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Some Synoptic Aspects and Dynamic Features of Vortices Associated with the Tornado Outbreak of 3 April 1974

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  • 1 Department of Geosciences, Purdue University, West Lafayette, Ind. 47907
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Abstract

A study has been made of certain aspects of the “jumbo” tornado outbreak of 3 April 1974. The region characterized by a) surface dew point temperature Td⩾55°F, b) 500 mb winds ⩾60 kt, and c) veering winds ⩾15° in the 500 mb to 850 mb layer, contained 85 of the 93 tornado tracks determined in Fujita's initial aerial survey.

Analyses of three storm cells with hook echoes, associated with tornado families (a total of nine tornadoes) in southeast Indiana, showed that 1) the velocity of a storm cell and accompanying hook are approximately the same, 2) tornadoes may move across and to the left of parent thunderstorm track at angles up to 21°, where 3) this angle tends to be less for longer tornado tracks, and 4) the tornado speed can differ from the speed of the hook. usually beginning close to it but ending several miles to the north of the hook (for left-moving tracks). Selected radar sequences show the prominent features of developmental, mature, and dissipative stages for each storm cell.

A motion picture study provided the first conclusive documentation of multiple (suction) vortices occurring within a parent vortex system, with as many as four suction vortices that showed a behavior similar to laboratory vortices studied by Ward (1972).

A revised form of Fujita's suction vortex model is presented based on the movie of the Parker tornado. A life cycle is described, in which suction vortices a) generate and accelerate on the left flank, b) achieve maximum intensity at the rear, c) weaken and decelerate on the right flank, and d) dissipate on the leading edge (with respect to the parent vortex). Velocity vectors identified in the multiple vortex system are the translational velocity of the parent vortex VT; and the tangential velocities of the parent vortex Vt, of the suction vortex vT with respect to the center of the parent vortex, and of the suction vortex vT with respect to its own center. Estimates of 50 m/s for maximum vT and 25 m/s for VT give maximum wind speeds relative to the ground of at least 75 m/s.

Abstract

A study has been made of certain aspects of the “jumbo” tornado outbreak of 3 April 1974. The region characterized by a) surface dew point temperature Td⩾55°F, b) 500 mb winds ⩾60 kt, and c) veering winds ⩾15° in the 500 mb to 850 mb layer, contained 85 of the 93 tornado tracks determined in Fujita's initial aerial survey.

Analyses of three storm cells with hook echoes, associated with tornado families (a total of nine tornadoes) in southeast Indiana, showed that 1) the velocity of a storm cell and accompanying hook are approximately the same, 2) tornadoes may move across and to the left of parent thunderstorm track at angles up to 21°, where 3) this angle tends to be less for longer tornado tracks, and 4) the tornado speed can differ from the speed of the hook. usually beginning close to it but ending several miles to the north of the hook (for left-moving tracks). Selected radar sequences show the prominent features of developmental, mature, and dissipative stages for each storm cell.

A motion picture study provided the first conclusive documentation of multiple (suction) vortices occurring within a parent vortex system, with as many as four suction vortices that showed a behavior similar to laboratory vortices studied by Ward (1972).

A revised form of Fujita's suction vortex model is presented based on the movie of the Parker tornado. A life cycle is described, in which suction vortices a) generate and accelerate on the left flank, b) achieve maximum intensity at the rear, c) weaken and decelerate on the right flank, and d) dissipate on the leading edge (with respect to the parent vortex). Velocity vectors identified in the multiple vortex system are the translational velocity of the parent vortex VT; and the tangential velocities of the parent vortex Vt, of the suction vortex vT with respect to the center of the parent vortex, and of the suction vortex vT with respect to its own center. Estimates of 50 m/s for maximum vT and 25 m/s for VT give maximum wind speeds relative to the ground of at least 75 m/s.

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