How does terrain influence the upscale convective growth of orographic deep moist convection?
Satellite observations have revealed that some of the world’s most intense deep convective storms occur near the Sierras de Córdoba, Argentina, South America. A ground-based radar climatology during two austral spring and summer seasons (2015–2017) revealed that most of the storms were multicellular and initiated most frequently during the early afternoon and late evening hours just east of the Sierras de Córdoba. The peak occurrence of these storms was between December-February. Storm environments in Argentina tend to be characterized by larger convective available potential energy and weaker low-level vertical wind shear compared to the United States. One of the more intriguing results is the relatively fast transition, and close proximity to terrain, from first storms to larger mesoscale convective systems compared with locations in the United States. A canonical upscale convective growth case was simulated with the Weather Research and Forecasting model to understand the role of topography in this transition process. This case featured an orographic supercell that transitioned into a bowing mesoscale convective system over three-to-four hours. The simulation revealed enhanced low-level vertical wind shear along the eastern slopes of the Sierras de Córdoba that aided in the formation of a left moving supercell. Shortly thereafter, strong downdrafts and expansion of the cold pool resulted in a rapid transition to a bowing mesoscale convective system. Terrain height sensitivity experiments were conducted with only the control and higher terrain experiments resulting in a supercell-to-bowing mesoscale convective system transition. The control simulation, with the real terrain of the Sierras de Córdoba, resulted in the faster upscale convective growth owing to both terrain-driven environmental and storm-scale effects, such as variations to thermodynamic/kinematic profiles and terrain blocking of cold pools, respectively. Inspired by the aforementioned ground-based radar climatology and in-depth numerical modeling upscale convective growth case study in north central Argentina, a set of different initial terrain height idealized numerical modeling experiments were conducted. These experiments were devised to determine the relative roles of both direct and indirect influences of terrain on upscale convective growth of a supercell in a model configuration similar to those observed near the Sierras de Cόrdoba in Argentina. The experimental results indicated that when the terrain was systematically raised, convection initiation occurred earlier, supercells were wider and more intense, and upscale convective growth generally occurred faster. A direct influence of terrain was blocking of cold pools leading to a deepening of the cold pools that drove surging outflow and more rapid upscale convective growth. Indirect influences of terrain included modifications to the surrounding thermodynamic and kinematic profiles, with terrain-enhancements to the vertical wind shear profile prompting wider updrafts in higher terrain supercells. These wider supercell updrafts were accompanied by greater vertical mass flux, wider and stronger downdrafts, and deeper cold pools, promoting more rapid upscale convective growth.
Upscale convective growth, mesoscale convective system, supercell, terrain, orographic
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign