Tornadic Debris Signatures in Iowa

Between 2011 and 2013, the National Weather Service WSR-88D Doppler radar network underwent a major upgrade to dual-polarization (dual-pol). Now, instead of sending out just one radio wave oriented in the horizontal, the radar simultaneously sends out a horizontal and vertically polarized wave. This enables the radar to take a cross-section of whatever particles it samples and assists meteorologists in determining their size, shape, and concentration. It also helps delineate which scatterers are meteorological (rain, hail, snow, etc.) or biological (birds, dust, and insects).

The dual-pol upgrade introduced three new products on top of the legacy reflectivity, velocity, and spectrum width data. The first, differential reflectivity (ZDR), simply calculates the difference between the horizontal and vertical channel reflectivity values. Positive numbers indicate objects oriented in the horizontal, negative values denote vertically oriented objects, and values near 0 signify spherical objects. The radar samples millions of particles multiple times within a single range bin, and correlation coefficient (CC) measures the similarity of these objects to one another. A value of 1 indicates uniformly shaped particles, while the closer one gets to 0, the more random the shape and size of the scatterers. Usually anything below 0.8 is non-meteorological in nature (the exception being large hail). Finally, differential phase shift (KDP) calculates the attenuation difference between the horizontal and vertical channels. Since rain drops become flattened as they fall and thus will attenuate the horizontal channel more than the corresponding vertical channel, KDP does an excellent job of locating regions of heavy rainfall.

One special phenomenon that has been observed on dual-pol radars with some tornadoes is the tornadic debris signature, or TDS. As the name implies, the radar is actually sampling the debris being lofted thousands of feet into the air by a tornado. Debris identification was possible before the implementation of dual-pol, but involved correlating a small but intense area of higher reflectivity values with a tight velocity couplet. Known then as a “debris ball”, it was difficult to determine in real-time and sampled on only a select few tornadoes. Now, the CC and ZDR products make locating a debris signature much easier. Debris will present a very low CC signal owing to their plethora of shapes and sizes. The tumbling nature of the debris will also result in a near 0 ZDR value since the objects “appear” circular to the radar beam. The colocation of the high reflectivity values, a tight velocity couplet, and low CC/ZDR values together form the text-book TDS. The stronger and closer a tornado is to a radar site, the more likely it is that the radar will display a TDS.

The Des Moines WSR-88D radar was modernized with dual-pol capabilities in September 2012.  A review of radar data for the 49 tornadoes that have been recorded in the NWS Des Moines service area (central third of Iowa) in the last two years turned up six definitive TDSs and four likely candidates whose radar characteristics did not quite fit the traditional TDS model and are still being investigated. Thus, TDSs were only found for 12% of the total number of tornadoes sampled by the radar (20% if the probable TDSs are included). All but one of these signatures were noted during the 2014 tornado season, which was significantly more active than 2013. Each TDS, like the tornadoes that produced them, was unique in its size, shape, and duration. However, many of the signatures behaved like a plume, originating from the tornado and spreading out over time.

There was little correlation between the strength/duration of the tornado and whether it produced a TDS. The Lake Panorama tornadoes of May 11, 2014 and the Zearing to Union tornado of June 30 were long-tracked tornadoes relatively close to the radar that caused substantial damage, yet failed to produce a TDS. Meanwhile, brief and weak tornadoes that hit didn’t strike any major objects produced TDSs. Four TDSs alone were sampled with just one storm system on June 30, 2014 in Adair, Madison, and Warren counties. The strongest and most persistent TDS was sampled on July 6, 2014 with a strong EF1 tornado over northern Tama County near Traer.

A prominent TDS (black circle) with a tornado north of Traer on July 6, 2014

Blog post by Kevin Skow