How Much Does “Backing Aloft” Actually Impact a Supercell?

Abstract

AbstractAmong forecasters and storm chasers, there is a common perception that hodographs with counterclockwise curvature or kinking in the midlevels (sometimes called backing aloft or veer?back?veer profiles) are unfavorable for long-lived supercells and tornadoes. This study reviews and then evaluates several possible explanations for the purported negative effect of backing aloft. As a controlled hypothesis test, simulated supercells are initiated within a range of idealized wind profiles, many of which include representative counterclockwise kinks or bends in their hodographs. In these experiments, the short-term, direct impacts of backing aloft upon supercell maintenance are generally small. Backing aloft does modify the component of vertical accelerations linked to updraft?shear interactions, but these changes generally occur well above the level of free convection (LFC), and they are generally offset by substantial upward accelerations attributable to other processes (e.g., within-storm rotation and positive buoyancy). In these simulations, the longevity of isolated supercells seems to be most directly hindered in environments with very low storm-relative helicity (SRH) or else (for a line of supercells) substantial along-line flow in the upper troposphere. Although these two disrupting properties can accompany backing aloft, they are neither universally nor exclusively associated with it. From the perspective of storm dynamics, it seems advisable to focus on SRH and along-line flow in the environment, rather than the presence (or absence) of backing aloft in the wind profile.