Atmospheric rivers (ARs) are long, narrow plumes of concentrated water vapor that are a critical factor in the transport of moisture from oceans to continents in the mid-latitudes. Much of the existing research on the impact of ARs on the United States focuses on the Pacific Coastal states and their importance as contributors to precipitation, their impact on water resources, and their role as flood producers. The objective of this study is to determine the importance of Pacific Ocean ARs penetrating further inland and affecting flooding in the state of Arizona. The following questions were addressed: (1) Are certain regions in Arizona more susceptible to AR-related flooding? (2) Do ARs produce flooding events of greater magnitude in Arizona than floods produced by other mechanisms? (3) Are there identifiable variables or conditions that influence the frequency, magnitude, and location of AR-related flooding in the state? Based on a study of selected watersheds throughout Arizona, results showed that the most active region of AR-related flooding in Arizona is associated with the abrupt increase of elevation along the Mogollon Rim of the state's Central Highlands Transition Zone physiographic region. The percentage of AR-related flooding events in this region can reach over 50% for some watersheds, such as the Verde and the Salt. The influence of ARs on flooding is weaker to the north, in the Colorado Plateau region, and to the south, where summer convective storm activity in southeastern Arizona's Basin and Range physiographic region is a more common flood producer, and where the most extreme floods are associated with late-summer tropical cyclones. When ARs did affect northern or southern Arizona, they did not have the same degree of influence on flood magnitude and frequency as in the Mogollon Rim/Central Highlands watersheds, which implies that watersheds in the Mogollon Rim/Central Highlands have characteristics that are favorable for AR-related flooding. Lastly, in addition to the importance of the Central Highlands' orography on AR flooding, another finding of this study points to the importance of the trajectory of the inland-penetrating AR as a factor. The corridor along which the AR enters the region can strongly affect which ARs will produce floods and which ARs will not, with a south/southwesterly trajectory across Baja California producing the largest percentage of AR floods in Arizona in this study.

Data from satellites and reanalysis products are analyzed to study the behavior of wave trains in the three major Southern Hemisphere Convergences zones: the South Pacific, the South Atlantic, and the South Indian. Using composites on high rain-rate days, a wave pattern is identified that is characteristic of high rain events. This wave pattern is then compared to the patterns of variability of brightness temperature using empirical orthogonal functions. A linear regression technique is used to examine the behavior of potential vorticity corresponding to the patterns of maximum variance. Planetary-scale waves, propagating in favorable regions, slow and break, dragging streamers of moisture from the tropics into higher latitudes. These streamers, combined with lifting, lead to the enhanced rain seen in the Southern Hemisphere’s convergence zones. It is concluded that the convergence zones are areas of enhanced streamer activity and that a more thorough study of streamers will yield more information on the structure and behavior of the convergence zones.

This study addresses the susceptibility of atmospheric rivers (ARs) to the behavior of the North Atlantic Subtropical High (NASH). ARs are a major mechanism for meridional moisture transport often connected to heavy precipitation and mid-latitude troughs. The NASH, a semi-permanent anticyclone over the subtropical North Atlantic Ocean, has been shown to be significantly influential on precipitation variability over the southeastern United States. A self-organizing map (SOM) was trained on a 4 x 3 regular grid over 250 iterations using ERA5 derived 6-hourly 850 hPa Geopotential Heights ≥ 1535 gpm from 1979-2020. The 12 resulting "nodes" were analyzed with respect to ARs defined by objects of ERA5 derived integrated water vapor transport (IVT) > 500 m-1 s-1 with lengths > 2000 km. Composites of thresholded 850 hPa heights, AR-concurrent PRISM precipitation, AR spatial frequency distribution maps, and seasonal AR frequency histograms per node illustrate seasonal interactions between the NASH and ARs that demonstrate a tendency of more frequent ARs and higher mean AR-driven precipitation over the Mississippi embayment and Ohio River Valley in the summer months, believed to be representative of extreme moisture transport events, when the NASH exhibits increased intensity, spatial expansion, and southwestward migration. Conversely, AR frequency and AR-concurrent precipitation composites suggest wintertime events are mainly supported by dynamically-driven nor'easter and bomb type cyclones when the NASH is constricted, at higher latitudes, and further east. Findings suggest that extreme summertime water vapor transport events associated with an AR are enhanced by the warm season NASH due to its increased intensity and proximity to the eastern US that acts as a supplementary lifting mechanism amidst low dynamic influence. / Master of Science / This study aims to investigate the response of atmospheric rivers (ARs) to the behavior of the North Atlantic Subtropical High (NASH). ARs are a major vehicle for the poleward transport of moisture from the tropics and subtropics. ARs are often affiliated with heavy precipitation and mid-latitude cyclones and frontal boundaries. The NASH, a semi-permanent anticyclone over the subtropical North Atlantic Ocean, has been shown to be significantly influential on precipitation variability over the southeastern United States. A self-organizing map (SOM), a method of vector quantification, was trained on a 4 x 3 regular grid over 250 iterations using ERA5 derived 6-hourly 850 hPa Geopotential Heights ≥ 1535 meters from 1979-2020. The 12 resulting "nodes" were analyzed with respect to ARs defined by objects that result from masking the rate of transport of water vapor within a vertical column from 1000 hPa to 300 hPa of which that are greater than 2000 km long. Composites of thresholded 850 hPa heights, AR-concurrent precipitation, AR spatial frequency distribution maps, and seasonal AR frequency histograms per node illustrate seasonal interactions between the NASH and ARs that demonstrate a tendency of more frequent ARs and higher mean AR-driven precipitation over the Mississippi embayment and Ohio River Valley in the summer months, believed to be representative of severe precipitation events, when the NASH is stronger, larger, and further southwestward. Conversely, AR frequency and AR-concurrent precipitation composites suggest wintertime events are mainly supported by nor'easter and bomb type cyclones that occur when the Polar jet stream is strongest and when the NASH is constricted, at higher latitudes, and further east. Findings suggest that extreme summertime water vapor transport events associated with an AR are enhanced by the warm season NASH due to its increased intensity and proximity to the eastern US that acts as a supplementary lifting mechanism amidst low dynamic influence.

