6A.6 How Much does “Backing Aloft” Actually Impact a Supercell?

Tuesday, 8 November 2016: 11:45 AM
Pavilion Ballroom East (Hilton Portland )
Matthew D. Parker, North Carolina State Univ., Raleigh, NC

Although it has rarely been discussed in the formal literature, among forecasters and storm chasers there is a common perception that hodographs with counter-clockwise curvature in the mid-levels (sometimes called “veer-back-veer” profiles) are unfavorable for long-lived supercells and/or tornadoes.  This presumed un-favorability has been rationalized in terms of one or more of the following: a) presumed disruption of the dynamical vertical perturbation pressure gradient accelerations in supercells; b) presumed smaller values of storm-relative helicity in such hodographs (because storm motions often fall quite close to the hodograph); c) presumed propensity for storm interactions/mergers to occur more rapidly; or d) some aspect of the larger-scale environment that accompanies the backing of winds/shear vectors with height.  

The present experiments initiate an isolated supercell in environments using the composite thermodynamic sounding from VORTEX2 tornadic supercells, combined with a set of 9 idealized hodographs that have identical length and curvature in the lowest 2 km AGL, but varying structures farther aloft.  These variations include simple quarter-turn and half-circle hodographs, as well as variations than include counter-clockwise kinks or bends above 2 km AGL (but similar 0-6 km bulk vertical wind shear).  The simulations are then repeated with a line of three closely-spaced storms.  The experiments show that there is no “internal” storm process that substantially hinders the supercells in the environments with backing aloft.  In terms of updraft strength, low-level and mid-level vertical vorticity, and vertical accelerations, the simulations are all rather similar to one another.  Most of the runs with backing aloft actually exceed the simple quarter-turn and half-circle hodographs in terms of updraft strength and the areal coverage of significant upward accelerations.  In fact, backing aloft may cause storms to tend toward the dryer end of the HP-CL-LP supercell spectrum.  If the anecdotal claims about un-favorability are to be believed, then either the hindrance is highly situational, or else the correct explanation is something other than within-storm processes or simple storm-environment interactions.

- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner