The Interaction of Numerically Simulated Supercells Initiated along Lines

Howard B. Bluestein School of Meteorology, University of Oklahoma, Norman, Oklahoma*


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Morris L. Weisman School of Meteorology, University of Oklahoma, Norman, Oklahoma*

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Abstract

Supercells in the southern plains are often localized, forming as cells along a convective line, even though the environment may support supercell formation over a much broader, mesoscale region. A set of numerical experiments is devised in which it is demonstrated that the evolution of supercells in a homogeneous environment depends upon the orientation of the vertical-shear profile with respect to the orientation of the line along which convection is initiated (the “line of forcing”). This work is motivated by the observations that the nature and consequences of the interaction of neighboring cells depend upon differential cell motion, which in turn is a function of the characteristics and orientation of the vertical-shear profile and its impact on the behavior of outflow boundaries.

Results for various orientations of the vertical-shear vector with respect to the line along which cells are initiated are described and interpreted physically. It is found that in idealized numerical simulations, shear oblique to (45° from) the line of forcing is most apt to support neighboring cyclonic supercells within the line, but also supports an anticyclonic supercell at the downshear end of the line; shear normal to the line of forcing is favorable for the maintenance of a squall line with isolated supercells at either end; shear parallel to the line of forcing is favorable for isolated supercells only on the downshear side of the line. The effects of low-level clockwise curvature in the hodograph vary from case to case, depending upon the orientation of the leading edges of the system cold pool with respect to the low-level shear. Differences in low-level static stability and the dryness of air aloft affect storm behavior less than differences in the orientation of the vertical shear. The process of storm collision is examined in detail.

Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 100 E. Boyd, Rm. 1310, Norman, OK 73019.

Email: hblue@ou.edu

Abstract

Supercells in the southern plains are often localized, forming as cells along a convective line, even though the environment may support supercell formation over a much broader, mesoscale region. A set of numerical experiments is devised in which it is demonstrated that the evolution of supercells in a homogeneous environment depends upon the orientation of the vertical-shear profile with respect to the orientation of the line along which convection is initiated (the “line of forcing”). This work is motivated by the observations that the nature and consequences of the interaction of neighboring cells depend upon differential cell motion, which in turn is a function of the characteristics and orientation of the vertical-shear profile and its impact on the behavior of outflow boundaries.

Results for various orientations of the vertical-shear vector with respect to the line along which cells are initiated are described and interpreted physically. It is found that in idealized numerical simulations, shear oblique to (45° from) the line of forcing is most apt to support neighboring cyclonic supercells within the line, but also supports an anticyclonic supercell at the downshear end of the line; shear normal to the line of forcing is favorable for the maintenance of a squall line with isolated supercells at either end; shear parallel to the line of forcing is favorable for isolated supercells only on the downshear side of the line. The effects of low-level clockwise curvature in the hodograph vary from case to case, depending upon the orientation of the leading edges of the system cold pool with respect to the low-level shear. Differences in low-level static stability and the dryness of air aloft affect storm behavior less than differences in the orientation of the vertical shear. The process of storm collision is examined in detail.

Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 100 E. Boyd, Rm. 1310, Norman, OK 73019.

Email: hblue@ou.edu

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