Changes in Extreme IVT on the US West Coast in NA-CORDEX, and Relationship to Mountain and Inland Extreme Precipitation

Western US rainfall and snowpack vary greatly on interannual and decadal timescales; this combined with their importance to water resources makes future projections of these variables highly societally relevant. Previous studies have shown that precipitation events in the western US are influenced by the timing, positioning, and duration of extreme integrated water vapor transport (IVT) events (e.g., atmospheric rivers) at the coast, and also by the pathways which this moisture-rich air takes through the complex terrain of the western US. We investigate end-of-21st-century projections of western US precipitation and IVT in a collection of regional climate models (RCMs) forced by several global climate models (GCMs) from the North American Coordinated Regional Downscaling Experiment (NA-CORDEX).

Several of the NA-CORDEX RCMs project a decrease in cool season precipitation at high elevation (e.g., across the Sierra Nevada) with a corresponding increase in the Great Basin of the US. We explore the causes of this terrain-related precipitation change in a subset of the NA-CORDEX RCMs through an examination of IVT events. Projected changes in frequency of IVT events depend on their intensity. By the end of the century extreme IVT events increase in frequency whereas moderate IVT events decrease in frequency. Projected precipitation changes during IVT events also depend on event intensity. In the future, precipitation across the Sierra Nevada generally increases during extreme IVT events and decreases during moderate IVT events. Thus we argue that the mean cool season decrease at high elevation is largely determined by the response of moderate IVT events which are projected to be less frequent and bring less high elevation precipitation.

We also present preliminary results from NA-CORDEX-forced high-resolution Intermediate Complexity Atmospheric Research (ICAR) model simulations. These simulations attempt to explore the impact of some of the limitations of the NA-CORDEX modeling framework on precipitation projections, namely the somewhat coarse NA-CORDEX grid spacing and the extremely simple microphysical parameterizations used in those simulations.