29 January 2018

The Changing Character of Twenty-First-Century Precipitation over the Western United States in the Variable-Resolution CESM

Differences of precipitation indices between end-century, mid-century and historical.

Using the variable-resolution CESM, projections of the future precipitation climatology in the western US through the end-of-century are developed and explained using the large-scale meteorological fields.  Extreme precipitation is observed to increase throughout the US West, particularly in the Pacific Northwest; however, increases are modulated by limited soil water content.


This study opens the door for future climate assessments to use the variable-resolution CESM to achieve high resolution over regional domains, particularly in regions where rapid topographic variation is important to determining the local climatology.  Our work also provides high-resolution predictions of future precipitation climatology through the end-of-century that are valuable to water managers and stakeholders, who depend on climate data for adaptation planning.


It is well known that the impact of a changing climate on the character of precipitation is complicated, and changes in mean precipitation may not be informative to understanding changing extremes. Given so many climate models focus on global scales, this paper adds value to the literature on changing precipitation by addressing changes to regional scale features by the end of 21st-century over the western U.S. In order to accommodate the complex topography of the U.S. West and its various local climate zones, a series of long-term simulations with relatively high local grid resolution are produced covering different phases from the historical period through the end of 21st-century. An up-to-date climate model (CESM) with variable-resolution is employed which combines the benefits of two-way scale interactions and refined numerical downscaling.

According to the simulation results, over the Pacific Northwest, extreme precipitation events are observed to increase significantly because of increased cool-season integrated vapor transport associated with a moistening of the cool seasons and drying through the warm seasons. Extreme precipitation in this region appears to increase more rapidly than would be predicted by the Clausius-Clapeyron relationship. Over California, precipitation climatology is mainly attributed with large interannual variabilities that are tied closely to ENSO patterns.

Paul Ullrich
University of California
Ullrich, P.  2017.  "The Changing Character of Twenty-First-Century Precipitation over the Western United States in the Variable-Resolution CESM."  Journal of Climate 30(18): 7555-7575, doi:10.1175/jcli-d-16-0673.1.