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A numerical modeling study for estimating maximum precipitation sequence during the wet season and precipitation deficit conditions during the fire season in the Northwestern US

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This dissertation proposes a new numerical model-based methodology to estimate design precipitation sequence and Maximum Precipitation (MP) for long durations on the order of several months over a target region. The estimation of design precipitation for long durations is crucial especially for the regions where extreme floods are caused by long-term hydrological and meteorological processes, such as snow accumulation/melting or a series of extreme precipitation events, as they can all contribute to increasing volumes in reservoirs, leading to large water releases. The proposed approach using the Weather Research and Forecasting (WRF) Model successfully maximized the historical precipitation sequences for the 6-months wet season (i.e., winter season) over large drainage areas in the Columbia River Basin (MP being 961.0 mm for Bonneville Dam drainage area and 1101.7 mm for Libby Dam drainage area). As a result of maximizing precipitation of the identified 115 Atmospheric River (AR) storm events, the change in precipitable water as well as the integrated water vapor transport, was found to be not necessarily consistent with the change in precipitation depths over the basin, which questions the traditional assumption in Probable Maximum Precipitation estimations. Also, the atmospheric boundary condition shifting method was suggested to be an essential method in AR-induced precipitation maximization, rather than by solely increasing atmospheric moisture in model-based approaches, especially for long-duration MP estimates. In this dissertation, we also attempt to better understand the upper tropospheric flow and moisture transport control on fire season precipitation and its deficit, and to provide an insight on how to estimate precipitation deficit conditions during the wildfire season in the US Northern Rockies. Logistic regression analysis and composite analysis showed that the wetting rainy days, which exerted the strong control on wildfire area burned over the region during 1984-2018, may be well explained based on 500mb geopotential height (Z500) anomaly over the Pacific Northwest region and integrated water vapor transport (IVT) anomaly over the US Northern Rockies. In our numerical experiment using the WRF model, fire season precipitation over the US Northern Rockies and Z500 over the Pacific Northwest region were simulated well for the target events. Our numerical experiment suggested a possibility of estimating extreme wetting rain deficit conditions over the US Northern Rockies based on geospatially shifting atmospheric boundary conditions as precipitation depths and wetting rainy days decreased for the target events. Moreover, a numerical experiment increasing the sea surface temperature at the middle and southwest sectors of North Pacific Ocean resulted in changing the Z500 pattern, leading to the decrease in wetting rainy days over the Northern Rockies in a target year. Overall, this dissertation attempts to provide useful insights for estimating physically-based extreme hydrometeorological conditions in the northwestern US region toward regional and seasonal disaster risk assessments.

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This item is under embargo until December 9, 2028.