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Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell *

Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell * The authors use a high-resolution supercell simulation to investigate the source of near-ground vertical vorticity by decomposing the vorticity vector into barotropic and nonbarotropic parts. This way, the roles of ambient and storm-generated vorticity can be isolated. A new Lagrangian technique is employed in which material fluid volume elements are tracked to analyze the rearrangement of ambient vortex-line segments. This contribution is interpreted as barotropic vorticity. The storm-generated vorticity is treated as the residual between the known total vorticity and the barotropic vorticity. In the simulation the development of near-ground vertical vorticity is an outflow phenomenon. There are distinct “rivers” of cyclonic shear vorticity originating from the base of downdrafts that feed into the developing near-ground vortex. The origin of these rivers of vertical vorticity is primarily horizontal baroclinic production, which is maximized in the lowest few hundred meters AGL. Subsequently, this horizontal vorticity is tilted upward while the parcels are still descending. The barotropic vorticity remains mostly streamwise along the analyzed trajectories and does not acquire a large vertical component as the parcels reach the ground. Thus, the ambient vorticity that is imported into the storm contributes only a small fraction of the total near-ground vertical vorticity. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell *

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Publisher
American Meteorological Society
Copyright
Copyright © 2013 American Meteorological Society
ISSN
0022-4928
eISSN
1520-0469
DOI
10.1175/JAS-D-13-0123.1
Publisher site
See Article on Publisher Site

Abstract

The authors use a high-resolution supercell simulation to investigate the source of near-ground vertical vorticity by decomposing the vorticity vector into barotropic and nonbarotropic parts. This way, the roles of ambient and storm-generated vorticity can be isolated. A new Lagrangian technique is employed in which material fluid volume elements are tracked to analyze the rearrangement of ambient vortex-line segments. This contribution is interpreted as barotropic vorticity. The storm-generated vorticity is treated as the residual between the known total vorticity and the barotropic vorticity. In the simulation the development of near-ground vertical vorticity is an outflow phenomenon. There are distinct “rivers” of cyclonic shear vorticity originating from the base of downdrafts that feed into the developing near-ground vortex. The origin of these rivers of vertical vorticity is primarily horizontal baroclinic production, which is maximized in the lowest few hundred meters AGL. Subsequently, this horizontal vorticity is tilted upward while the parcels are still descending. The barotropic vorticity remains mostly streamwise along the analyzed trajectories and does not acquire a large vertical component as the parcels reach the ground. Thus, the ambient vorticity that is imported into the storm contributes only a small fraction of the total near-ground vertical vorticity.

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Apr 29, 2013

References