Self-sustaining intense vortices

Abstract

We seek to identify an accessible observable for anticipating tornadogenesis in mesocyclones, because such tornadoes are characterized by exceptionally extended lifespan, exceptionally long and wide path, and exceptionally high wind speed. We associate tornadogenesis with the transition from a one-cell vortex to a two-cell vortex. After such a transition, the core (“eye”) of the vortex consists of virtually nonrotating, slowly recirculating, relatively dry air. Rapidly swirling air swiftly ascends in an annulus (“eyewall”), situated at a small but finite distance from the axis of rotation. The swiftly ascending air is described by a locus of thermodynamic states well approximated by a moist adiabat. Such a transition from a one-cell vortex to a two-cell vortex, on vastly large lateral scale, is known to characterize the intensification of a tropical storm to a typhoon. We adopt a simplified, tractable model for our initial analytic efforts. We examine a quasisteady axisymmetric vortex with a four-part structure, consisting of a bulk potential vortex, a near-ground inflow layer, an “eyewall”, and an “eye”. We inquire under what conditions such a four-part intense vortex, formed in convectively unstable stratified air, is self-sustaining. In particular, we inquire whether the vertical profile of the angular momentum outside of the eyewall is a discriminant for identifying the conditions for which an intense (two-cell) vortex could be self-sustaining. Guidance from laboratory experiments would be helpful concerning the turnaround (the portion of the flow field in which the near-ground swirling inflow separates to form the swirling updraft of the eyewall annulus).