An Investigation of Supercell Longevity and Streamwise Vorticity Current Characteristics in Response to Varying Midlevel Shear
Despite an increased understanding of environments favorable for tornadic supercells, it is still sometimes unknown why one favorable environment produces many long-tracked tornadic supercells and another seemingly equally-favorable environment produces only short-lived supercells. One relatively unexplored environmental parameter that may differ between such environments is the degree of backing or veering of the midlevel shear vector, especially considering that such variations may not be captured by traditional supercell or tornado forecast parameters. We investigate the impact of the 3-6 km shear vector orientation on simulated supercell evolution by systematically varying it across a suite of idealized simulations. We found that the orientation of the 3-6 km shear vector dictates where precipitation loading is maximized in the storms, and thus alters the storm-relative location of downdrafts and outflow surges. When the shear vector is backed, outflow surges generally occur northwest of an updraft, produce greater convergence beneath the updraft, and do not disrupt inflow, meaning that the storm is more likely to persist and produce more tornado-like vortices (TLVs). When the shear vector is veered, outflow surges generally occur north of an updraft, produce less convergence beneath the updraft, and sometimes undercut it with outflow, causing it to tilt at low levels, sometimes leading to storm dissipation. These storms are shorter lived and thus also produce fewer TLVs. Our simulations indicate that the relative orientation of the 3-6 km shear vector may impact supercell longevity and hence the time period over which tornadoes may form.
These same simulations are then used to investigate an ensemble of streamwise vorticity currents (SVCs), which have been hypothesized to enhance low-level mesocyclones within supercell thunderstorms and perhaps increase the likelihood of tornadogenesis. Recent observational studies have confirmed the existence of SVCs in supercells and numerical studies have allowed for further investigation of SVCs using high-resolution model output. The SVCs in our simulations develop on the cool side of left-flank convergence boundaries, the position and orientation of which is partially dependent on the updraft-relative location of outflow surges. Trajectories initialized within outflow surges associated with SVCs reveal that baroclinic generation of horizontal vorticity occurs as air descends in downdrafts. If the baroclinically-generated horizontal vorticity is not already streamwise, it is reoriented through tilting or crosswise-to-streamwise exchange. The streamwise vorticity is then stretched as the air accelerates toward the updraft where it is tilted into the vertical and subsequently stretched. Lastly, a majority of the tornado-like vortices in the simulations are preceded by SVCs, supporting the hypothesis that SVCs, especially persistent and deep ones, may increase the likelihood of tornadogenesis by strengthening low-level mesocyclones. An investigation of how surface drag impacts the results is left for future work and preliminary results will be presented.
