Theory and observations of bores in the nocturnal environment of the Great Plains
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Accurate representation of bores in forecast models is a challenge. This challenge is of considerable interest to both the operational and the research communities because bores are capable of producing vertical displacements of air sufficient for initiating convection. While the research presented here does not offer improvements to forecast models, it lays a foundation for future numerical studies focused on the life cycle of nocturnal thunderstorms and the development of forecast tools designed to predict the onset of bore-initiated convection. Although the current research may be relevant to similar nocturnal convection systems in other regions of the world, the scope of this research is limited to the Southern Great Plains of the United States, where the forecast skill for nocturnal thunderstorms is relatively poor. The datasets for the current study are predominantly from two field projects over the Great Plains. The first dataset is from the International H20 Project (IHOP_2002), and the dataset is used to characterize the origin of radar fine lines in reflectivity as density currents, bores or other nocturnal phenomena. Subsequently, the frequency of bores observed in IHOP_2002 data is compared with a statistical model applied to hydraulic theory. The longevity of the observed bores and their preferred direction is compared with environmental winds and wave ducting properties using linear wave theory. Next, using the Plains Elevated Convection At Night (PECAN) dataset, a method for forecasting the generation of a bore and subsequent bore-initiated convection is proposed and tested on a 3 June 2015 case study. Two techniques based on hydraulic and linear wave theory are used as part of the method to forecast the vertical displacement of parcels. The results indicate that density currents often generated bores in the nocturnal environment observed during IHOP_2002. This result is consistent with hydraulic theory which characterizes the interaction between a density current and the observed environment as partially blocked, leading to the generation of a bore. Of the parameters used to evaluate the flow regime, the inversion properties had the most influence over changes in the flow regime with time. Once a bore developed, the maintenance of a wave duct is diagnosed with a two-layer model based on the Scorer parameter. The curvature of the horizontal wind with height is a component of the Scorer parameter and the curvature associated with the nocturnal low level jet was found to be the primary mechanism for maintaining a wave duct. Convective instability parameters calculated from pre- and post-bore environment soundings are compared to gauge if a bore would initiate convection. The post bore soundings are generated with one of two techniques that mimic the parcel displacement through a bore. The technique based on hydraulic theory overestimates the displacement while the technique based on linear wave theory severely underestimates the displacement. These findings are part of a new line of investigation into the development of reliable tools to predict bore-initiated and bore-maintained convection.
- OU - Dissertations