Large-scale atmospheric disturbances play important roles in determining the general circulation of the atmosphere during the North Pacific boreal winter. A number of scientific questions have been raised due to these disturbances’ spatial and temporal complexity as well as the hydrological implication associated with them. In this dissertation, the principal goal is to further improve our understanding of the atmospheric high frequency (HF) and intermediate frequency (IF) disturbances active over the North Pacific. The study focuses on their energetics, intraseasonal and interannual variability, and the resulting hydrological impact over the eastern North Pacific and Western U.S. including extreme events. To delineate the characteristics of HF and IF disturbances in the troposphere, we first derive a new set of equations governing the local eddy kinetic energy (EKE), and assess the critical processes maintaining local budgets of the HF and IF EKE. The diagnosis assesses the 3-D patterns of energy flux convergence (EFC), barotropic conversion (BT), baroclinic conversion (BC), and cross-frequency eddy-eddy interaction (CFEI). The local EKE budget analysis is followed by an investigation of the modulation of HF and IF eddy activity by different modes of low frequency climate variability. On interannual timescales, the response of various local energetic processes to El Niño-Southern Oscillation (ENSO) determines the HF and IF EKE anomalies and the role of CFEI process is important in producing these anomalies. Also on interannual timescales, winter precipitation deficits associated with suppressed cyclonic activity, i.e., negative HF EKE anomalies, are linked to severe droughts over the U.S. Southern Great Plain (SGP) region. The suppressed cyclonic activity is, in turn, tied to phase changes in the West Pacific (WP) teleconnection pattern. On intraseasonal timescales, variations in HF disturbances (a.k.a. storm tracks) over the North Pacific are closely coupled with tropical convection anomalies induced by the Madden-Julian Oscillation (MJO), and partly drive larger scale intraseasonal flow anomalies in this region through eddy-eddy interactions. Anomalous HF eddy activity induces subseasonal transitions between “wet” and “dry” regimes over the west coast of North America. Also on intraseasonal timescales, the East Asian cold surge (EACS) is found to provide a remote forcing of the winter precipitation anomalies in the western U.S. This modulation is achieved through “atmospheric rivers” (ARs), which are narrow channels of concentrated moisture transport in the atmosphere and are responsible for over 70% of the extreme precipitation events in the western U.S.. EACS effectively modulates the IF disturbance activity over the North Pacific, and the anomalous IF disturbances lead to the formation of an AR over the eastern North Pacific that ultimately induces precipitation anomalies in the western U.S. Analyses of the simulations from the NCAR Community Climate System Model version 4 (CCSM4) demonstrate that the connections among the EACS, AR and western U.S. precipitation are better captured by a model with higher spatial resolutions. The improved simulation of these connections is achieved mainly through a better representation of the IF disturbances, and the associated scale-interaction processes in the higher resolution model.