The multiple-vortex structure of a tornado

Abstract

The structure and behavior of multiple subtornadic-scale vortices in a tornado were examined and were compared with laboratory, conceptual, and numerical models. Unique radar observations of an exceptionally large and violent tornado obtained with a Doppler on Wheels mobile radar on 3 May 1999 in northern Oklahoma provided the opportunity, for the first time ever with quantitative radar measurements, to characterize the size, strength, motion, horizontal and vertical structure, and persistence of multiple vortices in a tornado. Doppler velocity, received power, and spectral-width data were used to study the vortices. The structures of the multiple subtornadic-scale vortices were similar to that of tornadic vortices in certain respects. They exhibited doughnut-shaped received power maxima and/or hooks surrounding comparatively clear central eyes. Doppler velocity differences across the vortices decreased with height. However, the vortices exhibited intense small-scale shears at their centers that could not be explained by the inability to resolve core flow regions adequately. Even though the distances between wind speed maxima were typically about 250 m. approximately one-half of the total shear in most vortices was concentrated across 50 m or less. This was in contrast to the approximately solid-body rotation exhibited in the core flow region of the parent torando. It is hypothesized that either the very rapid motion of the vortices or small-scale transient updrafts caused this phenomenon. The shear across the vortices, about 100 m s-1. was about one-half of the total shear across the tornado, about 170 m s-1. The amplitude of the vortices was consistent with some, but not all, numerical and laboratory predictions. The central shear regions of the vortices exhibited estimated vertical vorticities of 4-8 s-1, the highest ever observed in tornadic flows. Wind speed changes of 50 m s-2, corresponding to 5 times the acceleration of gravity, would have been experienced by stationary observers impacted by the multiple vortices. The vortices appeared to tramslate around the tornado at a fraction of the peak azimuthally averaged tangential velocity of the parent tornado, consistent with some theoretical and computational predictions. It was not possible to rule out, however, that, in the absence of any upstream propagation, the vortices merely translated at the peak azimuthally averaged tangential velocity of the parent tornado at the radius of the vortices as predicted in other studies. Individual vortices were trackable for at least 40 s., revolving at least 180° around the parent tornado. The multiple vortices were most prominent during the weakening phase of the tornado, as peak azimuthally averaged tangential winds dropped from over 80 to less than 70 m s-1, and just after the radius of the peak flow region had contracted somewhat. possibly indicating changes in the swirl ratio.