A Severe Frontal Rainband. Part II: Tornado Parent Vortex Circulation

Abstract

By means of multiple Doppler radar analysis, Part I established the stormwide hydrodynamic structure as an intense gravity current which advances on a prefrontal low-level jet. Part II examines the initiation and evolution of a tornado parent vortex circulation in the context of stormwide vorticity production. Tilting of the prefrontal inflow below the low-level jet axis accounts for production of observed vorticity (10?2 s?1) at the surface front. The two-dimensional flow pattern is modified by the development of an inflectional instability with a wavelength of ?13 km. Breaking (or roll-up) of this instability occurs on a time scale of roughly 500 s and results in small-scale vortices along the surface front. One such vortex is examined in detail. This vortex is the parent circulation of a small tornado. The parent vortex structure is determined to be quite similar to those observed in summertime deep convection. This suggests a generality to the findings concerning parent vortex initiation and tornadogenesis. Net vorticity production within the parent vortex is small but various terms force substantial redistribution and concentration of vorticity subsequent to inflow tilting. Stretching at the gust front inflection and along trailing portions of the gust front is shown to be constructive at mid-levels. Stretching at the apex of the downdraft is also an important constructive feature aloft. Vertical transport of vorticity in the surrounding updrafts as well as in the intervening downdraft provides positive forcing which heretofore has not been reported. Downdraft initiation within the parent vortex is examined with respect to dynamical and microphysical forcing mechanisms. Spreading of downdraft air at the surface is determined to be temporally coincident with the tornadic phase. Interpretation of tornadogenesis in supercells is made in the perspective of these observations and the findings associated with fine-scale numerical simulations. A concluding hypothesis is that the major features of tornadogenesis are not particularly sensitive to many aspects of storm-scale circulations but rather they require creation of specific localized conditions along the storm outflow boundary. One such condition is the existence of a low-level inflow ?jet? which is a direct consequence of powerful storm-scale updrafts in buoyantly-driven storms.