The Numerical Simulation of Nonsupercell Tornadogenesis. Part II: Evolution of a Family of Tornadoes along a Weak Outflow Boundary

Abstract

Nonsupercell tornadogenesis along a weak outflow boundary has been simulated using a three-dimensional moist convective cloud model. Thermodynamic conditions similar to those observed for nonsupercell tornado (NST) events of the High Plains were utilized in the model initialization. As the ensemble system of storm, outflow boundary, and leading edge vortices evolve, six distinct life cycle stages for the development and decay of NSTs are documented that span a period of about 35 min. Consistent with the results of Part I of this numerical study, vortex sheet dynamics exert considerable control over the outflow leading edge. The progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the outflow boundary in discrete misocyclone circulations aligned in a 3-km wavelength pattern. The organization of larger-scale misocyclones and ultimate intensification to initial tornadic intensity occurs coincident with the rapid development of deep convection overhead. The strongest members of a family of NSTs that develop in the model maintain ground-relative surface wind speeds of greater than 30 m s?1 for approximately 11 min within which wind speeds meet F1 severity criteria for 6 min. The mature vortices reach the midlevels of the moist convection and display deep, rotationally induced axial downdrafts. The rapid transition to a predominant downdraft character for the storm complex and to an outflow dominated subcloud air mass heralds the onset of NST dissipation. The NST evolution simulated here compares very favorably to observational NST studies. The misocyclones are shown to provide an asymmetric pattern of convective forcing along the outflow boundary, which supports the formation of deep moist convection directly over them. Vertical vorticity in the boundary layer misocyclones is redistributed upward into the midlevels of the moist convection (?6 km) by developing deep updrafts. The mature vortices are maintained by vertical vorticity transported from a vortex sheet located along the outflow boundary and by vertical vorticity produced from the tilting of horizontal vorticity in the inflow region southeast of the NSTs. Low-level vortex stretching is the dominant vorticity tendency term as the vortices intensify to and maintain tornadic strength. No significant vertical tilting of baroclinically generated horizontal vorticity was indicated even after convective downdraft-associated new outflow pools formed in the environment surrounding the misocyclones. These new outflow pools play a major role in NST intensification by increasing convergence and resultant vortex stretching along the periphery of the tornadic circulations. A comparative simulation with rain production turned off, which precluded the possibility of new outflow development, revealed that NST intensity was roughly 25% greater in the baseline simulation. In an additional comparative simulation where no moist convection was allowed to develop, the resultant misocyclones markedly lacked the coherent organization and intensity of the misocyclones and NSTs of the baseline simulation and no NST-strength vortices developed. A six-stage ?refined? model of NST development and decay is presented.