Editorial Type:
Article
Article Type:
Research Article

The Use of Vertical Wind Shear versus Helicity in Interpreting Supercell Dynamics

Morris L. Weisman National Center for Atmospheric Research,* Boulder, Colorado

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Richard Rotunno National Center for Atmospheric Research,* Boulder, Colorado

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Print Publication:
01 May 2000
DOI:
https://doi.org/10.1175/1520-0469(2000)057<1452:TUOVWS>2.0.CO;2
Page(s):
1452–1472
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Abstract

A series of idealized simulations of supercell storms are presented for environments representing straight through circular hodographs to clarify the character of the storm dynamics over the large spectrum of hodograph shapes commonly observed. The primary emphasis is on comparing and contrasting recent theories of supercell dynamics, based on updraft–shear interactions, storm-relative environmental helicity (SREH), and Beltrami-flow solutions, to help clarify the degree to which each theory can represent the essential storm dynamics. One of the particular questions being addressed is whether storm dynamics are significantly different for straight versus curved hodographs, which has become a point of some controversy over recent years.

In agreement with previous studies, the authors find that the physical processes that promote storm maintenance, rotation, and propagation are similar for all hodograph shapes employed, and are due primarily to nonlinear interactions between the updraft and the ambient shear, associated with the localized development of rotation on the storm’s flank. Significant correlations between the updraft and vertical vorticity are also observed across the shear spectrum, and, in agreement with predictions of linear theories associated with SREH, this correlation increases for increasing hodograph curvature. However, storm steadiness and propagation must already be known or inferred for such concepts to be applied, thus limiting the applicability of this theory as a true predictor of storm properties. Tests of the applicability of Beltrami solutions also confirm reasonable agreement for purely circular hodographs, for which the analytical solutions are specifically designed. However, analysis of the model results indicates that the terms ignored for such solutions, representing the nonlinear effects associated with storm rotation, are more significant than those retained over most of the hodograph spectrum, which severely limits the general applicability of such analyses.

* The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Corresponding author address: Morris L. Weisman, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: weisman@ncar.ucar.edu

Abstract

A series of idealized simulations of supercell storms are presented for environments representing straight through circular hodographs to clarify the character of the storm dynamics over the large spectrum of hodograph shapes commonly observed. The primary emphasis is on comparing and contrasting recent theories of supercell dynamics, based on updraft–shear interactions, storm-relative environmental helicity (SREH), and Beltrami-flow solutions, to help clarify the degree to which each theory can represent the essential storm dynamics. One of the particular questions being addressed is whether storm dynamics are significantly different for straight versus curved hodographs, which has become a point of some controversy over recent years.

In agreement with previous studies, the authors find that the physical processes that promote storm maintenance, rotation, and propagation are similar for all hodograph shapes employed, and are due primarily to nonlinear interactions between the updraft and the ambient shear, associated with the localized development of rotation on the storm’s flank. Significant correlations between the updraft and vertical vorticity are also observed across the shear spectrum, and, in agreement with predictions of linear theories associated with SREH, this correlation increases for increasing hodograph curvature. However, storm steadiness and propagation must already be known or inferred for such concepts to be applied, thus limiting the applicability of this theory as a true predictor of storm properties. Tests of the applicability of Beltrami solutions also confirm reasonable agreement for purely circular hodographs, for which the analytical solutions are specifically designed. However, analysis of the model results indicates that the terms ignored for such solutions, representing the nonlinear effects associated with storm rotation, are more significant than those retained over most of the hodograph spectrum, which severely limits the general applicability of such analyses.

* The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Corresponding author address: Morris L. Weisman, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: weisman@ncar.ucar.edu

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