Low-level shear in the near-storm environment of simulated supercells and impacts of shear orientation on outflow characteristics.

The magnitude of low-level (0-1 km) vertical shear in supercell environments has been shown to be an important predictor for tornadoes associated with supercell mesocyclones. The dynamical link between the environmental shear and tornadogenesis has yet to be conclusively determined. Previous studies have demonstrated that increased low-level shear (provided that streamwise vorticity also increases) may increase vertical vorticity in supercell mesocyclones above the ground. Strengthening of the overlying mesocyclone may increase the upward-directed dynamic vertical perturbation pressure gradient force, providing favorable conditions for tornadogenesis if a near-ground circulation is located below the strengthened mesocyclone.

The hypothesis motivating the current study is that changes to the orientation and magnitude of the low-level shear vector may alter the structure and motion of supercell outflow, such that the position of the near-ground circulation relative the overlying mesocyclone may be modulated by shear, regardless of effects on the vertical perturbation pressure gradient. Results from preliminary numerical simulations in which the low-level shear is varied with minimal changes to near-ground storm-relative helicity (and minimal effect on mesocyclone strength aloft) will be presented.

Furthermore, the strength and orientation of the low-level shear in the vicinity of supercell gust fronts may also be significantly altered by the storm itself, through horizontal accelerations in proximity to the updraft or reductions in vertical mixing beneath the storm anvil. Consequently, measures of low-level shear provided by proximity soundings may not adequately reflect local shear conditions encountered by supercell gust fronts. These effects (as depicted by simulations) and their potential ramifications for tornadogenesis will be discussed.