The purpose of this study was to evaluate the influence of horizontal moisture fluxes from Atmospheric Rivers (ARs) on extreme precipitation (EP) events in the continental United States (CONUS). Climatological results for both EP, objectively defined using a peaks-over-threshold and block maxima approach, and ARs were processed and analyzed for co-occurrence. EP analyses produced a positive linear trend in magnitude, determined through the block maxima approach, in the Central US and a positive linear trend in frequency, determined by the peaks-over-threshold approach, predominantly for the Northern half of the CONUS. AR results show over 70 AR days throughout the country, and a linear trend of 10 less days per decade in the Central US. Results of the co-occurrence analysis suggest an increasing trend of about one instance of co-occurrence per decade throughout much of the Eastern Coast, Midwest and Pacific Northwest, with a corresponding negative linear trend of about one instance of co-occurrence per decade for much of the Southwest US to Louisiana. Throughout the world, the study of EP, and the careful analysis of its behavior, and possible amplification sources such as ARs, at the national and regional scale is imperative to obtain a comprehensive understanding of hydrometeorological impacts.

Flood frequency analysis over the western United States is complicated by annual peak flow records that frequently contain annual flows generated from distinctly different flood generating mechanisms. Bulletin17B (B17B) and its update Bulletin 17C (B17C) recognized the difficulties in determining flood frequency estimates with streamflow records that contain a mixed population of flood generated peaks, and recommend developing separate frequency curves when the hydrometeorologic mechanisms that generated the annual peak flows can be separated into distinct populations. Yet challenges arise when trying to consistently quantify the physical process that generated the observed flows. This thesis examines the role played by different flood producing mechanisms in generating annual maximum floods throughout the western United States using process-driven mixed populations. First I evaluate the impacts of hydrometeorological processes on flood frequency in the western United States, with emphasis on the spatial and fractional contributions of atmospheric rivers (ARs) and eastern North Pacific tropical cyclones and their remnants (TC events) on annual maximum flows throughout this area. Six main areas in which flooding are impacted by ARs at varying degrees are found throughout the western United States. The Pacific Northwest and the northern California coast have the highest fraction of AR-generated peaks (~80–100%), while eastern Montana, Wyoming, Utah, Colorado, and New Mexico have nearly no impacts from ARs. The individual regions of the central Columbia River Basin in the Pacific Northwest, the Sierra Nevada, the central and southern California coast, and central Arizona all show a mixture of 30–70% AR-generated flood peaks. Analyses related to the largest flood peaks on record highlight the strong impact of ARs on flood hydrology in this region. Conversely, TC events play a limited role in controlling the upper tail of the flood peak distributions across the western United States. Southern California, Arizona, southernmost Nevada and Utah, southern and western New Mexico, central Colorado, and Texas have the highest fractional contributions of TC-event-generated annual maximums flows (~5-14%). I then build on these insights to develop a statistical framework to perform a process-driven flood frequency analysis using the AR/non-AR-generated annual peak flows identified at 43 long-term U.S. Geological Survey (USGS) streamgages in the western United States. I use a simulation framework to perform flood frequency analyses in terms of mixed distributions and quantify the corresponding uncertainties by accounting for mixed populations. Sites with notably different quantile estimates in the upper tail of the distribution between the single (homogeneous) and the weighted (heterogeneous) population methodologies are found when (i) potentially influential low floods (PILFS) are identified and/or (ii) when the composite distribution contains markedly different at-site log-unit skews (shape parameter) among the AR/non-AR subpopulations compared to the single homogeneous population.

Atmospheric rivers (ARs) are important contributors to cool season precipitation in the Southwestern US, and in some cases can lead to extreme hydrometeorological events in the region. We performed a climatological analysis and identified two predominant types of ARs that affect the central mountainous region in Arizona: Type 1 ARs originate in the tropics near Hawaii (central Pacific) and enhance their moisture in the midlatitudes, with maximum moisture transport over the ocean at low-levels of the troposphere. On the other hand, moisture in Type 2 ARs has a more direct tropical origin and meridional orientation with maximum moisture transfer at mid-levels. We then analyze future projections of Southwest ARs in a suite of global and regional climate models used in the North American Regional Climate Change Assessment Program (NARCCAP), to evaluate projected future changes in the frequency and intensity of ARs under warmer global climate conditions. We find a consistent and clear intensification of the water vapor transport associated with the ARs that impinge upon Arizona and adjacent regions, however, the response of AR-related precipitation intensity to increased moisture flux and column-integrated water vapor is weak and no robust variations are projected either by the global or the regional NARCCAP models. To evaluate the effect of horizontal resolution and improve our physical understanding of these results, we numerically simulated a historical AR event using the Weather Research and Forecasting (WRF) model at a 3-km resolution. We then performed a pseudo-global warming experiment by modifying the lateral and lower boundary conditions to reflect possible changes in future ARs (as projected by the ensemble of global model simulations used for NARCCAP). Interestingly we find that despite higher specific humidity, some regions still receive less rainfall in the warming climate experiments - partially due to changes in thermodynamics, but primarily due to AR dynamics. Therefore, we conclude from this analysis that overall future increase in atmospheric temperature and water content as projected by global climate models will not necessarily translate into generalized heavier AR-related precipitation in the Southwestern US.