Correlation coefficient and tornado debris signatures

Correlation coefficient (CC) tells you how alike the targets in a radar pixel are. Rain is very uniform; tornado debris is not — so a sudden drop in CC inside a rotating storm can show a tornado is on the ground.

What CC measures

NEXRAD radars are dual-polarization: they send pulses that are both horizontal and vertical. Correlation coefficient compares the two echoes. When every target in a pixel is similar in size and shape — raindrops, snowflakes — the two echoes match closely and CC is near 1.0. When targets are a jumble of shapes and sizes, CC drops.

The tornado debris signature (TDS)

A tornado that is damaging buildings and trees throws leaves, insulation, wood and other debris into the air. Debris has every shape and size, so CC falls sharply. Forecasters look for all three of these in the same spot:

1. A low-CC area — typically well below the surrounding rain,
2. co-located with a strong velocity couplet (see velocity radar), and
3. with meaningful reflectivity, often at the tip of a hook echo.

A TDS is confirmation, not prediction: it means a tornado is already on the ground and causing damage. Larger and taller debris signatures generally go with stronger tornadoes.

Things that look like debris but aren't

• The melting layer — a ring of lower CC where snow melts into rain.
• Hail cores — lower CC with very high reflectivity.
• Edges of storms and weak echoes — CC is noisy where the signal is weak.
• Biological targets and clutter — very low CC, but no rotation.

CC and debris in TALON

TALON Pro shows each radar's super-resolution CC and automatically checks for debris signatures inside detected rotation — requiring low CC, enough reflectivity and a strong couplet together — and marks them on the map in pink.

Quick answers