Graduate Thesis Or Dissertation
 

Large Ensembles of Regional Climate Simulations over the western United States

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ng451m360

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  • Large ensembles of regional climate simulations were generated from the weather@home distributed volunteer computing project over the western US domain. Weather@home uses the U.K. Met Office Hadley Centre’s regional climate model HadRM3P (~0.22°) nested within the atmospheric global model HadAM3P (1.875 longitude° by 1.25 latitude°). Simulations from HadRM3P were evaluated against observational datasets, and were found to be able to faithfully reproduce various properties of the recent past climate of the western US. In an effort to explore local anthropogenically forced climate responses, especially precipitation response, over the complex terrain, western US climate was simulated for the recent past (1986-2014) and future (2031-2059). The large initial-condition ensemble of regional climate simulations provides detailed information of precipitation changes at local grid point level, with high signal-to-noise ratio, while results from the host global climate model are analyzed for synoptic-scale mechanisms driving the regional changes. Both winter and summer precipitation changes have a large dynamic origin. Winter precipitation changes are associated with a southeastward extension of the Aleutian low-pressure center and strengthening and eastward expansion of the upper subtropical jet stream. Summer precipitation changes are associated with a high-pressure anomaly centered over the northwest at the 500-hPa level, contributing to drying in the northwest, and the wetting in the southwest is associated with stronger increase in water vapor. The pattern of circulation change associated with changes in extreme (wet) monthly precipitation is similar to that for changes in mean monthly precipitation, but the changes are more intense. The changes of extreme precipitation vary spatially, with the least relative increase in Western Oregon-Washington, and the most relative increase in the Great Basin. Precipitation increase more on the leeward side of the Cascade Range than on the windward side, and this difference across mountain barrier is present in changes of both seasonal mean and extreme precipitation, suggesting common physical drivers. These large ensembles also present unique datasets to thoroughly assess the impact of internal variability on climate projections from a high-resolution regional model. We quantify the magnitude of changes forced by increasing greenhouse gas concentrations relative to internal variability and find that simulations spanning 10 to 30 years (the timespans used in the majority of published studies of regional climate modeling) often lead to unfounded confidence in results because of low grid-point level signal-to-noise ratios that furthermore vary considerably by season, variable and location. We offer a rule of thumb for determining the minimum adequate ensemble size N[subscript min] to detect a response to anthropogenic forcing in different climate variables, and a minimum adequate ensemble size N[subscript min]⁡(△ₓ) required for detecting spatial heterogeneity of such responses. Our results underscore the problem that increasing model resolution without increasing ensemble size does not necessarily advance the understanding of regional and local climate response to anthropogenic forcing. To summarize, the combination of high resolution over the complex terrain of western US and large numbers of simulations allow us to explore the effects of local and regional climate change in a way not otherwise possible.
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  • 2017-08-09 to 2019-03-23

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