Diurnal Modulation of Moist Processes in Summertime Droughts and Floods over the Central United States

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This team of investigators will evaluate both the relative and combined effects of various processes contributing to summer rainfall and its diurnal variability in the central United States. Diurnal variation has important effects on the summer hydrologic cycle in the mid-latitudes with the nocturnal low-level jet and mesoscale convective systems playing significant roles. Three hypotheses will be examined: (1) The large-scale diurnal variation changes from year to year such that rainfall-inducing processes peak during the nighttime (daytime) in flood (drought) years. (2) During flood years, the contributing processes phase together to produce enhanced precipitation and a stronger nocturnal maximum, while in drought years the components are out of diurnal phase. (3) Large-scale forcing in flood years favors organization and upscale growth of convection, which shifts peak precipitation to late night/early morning. The principal investigators will employ both observational analysis and numerical modeling approaches to examine the hypotheses. The numerical studies will be used to isolate and filter perturbations in both the spatial and temporal domains so that their influences can be systematically examined in regional climate simulations. The study will cover a selection of normal, drought, and flood years in the Midwest. This research is supported as part of NSF's Water and Energy: Atmospheric, Vegetative, and Earth Interactions (WEAVE) effort.