Radar can now confirm that a tornado is on the ground without anyone seeing it. The capability comes from transmitting in two orientations rather than one, and comparing what returns.
What the second orientation adds
Conventional radar transmits a pulse oscillating in a single plane and measures how much energy is reflected back.
Dual polarization transmits both horizontally and vertically oriented pulses, producing two measurements of the same target.
Comparing the two reveals the shape and consistency of what the beam encountered, which a single measurement cannot describe.
Why raindrops look uniform
Falling raindrops flatten slightly under air resistance, becoming wider than they are tall, and drops in a given volume are broadly similar to one another.
The two returns are therefore consistent with each other and consistent across successive pulses, giving a high correlation value.
Hail and mixed precipitation lower that value somewhat, since the targets are more varied in shape and orientation.
What a debris field looks like
Material lofted by a tornado includes insulation, timber, sheet metal, soil and vegetation, in every conceivable shape and size, tumbling with no common orientation.
Returns from that mixture are inconsistent between the two orientations and between pulses, producing a distinctly low correlation value in a small area.
When that signature appears at low levels, collocated with a tight rotation signature on the velocity display, it indicates a tornado lofting material from the surface.
Why confirmation changes the warning
A rotation signature alone indicates a storm capable of producing a tornado, and many such storms never do.
A debris signature indicates damage occurring at that moment, which justifies the strongest wording available and can trigger emergency notifications reserved for confirmed events.
It works in darkness, in heavy rain and in unpopulated areas, which are precisely the situations where ground reports are least likely to arrive.
What the technique cannot do
Debris must be lofted high enough to reach the radar beam, which sits well above the ground at distance, so tornadoes far from a radar may produce no detectable signature.
A tornado crossing open ground with little to pick up may generate too little debris to register, even at close range.
The signature also lags the tornado's arrival, since material must be lifted before it can be seen, which means it confirms rather than predicts.