The Impact of Midlevel Shear Orientation on the Longevity of and Downdraft Location and Tornado-Like Vortex Formation within Simulated Supercells

Abstract

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.

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“…Tornadic‐like vortices (TLVs) are identified by examining near‐surface fields of windspeed, VV, and the Obuko‐Weiss (OW) parameter. Adapting the approaches of Sherburn and Parker (2019), Gray and Frame (2021), and others, TLV identification requires VV, windspeed, and OW to exceed 0.1, 30 m, and 0.03 s −2 , respectively, and be collocated with low‐level updraft speeds exceeding 5 m s −1 . Upon locating the strongest TLV, maximum and minimum of x ‐direction and y ‐direction wind components are found within 500 m of the vortex center.…”