Dynamics of Simulated High-Shear Low-CAPE Supercells

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  • 1 Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC
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Abstract

High-shear low-CAPE environments prevalent in the southeastern U.S. account for a large fraction of tornadoes and pose challenges for operational meteorologists. Yet, existing knowledge of supercell dynamics, particularly in the context of cloud-resolving modeling, is dominated by moderate- to high-CAPE environments typical of the Great Plains. This study applies high-resolution modeling to clarify the behavior of supercells in the more poorly understood low-CAPE environments, and compares them to a benchmark simulation in a higher-CAPE environment. Simulated low-CAPE supercells’ main updrafts do not approach the theoretical equilibrium level; their largest vertical velocities result not from buoyancy, but from dynamic accelerations associated with low-level mesocyclones and vortices. Surprisingly, low-CAPE tornado-like vortex parcels also sometimes stop ascending near the vortex top instead of carrying large vorticity upward into the midlevel updraft, contributing to vortex shallowness. Each of these low-CAPE behaviors is attributed to dynamic perturbation pressure gradient accelerations that are maximized in low levels, which predominate when the buoyancy is small.

Corresponding author address: Andrew Wade, North Carolina State University, 2800 Faucette Blvd., Raleigh, NC 27607. E-mail: arwade@ncsu.edu

Abstract

High-shear low-CAPE environments prevalent in the southeastern U.S. account for a large fraction of tornadoes and pose challenges for operational meteorologists. Yet, existing knowledge of supercell dynamics, particularly in the context of cloud-resolving modeling, is dominated by moderate- to high-CAPE environments typical of the Great Plains. This study applies high-resolution modeling to clarify the behavior of supercells in the more poorly understood low-CAPE environments, and compares them to a benchmark simulation in a higher-CAPE environment. Simulated low-CAPE supercells’ main updrafts do not approach the theoretical equilibrium level; their largest vertical velocities result not from buoyancy, but from dynamic accelerations associated with low-level mesocyclones and vortices. Surprisingly, low-CAPE tornado-like vortex parcels also sometimes stop ascending near the vortex top instead of carrying large vorticity upward into the midlevel updraft, contributing to vortex shallowness. Each of these low-CAPE behaviors is attributed to dynamic perturbation pressure gradient accelerations that are maximized in low levels, which predominate when the buoyancy is small.

Corresponding author address: Andrew Wade, North Carolina State University, 2800 Faucette Blvd., Raleigh, NC 27607. E-mail: arwade@ncsu.edu
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