Kevin Reza Haghi-May 3

Contextualizing the presence of bores in the Southern Great Plains and proposing a method for forecasting bore-initiated convection

Accurate representation of bores in forecast models is a challenge.  This is of considerable interest to the research community because bores produce vertical displacements sufficient for initiating convection.  While the research presented here does not offer improvements to forecast models, it lays a foundation for either future numerical studies focused on the life cycle of nocturnal thunderstorms or the development of forecast tools designed to predict the onset of bore-initiated convection. The scope of this research is limited to the Southern Great Plains of the United States, where nocturnal thunderstorms are poorly forecast and the role of bores in convection needs to be addressed.

Part of this research uses the International H₂0 Project (IHOP_2002) dataset to classify the origin of radar convergence lines as density currents, bores or other nocturnal phenomena.  Subsequently, the frequency of bores in observations is compared with hydraulic and linear wave theory.  The longevity of observed bores and their preferred direction is related to favorable environmental winds and wave ducting properties.  Based on hydraulic and linear wave theory, a methodology for forecasting the generation of a bore and subsequent bore-initiated convection is proposed and tested on a 3 June, 2015 case study using the Plains Elevated Convection At Night (PECAN) dataset.  Given displacements based on theory, vertical thermodynamic profiles are constructed to forecast the impact a bore has on destabilizing the nocturnal environment.

The results discussed herein indicate that density currents often generated bores during IHOP_2002.  This result is consistent with hydraulic theory that characterizes the interaction between a density current and the observed environment as partially blocked.  Of the parameters used to evaluate the flow regime, the inversion properties were the determining factor in the development of a bore, and the curvature of the horizontal wind with height associated with the nocturnal low level jet determined the direction of the observed trapped wave mode within a wave duct adjacent to the ground.

Based on data from instrumentation readily accessible to forecasters, a prognostic methodology is developed to determine if a bore will form and the procedure is applied to an observed bore during PECAN.  Using two techniques based on the variation of hydraulic theory and linear wave theory, theoretical parcel displacements are calculated and then used to modify a sounding to demonstrate how the bore may destabilize the environment.  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.