Use of Dual-Polarization Radar to Assess Low-Level Wind Shear in Severe Thunderstorm Near-Storm Environments in The Tennessee Valley
Crowe, Christina C; Schultz, Christopher J; Kumjian, Matthew ; Carey, Lawrence D; Petersen, Walter A

The dual-polarization upgrade of the National Weather Service network of S-band radars is currently underway, and the incorporation of polarimetric information into the real-time forecasting process will enhance the forecaster s ability to assess thunderstorms and their near-storm environments. 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, an often critical factor in tornado formation. In an environment with strong low-level veering of the wind, ZDR is greatest along the right inflow edge of the thunderstorm near a large gradient in horizontal reflectivity (indicative of primarily large raindrops falling with few small drops), and take the shape of an arc. Meanwhile, KDP, which is proportional to liquid water content and indicative of many smaller drops, is greatest deeper into the forward flank precipitation shield than the arc of maximum ZDR, as the smaller drops are advected further from the updraft core by the low level winds than the larger raindrops.

Three severe weather events that occurred 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, when tornadic storms were present early, but then the tornado threat transitioned to a high wind threat; and 27 April 2011, when multiple rounds of tornadic storms ransacked the Tennessee Valley. The latter event had multiple modes of tornadic development, including Quasi Linear Convective Systems and classic supercells.

The overarching goal of this study was to compare the dual-polarization features from these events to those of previous work and to determine whether the ZDR and KDP signatures can be useful indicators of the potential for tornadic activity associated with the low-level wind shear in the near-storm environment.