Thursday, 7 October 2004: 10:45 AM
Using a mobile X-band radar during the Spring 2001 severe storm season on the Southern Plains, we encountered a curious, recurring reflectivity feature on the back side (with respect to storm motion) of developing supercell thunderstorms which we have called the Owl Horn signature (because it resembles the profile of the Great Horned Owl). In past forums, we have presented: (1) observational results of a cursory study in which the Tracking Radar Echoes by Correlation (TREC) technique was used to obtain an estimated wind field in and around the Owl Horn echo; and (2) initial findings concerning the structure of the "Owl Horn feature in numerical simulations using the Advanced Regional Prediction System (ARPS) model. It was seen in these simulations that the Owl Horn signature in reflectivity is situated atop narrow cold-air protrusions in the storm outflow. These protrusions were flanked by elongated bands of vertical motion, and elongated couplet bands of vertical vorticity. It was suggested that the vorticity bands may be a consequence of the cold-air protrusions, and yet at the same time, may help to advect the cold protrusions farther rearward in a positive-feedback mechanism.
In this study, we continue our earlier research through a series of sensitivity tests, to determine the environmental conditions conducive to Owl Horn formation in a numerical simulation. We then return to our initial simulation (using the composite Del City sounding from May 20, 1977) to calculate parcel trajectories and ultimately to make a vorticity budget analysis for the banded couplets of vorticity. The outflow configuration, as well as directional wind shear, are determined to be of direct consequence to the production of the echo. It is shown that the vorticity couplets are a result of the tilting of horizontal vorticity into the vertical as air is lifted over the outward-expanding outflow. The tilted vorticity is then stretched in the region of vertical motion ahead of the outflow, and the process is reversed on the storm-interior side of the supercritical head of the boundary.
These results allow us to explain the formation of the Owl Horn signature, present the forecasting utility of the feature, and, by comparison with the work of Xu (1992), suggest further research opportunities in gust front dynamics and supercell evolution.
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