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Browsing by Author "Rasmussen, Kristen, advisor"

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    Changes in the snowpack of the Upper Colorado River basin in a warmer future climate
    (Colorado State University. Libraries, 2023) Sherman, Erin Alexys, author; Rasmussen, Kristen, advisor; Schumacher, Russ, committee member; Fassnacht, Steven, committee member
    Water is a crucial factor to sustaining life on Earth. Snow acts as a reservoir for water, providing storage during the cold seasons and freshwater resources throughout the warmer months. Streamflow in the upper Colorado River Basin is primarily contributed by seasonal mountain snowmelt that provides critical freshwater resources to humans and wildlife, effectively connecting ecological, hydrological, and atmospheric systems. Global Climate Models (GCMs) and regional climate models do not represent the complex processes that can impact snowpack growth, evolution, and melting, thus they often rely on parameterizations to represent such processes. SnowModel is a high-resolution snowpack-evolution modeling system that can simulate processes such as blowing snow redistribution and sublimation, forest canopy interception, and snow-density evolution. To investigate how snowpack in the Upper Colorado Basin may change in a future warmer climate, high-resolution convection-permitting regional climate atmospheric model simulations at 4-km horizontal grid spacing are used to provide input conditions to drive SnowModel at 100-m in the current and future climate for 13 years. Results show that the average snow season will be shorter in the future, reducing the days that the snowpack can accumulate. In addition, analysis of the characteristics of precipitation in the simulations shows a ~150% increase in convective precipitation frequencies in the winter months, indicating shifts in the character of precipitation in a future climate. Liquid precipitation in winter increases ~200% in a future climate as a result of warmer air temperatures. In contrast, solid precipitation stays roughly the same in the winter, but decreases about 25 percent in the fall and spring. A case study analysis of the high-impact snowstorm on 17-19 March 2003 that delivered between 30-70 inches of snow along the Colorado Front Range in a current and future climate shows a shift from a snow-dominant to a rain-dominant event, as well as increases in moisture and convective precipitation frequencies. The simulated changes in the snowpack of the Upper Colorado River Basin will likely have detrimental impacts on freshwater resources and food production in a future climate that will undoubtedly impact a multitude of humans and ecosystems in the western United States.
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    Characteristics of current and future flood-producing storms in the continental United States
    (Colorado State University. Libraries, 2020) Dougherty, Erin M., author; Rasmussen, Kristen, advisor; Schumacher, Russ, committee member; Maloney, Eric, committee member; Morrison, Ryan, committee member
    Understanding the changes to extremes in the hydrologic cycle in a future, warmer climate is important for better managing water resources and preventing detrimental impacts to society. The goal of this dissertation is to contribute to this understanding by examining the precipitation characteristics of flood-producing storms in the current climate over the continental United States (CONUS) and how these will change in a future, warmer climate. Numerous storm types are responsible for floods over the CONUS, so quantifying how their characteristics will change among a large number of flood-producing storms in the future provides a spectrum of possible changes and impacts to flood-prone regions across the country. To understand flood-producing storms in the current climate over the CONUS, a climatology of these storms from 2002–2013 is created by merging storm reports, streamflow-indicated floods, and Stage-IV precipitation data (Chapter 2). From this climatology, it is observed that flash flood-producing storms preferentially occur in the warm-season in the Mississippi River Basin, with intense rain rates and short durations. Slow-rise floods occur mostly during the cool-season, concentrated in the Ohio River Valley and Pacific Northwest, and are long-duration, low-intensity rainfall events. Hybrid floods, having characteristics of both flash and slow-rise flood-producing storms, tend to occur in the spring and summer notably in the central CONUS and Northeast, with moderate durations and rain rates. Examining these floods on a sub-basin scale in the Wabash and Willamette basins, precipitation and instantaneous streamflow correlations are spatially variable, with strong positive correlations in areas of complex terrain and urbanization (Chapter 3). These studies show that in the current climate, flood-producing storm precipitation characteristics and their hydrologic response is nuanced, which is critical to document in order to understand their behavior in a future climate. A subset of nearly 600 flash flood-producing storms from the Chapter 2 climatology are examined using high-resolution convection-permitting simulations over the CONUS to understand how these historical storms might change in a future, warmer climate (Chapter 4). Both precipitation and runoff show widespread increases in the future over the CONUS, increasing by 21% and 50%, respectively, with maximum hourly rain rates becoming more intense by 7.5% K−1. In California, 45 flood-producing storms associated with atmospheric rivers also display a future increase (decrease) in precipitation (snow water equivalent) leading to increased runoff, particularly over the Sierra Nevada Mountains, implying a shift in future water resources in California (Chapter 5). In the Mississippi River Basin–a flash flood hotspot in the CONUS––nearly 500 flash flood-producing storms exhibit a 17% average increase in precipitation and 32% average increase in runoff primarily associated with warm-season convection, and to a lesser extent, tropical cyclones (Chapter 6). When stratified by vertical velocity, the storms with the strongest vertical velocity in the current climate exhibit the greatest (least) increase (decrease) in future rainfall (vertical velocity), suggesting a potential role of storm dynamics in modulating future rainfall changes.
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    Investigating the impact of forced and internal climate variability on future convective storm environments in subtropical South America: a large ensemble approach
    (Colorado State University. Libraries, 2023) Chakraborty, Anindita, author; Rasmussen, Kristen, advisor; Hurrell, James, advisor; Anderson, Brooke, committee member
    Subtropical South America (SSA) has some of the most intense deep convection in the world. Large hail and frequent lightning are just two of the hazards that profoundly affect people, agriculture, and infrastructure in this region. Therefore, it is important to understand the future convective storm environments over SSA associated with climate change and how these large-scale environmental changes are likely to change high-impact weather events in the future. Previous studies have used convection-permitting regional models and radar data to examine convective storm environments in the current climate across different regions of South America. Here, we use a large ensemble of Earth system model simulations to quantify anthropogenically-driven future changes in large-scale convective environments, as well as how those forced changes might be modified by unforced, internal climate variability. Specifically, we examine changes in different thermodynamic parameters of relevance to severe weather events over SSA in austral spring and summer (September-February). We use daily data from a 50-member ensemble from 1870-2100 performed with version two of the Community Earth System Model (CESM2). Results indicate that no forced changes in convective environments are evident until very late in the 20th century. However, increases in convective available potential energy and atmospheric stability, as well as an increase in lower tropospheric vertical wind shear, became apparent around 1990, and these trends are projected to continue throughout the rest of this century. The implication is that future large-scale environments may be favorable for less frequent, but perhaps more intense and severe convective modes and their associated hazards. Results also demonstrate that anthropogenic changes are likely to be significantly modified, regionally, by internal climate variability.