159 Radar Observations of a Small Tornado on 5 April 2017 during VORTEX-SE

Tuesday, 29 August 2017
Zurich (Swissotel Chicago)
Anthony W. Lyza, Univ. of Alabama, Huntsville, AL; and K. Knupp

In anticipation of a significant tornado event, the Verification of Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) deployed across north-central and northeastern Alabama on 5 April 2017 as part of intensive operations period (IOP) 3b. Despite a lack of the expected discrete supercellular activity and large, long-tracked tornadoes, teams gathered information on widespread quasi-discrete deep convective cells, some of which exhibited rotation and supercellular characteristics. As operations unfolded, a tighter circulation was noted near the intersection of Alabama, Georgia, and Tennessee, which then progressed northeastward into areas west of Chattanooga, Tennessee, near Nickajack Lake on the Tennessee River. While the peak rotational velocity of the circulation was only 15.5 m/s, a maximum of very high spectrum width up to 13.5 m/s was observed in the middle of the rotational couplet. And although no formal polarimetric tornado debris signature (TDS) was observed, a depression in correlation coefficient as low as 0.88 was observed. This depression remained shallow but was temporally consistent with the motion of the circulation over a 10-min period. A ground survey conducted by the University of Alabama in Huntsville’s Severe Weather Institute – Radar and Lightning Laboratories (UAH-SWIRLL) found a narrow, concentrated path of weak damage that consisted mostly of uprooted and snapped trees and damage to outbuildings ranking up to EF0-intensity on the Enhanced Fujita (EF) scale. This damage path was approximately 3.6 km long and up to 75 m wide, and featured signs of convergence along it. This presentation serves to present detailed evidence of this damage having been caused by a small tornado. A particular emphasis is given to the radar signatures from this event, which are compared to recent literature developments in the radar detection of small tornadoes. These signatures include sub-TDS-intensity correlation coefficient depressions and spectrum width maxima located with rotational couplets. These observations are compared to the observed damage from the ground survey to form a preliminary conclusion of the tornadic nature of the damage.
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