Browsing by Author "Harding, Keith John Iliff"

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    Examining the drivers of current and future changes in Central U.S. warm-season rainfall
    (2014-09) Harding, Keith John Iliff
    Warm-season precipitation in the Central U.S. is highly variable, as severe droughts and flooding often occur in consecutive years or simultaneously. Some of the most highly productive agricultural lands are present within the region despite susceptibility to warm-season rainfall extremes. Climate change is expected to increase precipitation extremes globally, but how warm-season Central U.S. precipitation will be affected is unclear. In this study, I examine the drivers of current and future warm-season precipitation in the region as well as how the basic characteristics of summer rainfall may be affected by climate change through the use of gridded observations, reanalysis datasets, and dynamical downscaling of global climate models (GCMs). It is demonstrated that the negative phase of the Pacific-North American (PNA) teleconnection pattern enhances heavy precipitation events over the Upper Midwest by modulating the strength of the Great Plains Low Level Jet (GPLLJ), possibly enabling greater medium range prediction of Midwest heavy rain events. Similarly, I aim to reduce uncertainty in long-term projections of how precipitation may be affected by climate change by examining shortfalls in GCM-simulated warm-season precipitation and demonstrating improvement with dynamical downscaling. Using the Weather Research and Forecasting (WRF) model, two GCMs are dynamically downscaled in one historical and three future timeslices with varying anthropogenic forcing. Future warm-season precipitation in these simulations is more intense, less frequent, and occurs with more days between rain events, similar to trends in observations that show large increases in extreme rainfall events and rainfall intensity. The intensification of extreme rainfall events in future simulations is the strongest during the April-July, associated with a strengthening of the GPLLJ during those months. Heavier rainfall rates during extreme precipitation events are related to a stronger cold pool and mesohigh, which force stronger moisture convergence above the cold pool in the presence of additional low-level moisture and a drier mid-troposphere. Overall, the identification of plausible physical mechanisms that might contribute to the enhancement of heavy rainfall events in the region enables greater confidence in future projections of extreme rainfall events.
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    Modeling the impact of iIrrigation on precipitation over the Great Plains.
    (2011-08) Harding, Keith John Iliff
    Since World War II, the rapid expansion of irrigation throughout the Great Plains has threatened the sustainability of the Ogallala Aquifer. Irrigation has been shown to modify the surface energy and water budgets over the Great Plains by altering the partitioning of latent and sensible heating. An increase in latent heating from irrigation contributes to a cooler and more humid surface, which has competing impacts on convection. In this study, the Weather Research and Forecasting model was modified to simulate the effects of irrigation at sub-grid scales. Nine April-October simulations were completed for different hydrologic conditions over the Great Plains. Data from these simulations was assimilated into a back-trajectory analysis to identify where evapotranspired moisture from irrigated fields predominantly falls out as precipitation. May through September precipitation increased on average over the Great Plains by 4.97 mm (0.91%), with the largest increases during wet years (6.14 mm; 0.98%) and the smallest increases during drought years (2.85 mm; 0.63%). Large precipitation increases occurred over irrigated areas during normal and wet years, with decreases during drought years. On average, only 15.8% of evapotranspired moisture from irrigated fields fell out as precipitation over the Great Plains, resulting in 5.11 mm of May-September irrigation-induced precipitation. The heaviest irrigation-induced precipitation occurred over north-central Nebraska, coincident with simulated and observed precipitation increases. While irrigation resulted in localized and region-wide increases in precipitation, large evapotranspiration increases suggest that irrigation contributes to a net loss of water in the Great Plains.