An Analysis of Tornado Debris Signatures in the 30 June - 1 July 2014 Quasi-Linear Convective System Tornado Outbreak

On 30 June - 1 July 2014, a tornadic quasi-linear convective system (QLCS) moved through northern Illinois and northern Indiana producing 29 confirmed tornadoes and numerous areas of significant straight-line wind damage.  Damage surveys, along with Weather Surveillance Radar - 1988 Doppler (WSR-88D) and terminal Doppler weather radar (TDWR) data from northern Illinois and northern Indiana (KLOT, TMDW, and KIWX), were used to identify 39 mesovortices that formed during the lifespan of the QLCS.  According to the damage surveys, eleven of the mesovortices were tornadic, producing EF0 to EF1 damage.

Since the NWS dual-polarization radar upgrade from 2011 to 2013, the identification of tornado debris signatures (TDSs) has become common practice in both operations and post-storm analysis.  While the scientific literature is rich in papers on polarimetric TDS identification, the emphasis has largely been focused on supercell tornadoes producing EF-3 or greater damage (e.g., Ryzhkov et al. 2005; Kumjian and Ryzhkov 2008; Schultz et al. 2012 a,b).  Previous studies utilized a cross-polar correlation coefficient (ρhv) threshold of < 0.8 to discriminate between hail and tornadic debris at S-band wavelength (< 0.7 at C-band).  Some confirmed tornadoes from this case, as well as other areas of known damage that were not confirmed as tornadoes or not surveyed, featured ρhv values that met the < 0.8 criteria for S-band debris detection, but other signatures (including confirmed tornadoes) featured ρhv values in the 0.8 - 0.95 range.  While melting hail is one additional common cause for lowered ρhv values aside from tornado debris, the wet-bulb zero height during this event was around 4.7 km AGL, as measured by aircraft soundings (AMDAR) from Chicago-Midway International Airport (KMDW).  The lowered ρhv values were generally recorded under 1.75 km AGL, with most at or below 0.75 km AGL.  Given the high wet-bulb zero height observed, it is unlikely that hail was present at the height of the lowered ρhv signatures observed during this event.

In this presentation, an analysis of TDSs associated with numerous mesovortices during this QLCS tornado outbreak will be offered.  Observed TDS characteristics of interest include depth and diameter details for each observed TDS, merging of multiple TDSs into one broad signature, and additional tornadoes forming within debris fallout and subsequent tightening of associated ρhv signatures.  Implications for real-time operational debris detection are discussed.