End-of-century simulations from eight models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) were downscaled with ICAR and compared to the widely utilized statistically downscaled dataset, localized constructed analogs (LOCAs), to understand where and why projections of cool-season (September–May) precipitation differed. ICAR and LOCA precipitation projections were similar, but their sign differed in hydrologically relevant regions likely due to ICAR’s simulation of microphysics and mesoscale dynamics with high-resolution topography (6 km). In the Pacific Northwest, cool-season precipitation projections from ICAR showed an increase on the windward side of the Cascades and no significant change within the lee. This difference between the windward and leeward side was attributed to reduced zonal wind speeds, allowing more time for microphysical processes within ICAR. This contrast is enhanced by rain’s faster fall speed compared to snow, limiting transport into the lee. Meanwhile, LOCA projected an increase in precipitation across the Cascades. In the Upper Colorado River basin, LOCA projected an increase in precipitation in high elevation regions (>3000 m), but ICAR projected no significant change or a decrease in precipitation. High elevation differences were most evident in the spring and fall and were also attributed to a snow-to-rain transition and dynamical processes that impacted orographic enhancement within ICAR. Idealized, controlled studies are needed to better isolate individual processes, but these results underscore the importance of including microphysics and mesoscale dynamics within regional-scale precipitation projections.

Significance Statement

A set of global climate model simulations was downscaled using an atmospheric model that contains key physical equations, referred to as Intermediate Complexity Atmospheric Research (ICAR). ICAR was used to examine projected changes in end-of-century cool-season precipitation over mountains in the western United States. Precipitation projections from ICAR were similar to projections that used statistical relationships to downscale climate projections. However, projections differed between ICAR and statistically downscaled datasets in whether they increased, decreased, or stayed the same in specific, hydrologically relevant regions such as the eastern Cascades and high elevation areas of the Upper Colorado River basin. These differences were attributed to the simulation of physical processes in ICAR. The results highlight the importance of kilometer-scale atmospheric processes in regional climate projections.

• Sponsoring Organization: USDOE
• OSTI ID: 2565985
• Journal Information: Journal of Hydrometeorology, Journal Name: Journal of Hydrometeorology Journal Issue: 5 Vol. 26; ISSN 1525-755X
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English