The “Owl Horn” Radar Signature in Developing Southern Plains Supercells

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.