Use of dual-polarization signatures in diagnosing tornadic potential.

Christina C. Crowe
National Weather Service, Huntsville, Alabama

Christopher J. Schultz
Department of Atmospheric Science, UAHuntsville, Huntsville, Alabama

Matthew Kumjian
Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, and NOAA/OAR/National Severe Storms Laboratory, Norman, OK

Lawrence D. Carey
Department of Atmospheric Science, UAHuntsville, Huntsville, Alabama

Walter A. Petersen
NASA Wallops Flight Center, Wallops, Virginia


     Recent research has suggested that the combination of differential reflectivity (ZDR) and specific differential phase (KDP) can be useful in the assessment of low-level wind shear within a thunderstorm, a critical factor in tornado formation. The two main polarimetric signatures indicated for this diagnosis include an arc of ZDR along the right inflow edge of the thunderstorm near or collocated with a large gradient in horizontal reflectivity, ZH, (indicative of regions of preferentially large raindrops) and a region of enhanced KDP located deeper into the forward flank precipitation shield than the ZDR arc (indicating that the smaller drops are preferentially advected farther from the updraft core by the low-level winds).

     Three severe weather events in North Alabama were examined to assess the utility of these ZDR and KDP signatures in determining the potential for tornadic activity. The cases were: 26 October 2010, when many storms indicated tornadic potential from a standard reflectivity and velocity analysis, but very few storms actually produced tornadoes; 28 February 2011, a broken line event that transitioned from a tornadic to high wind threat; and 27 April 2011, when multiple rounds of tornadic storms, associated with quasi-linear convective systems (QLCS) and supercells, thrashed the Tennessee Valley. All three cases displayed strong evidence of ZDR arcs and horizontal separation of KDP and ZDR during tornadic periods. In addition, non-tornadic storms showed consistent signatures of overlapping dual-pol fields. While some variations remain between supercell, broken line, and QLCS tornadoes, common signatures among all storm types indicate a potentially broad application of this type of signature recognition.

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