Abstract
The storm which spawned the devastating Union City, Okla., tornado of 24 May 1973 was observed by Doppler Plan Shear Indicator (PSI). The PSI, an analog device for display of velocity gradients, had been temporarily mated to the 10 cm Doppler radar operated by the National Severe Storms Laboratory. The first PSI picture showed intense shear at 8–9 km altitude, 45 min before the earliest tornado damage. About 40 min before the tornado struck, a vortex pattern with cyclonic rotation was recognizable at 5–8 km, but the wind field in the storm below 4 km was quite uniform. The base of the flow disturbance as well as the vortex descended and intensified during this early period prior to tornado touchdown. Vortex diameter was as large as 5 km when initially detected but decreased to less than 1 km at the surface when the tornado was on the ground. Maximum tangential speeds of the vortex were generally 10–25 m s−1, with the higher values measured just before and during the period of surface damage. Shears as large as 0.1 s−1 were measured. The vortex was tilted toward the left of the storm direction of travel, at an angle of about 30° from vertical. While the tornado was in progress the vortex was a highly organized, intense singularity in an otherwise smooth wind field near the ground; but in the upper third of the storm, at heights of 10–14 km, the disturbance was greatly enlarged, disorganized and unstable.
