Numeric and fluid dynamic representation of tornadic double vortex thunderstorms

Current understanding of a double vortex thunderstorm involves a pair of contra-rotating vortices that exists in the dynamic updraft. The pair is believed to be a result of a blocking effect which may occur when a cylindrical thermal updraft of a thunderstorm protrudes into the upper level air and there is a large amount of vertical wind shear between the low level and upper level air layers.

Eagleman developed a numerical tornado prediction scheme based on the double vortex thunderstorm. The Energy-Shear Index (ESI) is part of the scheme and is calculated from radiosonde measurements. The ESI incorporates parameters representative of thermal instability and blocking effect, and indicates appropriate environments for which the development of double vortex thunderstorms is likely.

The ESI and modifications of it were tested using data derived from NASA's Fourth Atmospheric Variability Experiment (AVE IV). The results showed that the index has some general usefulness in forecasting tornadic outbreaks over a large area, but is probably not definitive enough for operational use. At times the results indicated large areas of expectant tornadic activity where little actually occurred. This deficiency led the author to investigate the possibility of incorporating into the index an additional dimensionless parameter to take into account the contribution to tornadic development associated with the fluid mechanics and shear instabilities of the double vortex thunderstorm itself.

To demonstrate the feasibility of obtaining information on this parameter, a wind tunnel investigation was conducted. The cylinder and jet in a laminar crossflow at conditions represented by a Reynolds number (based on cylinder diameter, crossflow velocity, and kinematic viscosity) near 5,000 were investigated. The method of investigation was flow visualization. Transition waves in the region extending from the cylinder to near three radii downstream of the cylinder's center were photographed.

Since complete simulation of the atmosphere with this wind tunnel facility is not possible, the results of the investigation cannot be directly related to a physical description of a double vortex thunderstorm, The direction for future investigation, however, is suggested from this study.