Tornadogenesis by Nonprecipitation Induced Wind Shear Lines

James W. Wilson National Center for Atmospheric Research, Boulder, Colorado 80307

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Abstract

Five tornadoes occurred within a 40 min period on 18 May 1984 in eastern Colorado. The evolution of these tornadoes was documented by a single Doppler radar, research aircraft, mesonetwork and chase team. Three of these tornadoes were narrow (≈300 m), rotating dust columns extending from the surface to cloud base more than 5 km from the nearest precipitation. The Doppler-observed parent circulations were <2 km deep and <1 km in diameter. Tornadoes of this type do relatively minor damage and are frequently called gust front tornadoes or gustnadoes. It is believed this is the first Doppler radar documentation of this tornado type. In an operational environment, even at close radar range, it would be difficult to detect the parent circulation associated with these tornadoes. However, by closely monitoring wind shift boundaries and associated localized strong shear regions, preferred tornado areas can be identified.

The other two tornadoes were associated with condensation funnels and occurred near precipitation. The Doppler parent circulations were deeper and wider than the first three tornadoes but were relatively small compared to many of those reported in the literature. All five of these tornadoes occurred along two wind shift lines near the point where the lines intersected. These lines were of synoptic scale origin—a cold front and a trough line.

The low-level echo structure and wind field associated with the parent storm of the two larger tornadoes closely resemble those described in the literature for supercell storms. While the environmental vertical wind shear was less than normally expected for supercell storms, it is believed that the preexisting boundaries created the necessary vorticity and vorticity production mechanisms for tornadogenesis.

Abstract

Five tornadoes occurred within a 40 min period on 18 May 1984 in eastern Colorado. The evolution of these tornadoes was documented by a single Doppler radar, research aircraft, mesonetwork and chase team. Three of these tornadoes were narrow (≈300 m), rotating dust columns extending from the surface to cloud base more than 5 km from the nearest precipitation. The Doppler-observed parent circulations were <2 km deep and <1 km in diameter. Tornadoes of this type do relatively minor damage and are frequently called gust front tornadoes or gustnadoes. It is believed this is the first Doppler radar documentation of this tornado type. In an operational environment, even at close radar range, it would be difficult to detect the parent circulation associated with these tornadoes. However, by closely monitoring wind shift boundaries and associated localized strong shear regions, preferred tornado areas can be identified.

The other two tornadoes were associated with condensation funnels and occurred near precipitation. The Doppler parent circulations were deeper and wider than the first three tornadoes but were relatively small compared to many of those reported in the literature. All five of these tornadoes occurred along two wind shift lines near the point where the lines intersected. These lines were of synoptic scale origin—a cold front and a trough line.

The low-level echo structure and wind field associated with the parent storm of the two larger tornadoes closely resemble those described in the literature for supercell storms. While the environmental vertical wind shear was less than normally expected for supercell storms, it is believed that the preexisting boundaries created the necessary vorticity and vorticity production mechanisms for tornadogenesis.

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