A Numerical Investigation of Convection Initiation by Instabilities along Outflow Boundaries

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Three-dimensional aspects of storm outflow play an important role in the initiation of convective storms. In many forecast scenarios, the primary question is not whether deep convection will develop along a given boundary but where and when along this boundary will storms be favored. Preliminary and related modeling studies by the investigators as well as recent observations suggest a link between anomalies/instabilities along outflows and other convergence boundaries and convection initiation. A three-dimensional convective cloud model will be employed to improve understanding of the processes that prompt deep convection initiation at specific locations along outflow and other convergence boundaries. As part of this study, the detailed three-dimensional structure of outflow boundaries and related moist convective forcing will be investigated in environments favoring specific instabilities along the outflow leading edge. Observations will be used where possible to validate the results from the numerical simulations. The underlying objective of the project is to improve the prediction of initial storm location by applying the knowledge obtained from this research. Specific primary objectives of the project include:1. Investigating the role misocyclone circulations along outflow boundaries with marked horizontal shear (i.e., a vertical vortex sheet) play in moist convection initiation;2. Examining the moist convective forcing and basic structure and morphology of lobe and cleft instability along outflow boundaries. This second objective will include a comprehensive examination of the fine-scale three-dimensional structure of outflow boundaries in environments without marked leading edge horizontal shear (in contrast to item 1).These objectives involve critical elements for furthering our understanding of convection initiation along outflow boundaries. Further, these objectives are consistent with a key goal of the U.S. Weather Research Program involving the refinement of precipitation forecasts with improved knowledge of boundary layer processes.