How Does Terrain Impact Upscale Convective Growth of Orogenic Deep Moist Convection?

The environmental and internal storm dynamics that govern upscale convective growth of discrete deep moist convection into organized mesoscale convective systems (MCSs) are still not fully understood, particularly for orogenic convection. Satellite and ground-based radar studies have demonstrated that upscale convective growth near the complex terrain of the Sierras de Córdoba, Argentina, South America, is common and tends to occur closer to the terrain and more rapidly than in the Central USA. Here we examine the severe convective event of 29 November 2017 near the city of Córdoba; an orogenic supercell transitioned into a bowing MCS over a short time span of two to three hours. High-resolution Weather Research and Forecasting (WRF) model simulations are used to analyze the processes that aided in this rapid upscale convective growth. Radar observations indicate that a 333-m grid spacing control simulation accurately reproduces the evolution of convection initiation through supercell development and eventual upscale convective growth of the event. Enhanced antistreamwise horizontal vorticity along the eastern edge of the highest terrain appears to aid in the formation of a dominant left-moving supercell. Shortly thereafter, strong downdrafts and expansion of the cold pool result in a rapid upscale transition to a bowing MCS. Sensitivity experiments in which the height of the Sierras de Córdoba are artificially raised or lowered are conducted to examine the effects the terrain height have on the upscale convective growth process. All terrain height experiments result in supercell formation and eventual transition to a bowing MCS; however, the artificially higher terrain height experiments show earlier convection initiation, stronger backing of low-level winds, greater low-level vertical wind shear, weaker parcel buoyancy, lower lifting condensation level heights, stronger downdrafts and cold pools, greater terrain blocking of cold pools, and greater rainfall accumulation. The earlier convection initiation in high terrain experiments is due to enhanced upslope flow and quicker removal of convective inhibition. Low-level antistreamwise horizontal vorticity is locally augmented east of the terrain by increased upslope flow and backing low-level winds, allowing for a more steady mesocyclone that moves farther east of the Sierras de Córdoba as compared with the lower terrain experiments (Fig. 1). Further implications of the effects of complex terrain on environmental (both thermodynamic and dynamic) and storm-scale processes related to upscale convective growth will be discussed and trajectory analyses will be shown.