The Numerical Simulation of Non-Supercell Tornadogenesis. Part I: Initiation and Evolution of Pretornadic Misocyclone Circulations along a Dry Outflow Boundary

Abstract

High-resolution three-dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary. Model conditions were designed such that this development could be simulated independent of moist processes. The outflow boundary is allowed to propagate into a region of southerly low-level flow which results in a vertical vortex sheet along the outflow?s leading edge. Lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities. These growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1.6 to 3.2 km. The structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast Colorado. Vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge. The vertical vortex sheet passes through discrete evolutionary stages including vortex sheet roll up, subharmonic interaction, consolidation, and dissipation. All predissipation stages result in an increase in misocyclone circulation. Model simulations indicate that the pretornadic misocyclone circulations control the vertical velocity distribution along the leading edge with significant updraft maxima located adjacent to the downdraft centers (pressure lows) that are found in the middle of the misocyclones. These findings refine and add breadth to the observational hypothesis for non-supercell tornadogenesis by providing detail on the origin and development of the low-level misocyclone and by suggesting a locational relationship between deep convection and the misocyclone. A comparison between simulations employing semi-slip and free-slip surface layers revealed marked differences in misocyclone development. Other parameter studies varying the ambient vertical shear and the strength of the vortex sheet along with simulations testing the influence of a capping inversion were performed to identify ?(timal? conditions for producing strong misocyclones (? ≥ 0.1 s?1). This ?(timal? state was found to include 1) an ambient vertical shear profile of similar depth to the outflow that led to a supercritical outflow head, 2) substantial across-front horizontal shear leading to a strong vertical vortex sheet (?? > 0.02 s?1) that evolved into vigorous misocyclone centers, and 3) an environment of neutral or near-neutral stability for a layer considerably deeper than the outflow itself. The influence of a capping stable layer on misocyclone evolution was highly dependent on the effect the ambient vertical shear profile had on the outflow depth.