The Multiple Vortex Structure of a Tornado

The structure and behavior of multiple vortices in a tornado are examined. Unique radar observations of an exceptionally large and violent tornado obtained with a Doppler On Wheels (DOW) mobile radar on 3 May 1999 in northern Oklahoma provide the opportunity, for the first time ever, to characterize the size, strength, motion, horizontal and vertical structure, and persistence of multiple vortices in a tornado. Doppler velocity, reflectivity, and spectral width data are used to study the vortices.

The structure of the vortices are similar to tornadic vortices in some ways; they exhibit doughnut shaped reflectivity maxima surrounding relatively clear central eyes. Doppler wind speed differences across the vortices decrease with height. However, the vortices exhibit intense small-scale shears at their centers that cannot be explained by the inability to adequately resolve core flow regions. Even though the distances between wind speed maxima are approximately 250 m, approximately one half of the total shear in some vortices is concentrated across 50 m or less. It is hypothesized that either the very rapid motion of the vortices or small-scale transient updrafts cause this phenomenon. The shear across the vortices, about 100 ms-1, is about one half of the total shear across the tornado, about 170 ms-1. The central shear regions of the vortices exhibit vertical vorticities of 2-4 s-1, the highest ever observed in tornado-related flows. The vortices appear to translate around the tornado at approximately the same speed as the background flow, 50 ms-1, and do not propagate upstream as would Rossby-wave-like disturbances. Individual vortices persist for at least 30 s, translating at least 180° around the parent tornado.