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Matthew R. Kramar
,
Howard B. Bluestein
,
Andrew L. Pazmany
, and
John D. Tuttle

Abstract

During spring 2001 in the Southern Plains, a recurring, hitherto undocumented reflectivity signature that the authors have called the “Owl Horn” signature (because the radar reflectivity pattern resembles the profile of the Great Horned Owl) was observed on a mobile, X-band radar display. The reflectivity signature was always located at the rear side of a developing supercell, spanned the entire rear side of the storm, and was always seen on low-level plan position indicator (PPI) scans. It lasted on the order of only 5–10 min and was not an artifact of the radar.

A study of the Owl Horn signature was undertaken using the Tracking Radar Echoes by Correlation technique (TREC) to estimate the wind field. TREC has previously been applied to clear-air and hurricane environments, and to the internal motions of severe storms, but not to their evolution. The characteristics of the signature are presented, and then, through the application of TREC to the radar reflectivity data (Doppler wind data were not available in 2001) collected during May and June 2001, the horizontal wind field was estimated around and in the Owl Horn signature.

Instances of the Owl Horn in numerical model storm simulations were investigated. The numerical simulations were used to identify conditions under which the signature occurs, the process by which it is created is discussed, and its dependence upon the environmental wind shear is examined. Results indicate that the hodograph shape and magnitude influence the production of the Owl Horn signature. Supercell-magnitude shear is required, and some curvature—particularly low-level curvature—is essential to the production of the feature. The Owl Horn signature is formed when horizontal vorticity is tilted into the vertical by expanding outflow through a positive feedback mechanism with the outflow.

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Michael M. French
,
Howard B. Bluestein
,
Louis J. Wicker
,
David C. Dowell
, and
Matthew R. Kramar

Abstract

On 16 May 2003, two ground-based, mobile, Doppler radars scanned a potentially tornadic supercell in the Texas Panhandle intermittently from ∼0200 to 0330 UTC. The storm likely was tornadic, but because it was dark, visual confirmation of any tornadoes was not possible. A damage survey was completed after the storm moved through the area. The final conclusion of the damage survey prior to this analysis was that there were two tornadoes near Shamrock, Texas: one that formed prior to 0300 UTC and one that formed at or after 0300 UTC. High-resolution, mobile, Doppler radar data of the supercell were compared with the damage survey information at different times. The location of the first tornado damage path was not consistent with the locations of the low-level circulations in the supercell identified through the mobile, Doppler radar data. The damage within the first path, which consisted mostly of downed trees, may have been caused by straight-line winds in a squall line that moved through the area after the passage of the supercell. The mobile, Doppler radar data did not provide any supporting evidence for the first tornado, but the data did support the existence of the second tornado in Wheeler County on the evening of 15 May 2003. Ground-based, mobile, Doppler radar data may be used as an important tool to help to confirm (or deny) tornado damage reports in situations in which a damage survey cannot be completed or in which the survey does not provide clear evidence as to what phenomenon caused the damage.

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Michael M. French
,
Howard B. Bluestein
,
David C. Dowell
,
Louis J. Wicker
,
Matthew R. Kramar
, and
Andrew L. Pazmany

Abstract

On 15 May 2003, two ground-based, mobile, Doppler radars scanned a supercell that moved through the Texas Panhandle and cyclically produced mesocyclones. The two radars collected data from the storm during a rapid cyclic mesocyclogenesis stage and a more slowly evolving tornadic period. A 3-cm-wavelength radar scanned the supercell continuously for a short time after it was cyclic but close to the time of tornadogenesis. A 5-cm-wavelength radar scanned the supercell the entire time it exhibited cyclic behavior and for an additional 30 min after that. The volumetric data obtained with the 5-cm-wavelength radar allowed for the individual circulations to be analyzed at multiple levels in the supercell. Most of the circulations that eventually dissipated moved rearward with respect to storm motion and were located at distances progressively farther away from the region of rear-flank outflow. The circulations associated with a tornado did not move nearly as far rearward relative to the storm. The mean circulation diameters were approximately 1–4 km and had lifetimes of 10–30 min. Circulation dissipation often, but not always, occurred following decreases in circulation diameter, while changes in maximum radial wind shear were not reliable indicators of circulation dissipation. In one instance, a pair of circulations rotated cyclonically around, and moved toward, each other; the two circulations then combined to form one circulation. Single-Doppler radial velocities from both radars were used to assess the differences between the pretornadic circulations and the tornadic circulations. Storm outflow in the rear flank of the storm increased notably during the time cyclic mesocyclogenesis slowed and tornado formation commenced. Large storm-relative inflow likely advected the pretornadic circulations rearward in the absence of organized outflow. The development of strong outflow in the rear flank probably balanced the strong inflow, allowing the tornadic circulations to stay in areas rich in vertical vorticity generation.

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