Some Effects of the Shearing and Veering Environmental Wind on the Internal Dynamics and Structure of a Rotating Supercell Thunderstorm

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  • 1 Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis 63103
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Abstract

A three-dimensional severe thunderstorm model is employed to study some effects that shearing and veering environmental winds exert on the structure and internal dynamics of a typical supercell rotating storm during its quasi-steady mature stage. These environmental winds are analytically formulated to conform with observations deduced from seven well-documented supercell storms. Horizontal relative winds are generated using the Rankine vortex concept for the inner core region (radii 0–4 km), the potential flow concept for the outer portion (radii 8–25 km) and the transition zone in between (radii 4–8 km). The temperature field is considered to conform with the observed warm-core structured storm. Using these semi-realistic data as input, six numerical experiments are conducted allowing the environmental wind to veer and to shear systematically from one case to another. Vertical velocities are obtained by solving the scaled mass continuity equation. Values of total pressure and perturbation pressure are computed from the diagnostic pressure equation obtained from the horizontal momentum equations using the sequential relaxation method. Results show that fields of perturbation pressure and vertical velocity are quite sensitive to the veering and shearing environmental wind in the region surrounding the central updraft core. Specifically, pronounced upward and downward motions are found on the right and left flank of a storm's updraft core, respectively. The magnitude of these induced vertical velocities increases in proportion to the vertical wind shear and is found to be closely related to perturbation pressure gradients. These findings are in good qualitative agreement with observational evidence reported in the literature. The role of these perturbation pressure forces in protecting the storm's main updraft is emphasized.

Abstract

A three-dimensional severe thunderstorm model is employed to study some effects that shearing and veering environmental winds exert on the structure and internal dynamics of a typical supercell rotating storm during its quasi-steady mature stage. These environmental winds are analytically formulated to conform with observations deduced from seven well-documented supercell storms. Horizontal relative winds are generated using the Rankine vortex concept for the inner core region (radii 0–4 km), the potential flow concept for the outer portion (radii 8–25 km) and the transition zone in between (radii 4–8 km). The temperature field is considered to conform with the observed warm-core structured storm. Using these semi-realistic data as input, six numerical experiments are conducted allowing the environmental wind to veer and to shear systematically from one case to another. Vertical velocities are obtained by solving the scaled mass continuity equation. Values of total pressure and perturbation pressure are computed from the diagnostic pressure equation obtained from the horizontal momentum equations using the sequential relaxation method. Results show that fields of perturbation pressure and vertical velocity are quite sensitive to the veering and shearing environmental wind in the region surrounding the central updraft core. Specifically, pronounced upward and downward motions are found on the right and left flank of a storm's updraft core, respectively. The magnitude of these induced vertical velocities increases in proportion to the vertical wind shear and is found to be closely related to perturbation pressure gradients. These findings are in good qualitative agreement with observational evidence reported in the literature. The role of these perturbation pressure forces in protecting the storm's main updraft is emphasized.

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