Simulated Supercells in Nontornadic and Tornadic VORTEX2 Environments

Abstract

The composite near-storm environments of nontornadic and tornadic supercells sampled during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) both appear to be generally favorable for supercells and tornadoes. It has not been clear whether small differences between the two environments (e.g., more streamwise horizontal vorticity in the lowest few hundred meters above the ground in the tornadic composite) are actually determinative of storms’ tornadic potential. From the VORTEX2 composite environments, simulations of a nontornadic and a tornadic supercell are used to investigate storm-scale differences that ultimately favor tornadogenesis or tornadogenesis failure. Both environments produce strong supercells with robust midlevel mesocyclones and hook echoes, though the tornadic supercell has a more intense low-level updraft and develops a tornado-like vortex exceeding the EF3 wind speed threshold. In contrast, the nontornadic supercell only produces shallow vortices, which never reach the EF0 wind speed threshold. Even though the nontornadic supercell readily produces subtornadic surface vortices, these vortices fail to be stretched by the low-level updraft. This is due to a disorganized low-level mesocyclone caused by predominately crosswise vorticity in the lowest few hundred meters above ground level within the nontornadic environment. In contrast, the tornadic supercell ingests predominately streamwise horizontal vorticity, which promotes a strong low-level mesocyclone with enhanced dynamic lifting and stretching of surface vertical vorticity. These results support the idea that larger streamwise vorticity leads to a more intense low-level mesocyclone, whereas predominately crosswise vorticity yields a less favorable configuration of the low-level mesocyclone for tornadogenesis.

Horizontal Vortex Tubes near a Simulated Tornado: Three-Dimensional Structure and Kinematics

“…The increase in the degree of organization of the LFCB results in the formation of meridionally-oriented "vertical vorticity rivers" [35,36], which first appear at the base of downdrafts northwest of the low-level circulation (Figure 2d). These features, along with enhanced vertical vorticity in an RFD internal boundary, are thought to be the primary storm-scale sources of vertical vorticity to the developing low-level rotation in other simulations in the literature [35][36][37][38]. At 11,888.775 s, the first tornado forms at the tip of the hook as seen in the vertical vorticity field (Figure 2d, Figure 3a, and Figure 4a).…”

The Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercell Thunderstorms

“…Previous studies (i.e., Wicker 1996;Miller, 2006;Esterheld and Giuliano 2008;Nowotarski and Jensen 2013;Coffer and Parker 2017) have identified this shear profile as being unfavorable for tornadogenesis, suggesting that such a profile (with its weaker storm-relative winds opposing the gust front) results in outflow dominated storms in which low-level circulation is advected ahead of the primary midlevel updraft/mesocyclone. However, the three simulations containing the α = 0° wind profile correspond to the three highest near-surface vorticity maxima of all the simulations performed.…”

Axisymmetric Tornado Simulations with a Semi-Slip Boundary

“…In [1], we find this statement about including surface drag in a storm model: "Our philosophy is that the inclusion of modest surface drag represents a more physically consistent bottom boundary condition for tornadogenesis than does the habitually employed free-slip assumption." Several other recent storm modeling efforts have investigated surface drag in storm evolution and tornadogenesis [2][3][4].…”

“…As used here, semi-slip refers to the simplest application that we see atmospheric modeling, such as in [1,3,10]. It could also be called bulk drag.…”