Idealized Simulations of Supercell Thunderstorms Traversing the Appalachian Mountains

Abstract Substantial short-term forecasting challenges arise when supercell thunderstorms traverse through regions of complex terrain, such as the central and southern Appalachian Mountains. Prior work has documented the environmental and radar characteristics of supercells that cross or fail to cross complex terrain throughout this region. The current work builds upon these prior studies by conducting a series of high-resolution idealized simulations to further investigate how the terrain of the Appalachian Mountains impacts discrete supercell thunderstorms. Emphasis is placed on quantifying influences due to physical airflow over the mountains versus localized terrain-induced environmental heterogeneities experienced by supercells. Nine sensitivity experiments, rooted in prior observational work, explored how orographic features act to modulate the inflow environment as well as storm-scale morphology. The model’s background field was either horizontally homogeneous and fixed over time (steady state) or systematically allowed to evolve via base-state substitution (variable state). Three different frictionless lower-boundary conditions were tested, including no terrain, idealized terrain, and realistic terrain. Results effectively highlighted four key terrain-induced processes (i.e., blocking, channeling, upslope flow, and downslope flow) that modulate supercells at both the meso- γ and meso- β scales. A series of conceptual models were constructed to summarize the impacts these processes may present on supercells in real time. In short, terrain influences resulting in low-level kinematic enhancements are crucial for supercellular maintenance while traversing regions of complex terrain. Significance Statement The evolution of isolated supercells (thunderstorms with rotating updrafts) while traversing complex terrain is not well understood. This study continues a systematic analysis of numerous warm-season supercells that encountered the central and southern Appalachian Mountains. Idealized numerical simulations were run to highlight environmental and evolutionary differences among storms that maintained supercellular structures following terrain interaction (crossing) and those that did not (noncrossing). All simulations were rooted in observations, but with gradually increasing realism to systematically capture a multitude of impacts that supercells experience while traversing orographic features. Results affirm that both the physical reorientation of low-level airflow and terrain-induced environment heterogeneities act to modulate supercells. Such processes were summarized in a series of conceptual models to alleviate short-term forecasting challenges.