A new pathway for tornadogenesis exposed by numerical simulations of supercells in turbulent environments

Paul Markowski, Penn State University

Abstract:

A simulation of a supercell storm produced for a prior study on tornado predictability is reanalyzed for the purpose of examining the finescale details of tornadogenesis. It is found that the formation of a tornadolike vortex in the simulation differs from how such vortices have been understood to form in previous numerical simulations. The main difference between the present simulation and past ones is the inclusion of a turbulent boundary layer in the storm's environment in the present case, whereas prior simulations have used a laminar boundary layer. The turbulent environment contains significant nearsurface vertical vorticity, organized in the form of longitudinal streaks aligned with the southerly groundrelative winds. The ζ streaks are associated with corrugations in the vertical plane in the predominantly horizontal, westwardpointing environmental vortex lines; the vortexline corrugations are produced by the vertical drafts associated with coherent turbulent structures aligned with the aforementioned southerly groundrelative winds (longitudinal coherent structures in the surface layer such as these are wellknown to the boundary layer and turbulence communities). The ζ streaks serve as focal points for tornadogenesis, and may actually facilitate tornadogenesis, given how nearsurface ζ in the environment can rapidly amplify when subjected to the strong, persistent convergence beneath a supercell updraft.