Radar cannot see a tornado. It sees precipitation, and a tornado is not made of precipitation. What radar can see is what a tornado has picked up, and that turns out to be nearly as good: a tornado debris signature is the closest thing forecasters have to remote confirmation that a tornado is on the ground and destroying property, without anybody standing outside looking at it.
What is a debris ball?
On the reflectivity display, a debris ball appears as a compact, roughly circular blob of very high reflectivity at the tip of the hook echo. Reflectivity measures how much energy is bounced back, and a mass of splintered timber, insulation, roofing and sheet metal lofted into the air returns a great deal of it. On that display alone, though, a debris ball is ambiguous, because torrential rain and large hail also return high reflectivity. Reflectivity tells you something substantial is up there. It does not tell you what.
How does the radar know it is debris and not rain?
Through correlation coefficient, which came with the dual-polarization upgrade to the national radar network. The radar sends pulses both horizontally and vertically and compares how similarly the two returns behave. Raindrops are uniform: similar in size, similar in shape, all falling the same way. They return a correlation coefficient close to 1. Debris is the opposite of uniform. A sheet of roofing, a two-by-four and a fistful of insulation tumbling end over end return wildly different values, and the correlation coefficient collapses.
So the signature is a pairing. High reflectivity in the same place as a sharp drop in correlation coefficient, co-located with rotation, means lofted debris. That is a tornado debris signature, usually shortened to TDS. Our piece on the NEXRAD network covers the dual-polarization upgrade that made this possible.
What does a TDS actually prove?
It proves the tornado is on the ground and destroying things, which is a genuinely different claim from what rotation alone gives you. A velocity couplet shows a circulation aloft. Plenty of circulations never reach the surface, and issuing warnings on rotation alone is part of why so many warnings come to nothing. A TDS is physical evidence: something got picked up, so something got destroyed. That confirmation can be relayed to broadcasters and emergency managers in real time, at night, or when the tornado is rain-wrapped and nobody can see it.
What can it not tell you?
It cannot tell you the tornado's strength. It is tempting to read a large, bright debris ball as an intense tornado, and the correlation is loose at best. A TDS depends on how much there was to pick up. A violent tornado over open farmland may produce little debris signature, and a weaker tornado through a dense subdivision may produce a striking one. What it measures is what the tornado hit, not how hard it was turning.
It also cannot see a tornado that is too far from the radar. The radar beam rises with distance, so at long range it is sampling well above the ground and passes over the debris entirely. A TDS is a close-range signature.
Where has this been seen most clearly?
The 2011 Joplin tornado produced a textbook example as it crossed the city, with debris later recovered tens of miles downwind. It is on the Joplin archive page along with the rest of that event's record.