The Numerical Simulation of Nonsupercell Tornadogenesis. Part III: Parameter Tests Investigating the Role of CAPE, Vortex Sheet Strength, and Boundary Layer Vertical Shear

Abstract

Nonsupercell tornadogenesis has been investigated in a three-part numerical study. Building on the results of Parts I and II, Part III addresses the sensitivity of nonsupercell tornadogenesis to variations in convective available potential energy (CAPE), outflow boundary vortex sheet strength, and boundary layer vertical shear. A three-dimensional, nonhydrostatic, quasi-compressible convective cloud model has been employed to examine nonsupercell tornado (NST) development in an environment typical of the Colorado high plains. A strong relationship was shown to exist between the magnitude of the environmental CAPE and the structure and intensity of the misocyclones and nonsupercell tornadoes that developed. As CAPE was increased from 0 to 1700 J kg?1, the simulated vortices markedly contracted and intensified. Multiple CAPE thresholds were identified that yielded markedly different vortex intensity. The highest CAPE runs produced NST families with peak ground-relative surface winds of ?47 m s?1. Vortex sheet strength along the outflow boundary played a controlling role in the upscale progression of misocyclones. Higher sheet strength was related to an accelerated rate of vorticity concentration and generally larger misocyclone circulations along the outflow boundary. A threshold value of sheet strength existed that delineated conditions supportive of tornado strength vortex development from those only supportive of nontornadic misocyclones. The boundary layer vertical shear simulations revealed a marked variability in misocyclone/NST intensity and coherency as the ambient boundary layer vertical shear was varied. A vertical shear window approximately ranging from 80% to 120% of optimal boundary layer vertical shear was identified that was supportive of deep and intense tornadic circulations.