A debris ball, less frequently known as a tornadic debris signature (TDS), is an area of high reflectivity on weather radar images caused by large debris being lofted into the air, usually associated with a tornado or tornado vortex signature (TVS). Debris balls can be a result of anthropogenic or biomass debris, and as such, are highly probable after a tornado, seen as a TVS on radar, crosses a dense forest or city. As a result of the strong winds required to damage structures and loft debris into the air, debris balls are normally the result of EF3 or stronger tornadoes on the Enhanced Fujita Scale. Weaker tornadoes may also not contribute to debris balls due to their mostly short–lived nature, and thus any debris may not be sampled by radar. However, not all tornadoes meeting such strength requirements exhibit debris balls, depending on their vicinity to structures. A debris ball on radar images can verify tornadoes 70–80% of the time.
Debris balls are seen on radar reflectivity images as a small, round area of high reflectivity values. Research conducted on debris balls that were noted during the April 25–28, 2011 tornado outbreak suggested that horizontal reflectivity from debris balls ranged from 51 to 72 dBZ. Reflectivity values also decreased with increasing height. Due to the irregular size, variable size, and tumbling nature of tornadic debris, debris balls typically produce a correlation coefficient (ρhv) less than 0.80. Differential reflectivity (Zdr) values associated with debris balls are normally near or below 0 dB. Debris balls are always associated with a strong TVS.
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