Explaining the Nocturnal Maximum in Convection over the Great Plains Through Understanding the Interactions Between the Nocturnal Low-level Jet and Mesoscale Convective Systems

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Although thunderstorms can occur at any time of the day in response to weather disturbances, such as fronts and tropical cyclones, thunderstorm activity over most of the planet's land masses tends to most frequently occur in the late afternoon and early evening. The general timing of these thunderstorm is well understood as the daily heating of the earth's surface by the sun and the associated warming and moistening of the lower atmosphere provides the fuel for these storm systems. Remarkably, however, thunderstorms over the Great Plains during the summer do not follow this general tendency. Instead, the maximum in thunderstorm activity over the Great Plains takes place late in the night and into the early morning hours. This proposal seeks to understand why large thunderstorm complexes tend to occur at night over the Great Plains during the summer. The proposed effort focuses on determining whether the atmosphere over the Great Plains has relatively unique characteristics that lead to this nighttime maximum and on determining whether storm systems over this region are maintained through different processes than their late afternoon counterparts. A framework for understanding these nocturnal storms is important given that the numerical models utilized to predict weather have difficulty in representing these nocturnal storms. Accurate predictions of these events are important to society given that the nighttime thunderstorms can contain high winds, hail, and heavy rainfall. Specifically, this proposed effort seeks to explain the nocturnal maximum in organized thunderstorm activity over the Great Plains during the summer through advancing knowledge of the interaction between the spatial and temporal variations of the nocturnal low-level jet (NLLJ) and the dynamical processes maintaining nocturnal mesoscale convective systems (NMCSs). This proposal will utilize observations from the multi-agency PECAN (Plains Elevated Convection at Night) and the International H2O Project (IHOP_2002) field campaigns along with numerical simulations and theory. One aspect of this effort is to investigate the spatial and temporal variations in the thermodynamics and winds over this region within the context of the new framework for the NLLJ that shows that the jet is not simply a southerly wind maximum driven by an inertial oscillation, but a more complicated phenomenon with a westerly wind maximum above the peak southerly winds and the possibility of a slow, persistent ascent. This framework means that the NLLJ will impact convective instability through radiative processes controlling the formation of the stable nocturnal boundary layer and the heating and cooling on the sloping terrain of the region along with dynamical processes, such as local mixing, differential advection, and mesoscale ascent. A key component of the investigation will be to develop a framework that explains how the NLLJ impacts the vertical profiles of horizontal winds, Convective Available Potential Energy (CAPE), Convective Inhibition (CIN), and the Level of Free Convection (LFC). Another aspect of this effort is to focus on the role of bores and other gravity wave phenomena in initiating and maintaining convection in the NLLJ environment. Recent research has shown that bores and gravity waves are often inherent component of NMCSs over the Great Plains. The research team will investigate how the spatial and temporal variations in these vertical profiles impact the structure, movement, and evolution of NMCSs and how bores and gravity waves are generated in the NLLJ environment to impact generations of NMCSs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.