Characterization of landfalling atmospheric rivers and their impacts over California

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Hecht, Chad William
Atmospheric Rivers (ARs) are responsible for a majority of global poleward moisture transport and can result in extreme precipitation events and flooding along the U.S. West Coast. ARs are long (>2000 km) and narrow (500-1000 km) corridors of enhanced vertically integrated water vapor (IWV) and integrated water vapor transport (IVT) that may be found within a variety of synoptic-scale flow patterns. Observational evidence suggests that ARs within different flow patterns may contain different water vapor source regions, different orientations, different IVT magnitudes, and may result in different precipitation distributions. This study uses a K-means clustering technique to objectively identify different flow patterns that contain landfalling ARs along the U.S. West Coast. The K-means clustering algorithm used NCEP-CFSR and NCEP-GFS-derived IVT to cluster the different types of ARs that may make landfall over north-central California. For example, the clustering technique identified five different types of ARs: northwesterly, westerly, south/southwesterly, or southwesterly with moderate IVT magnitudes >200 kg m-1 s-1 or strong southwesterly with IVT values >400 kg m-1 s-1. Composite analyses of the synoptic-scale features present in conjunction with each AR type highlight the variety of conditions that influence the orientation and magnitude of each landfalling AR. The differences in synoptic-scale flow regimes between the AR types results in differences in quasi-geostrophic forcing for ascent/descent co-located over the terminus of the ARs at landfall. This thesis will discuss and present the roles that both upslope IVT magnitude and quasi-geostrophic forcing for ascent play on precipitation accumulations and distributions associated with each AR type. The second portion of this thesis objectively quantifies the impacts associated with these precipitation accumulations and distributions produced by landfalling ARs using an AR impact scale. This impact scale considers precipitation, population, and drought conditions when calculating the impacts associated with individual ARs. Results illustrate how high precipitation accumulations don't necessarily result in a high-impact event. The high precipitation must occur over large populations that are drought stricken in order to result in a high-impact event. Analysis of the AR impact scale indicates that it is possible to objectively quantify the impacts associated with past, present, and future AR events, and introduces several avenues for future work.