An Observational Study of the West Lafayette, Indiana, Tornado of 20 March 1976

Abstract

An observational study of the West Lafayette, Ind., tornado of 20 March 1976 has been made that presents photographic evidence of tornado formation accompanied by suction ?debris? vortices in the surface boundary layer that produced a cycloidal debris pattern. Furthermore, four case studies of cycloidal debris analysis form this pattern using aerial photography support the multiple-suction vortex model proposed by Agee et al. (1975). Average values of the tangential speed of a suction vortex according to loop shift estimates ranged from 26.4 to 44 m s?1 and maximum values according to loop width estimates ranged from 44.0 to 61.6 m s?1 as the tornado grew to its most intense stage. The aerial and ground surveys of the damage track also showed that the cycloidal debris pattern corresponded to a region of F3 and F4 wind-speed damage. Photographic evidence also documented a tail-cloud formation similar to the Fargo tornado (Fujita, 1959) that rotated cyclonically through the northeast to northwest sector of the tornado cyclone with a tangential speed of 16 m s?1 at a distance of 3 km from the tornado funnel. Analysis of cloud photography further showed the features of the tornado system at various stages of intensity. Notable was the formation of a broad descending bowl-shaped cloud accompanied by multiple ?condensation? funnels that corresponded to the region of strongest tornado intensity with F4 structural damage. This study also illustrates that multiple-suction debris vortices in the surface boundary layer can occur with only one condensation funnel present. This event is recognized as a similar but slightly different and smaller multiple-vortex phenomenon than that required for multiple-condensation funnels that fully extend through the planetary boundary layer. Features unique to cyclodial debris patterns and the role of suction vortices in producing certain types of damage are also brought out in this study. Also a concentrated region of damage along a pathline of the southerly flow into the multiple-vortex region may be associated with a strong vorticity feeder band similar to that reported by Golden and Purcell (1975). Several such feeder bands may concentrate the vorticity into the core of the tornado cyclone. The tail-cloud phenomenon appears to be related to this flow feature in the tornado cyclone wind field. Radar data from Marseilles, Ill., and Grissom Air Force Base, Ind., were analyzed for a portion of the lifetime of the parent thunderstorm system to determine the relationship between the path of the tornado cyclone and the Sadorus, Ill., and West Lafayette tornado tracks. Finally, a pressure drop of 44.6 mb was recorded in the path of the tornado cyclone at the time the West Lafayette tornado funnel was developing.