Classifying and analyzing tornado-like vortices far from axisymmetry

Much of our understanding of tornado structure and dynamics is based on analysis of vortices that are near axisymmetry. Vortices in the field can deviate significantly from the axisymmetric limit, but relaxing that symmetry increases greatly the difficulties of quantitative study. In this work we consider the classification and analysis of tornado-like vortices away from the axisymmetric limit, using sets of large-eddy simulations as examples for study. New features and complications arising from asymmetry are highlighted. Particular emphasis is placed on the embedding of a concentrated vortex within a larger-scale vortex circulation for different configurations. This category is of relevance for analyzing both tornadoes embedded within mesocyclones and secondary vortices embedded within tornadoes. A large variety of results are encountered for the vortex axis shape (including both left and right handed windings), axis motion (stationary or rotating in alignment with or opposed to the large-scale rotation), and embedding location (e.g., near-central within the large-scale vortex or well off axis). These qualitative features are governed largely by the basic cornerflow structure of the large-scale vortex and the nature of the vorticity source for the small-scale vortex.

A key issue for tornado-like vortices in general is the level of near-surface intensification present and the physical mechanisms producing it. Some of the possibilities for asymmetric vortices are the same as for their axisymmetric counterparts, but new possibilities arise as well. A secondary vortex rotating about a high-swirl primary vortex or the spiral vortex sometimes encountered within the vortex breakdown in a low-swirl corner flow both provide important examples of vortices that gradually weaken aloft without themselves necessarily undergoing a vortex breakdown. We have used simulation results and simpler analytic models to study how the near-surface intensification is achieved in these vortices or, equivalently, the nature of their "termination" aloft. This has provided estimates on the pressure drop intensification within these secondaries that is in general accord with that found in the simulations. This is of potential relevance in understanding the limits in some regimes to the strength of a tornado embedded within a mesocyclone or a secondary vortex embedded within a tornado.