Airflow Characteristics Around the Union City Tornado

Abstract

The NSSL Tornado-Intercept Project team intercepted and photographed an intense tornado that struck Union City, Okla., on 24 May 1973. The life cycle of the tornado was photographically documented. Photo-grammetric data permitted velocity measurements from debris and cloud-tag motion. When the tornado was at its maximum size and intensity 2 km west of Union City, maximum measured horizontal velocities in the debris cloud were 60?80 m s?1 at 90 m elevation and a radius of about 200 m. At the same time, cloud tags rotating around the upper periphery of the tornado funnel had horizontal velocities up to 30?45 m s?1 at radii of 400?700 m from the tornado's axis. A few representative calculations of upward velocities yield 13?30 m s?1 in the debris cloud below 100 m elevation and 10?15 m s?1 in a ?feeder? band of cloud tags which spiraled into the tornado near cloud base from the northeast. During the tornado's decay stage, tangential velocities of particles orbiting the funnel ranged from 40 to 65 m s?1, at radii generally between 25 and 50 m. The tornado's debris-cloud circulation decreased from 6.O ? 104 to 1.6 ? 104 m2 s?1 between the mature and decay stages. Finally, the tornado's apparent flow structure, as inferred from debris and cloud-tag trajectories, was strongly asymmetric in both rotational and vertical flow components.