Correlation coefficient is a dual-polarization radar variable that measures how consistently the horizontal and vertical returns from a sampled volume look alike. Its scale runs from 0 to 1, where 1 means the scatterers inside the volume are essentially uniform in shape, size, and orientation, and lower values mean the volume contains a chaotic mix.
What CC is physically saying
A modern WSR-88D transmits pulses in both horizontal and vertical polarizations. Rain drops flatten as they fall, so their horizontal returns look different from their vertical returns in a predictable way. When every drop in the volume is doing the same thing, the two returns stay tightly correlated pulse to pulse, and CC sits near 1.
When the volume contains a mix of scatterers, such as rain with hail, melting snowflakes with rain, or tumbling debris of many sizes and orientations, the two channels stop tracking together and CC drops. CC is not measuring shape directly. It is measuring how consistent the shape mix inside the pulse volume is.
Reading CC values
Uniform rain and uniform snow typically produce CC above about 0.97. Mixed precipitation, such as rain and hail together, wet snow, or melting graupel, tends to fall in the 0.85 to 0.97 range, and the melting layer often shows a horizontal band of reduced CC that helps meteorologists locate the freezing level in stratiform rain.
Values below about 0.80 usually indicate non-meteorological targets or highly chaotic mixtures. Birds, insects, dust, smoke, wildfire ash, chaff, and ground clutter all read low. So does lofted tornado debris, which is why CC is the anchor for the tornado debris signature.
Tornado debris signature
A tornado debris signature, or TDS, is a small area of CC below roughly 0.80 that is co-located with a strong low-level velocity couplet and moderate to high reflectivity. The low CC is the tornado lofting boards, insulation, soil, and vegetation of many shapes and sizes into the same radar volume.
A TDS is powerful confirmation because it shows physical debris in the air. It has important limits, though. It cannot appear until a tornado has lofted debris high enough to be sampled by a lifted radar beam, which means weak, brief, or open-country tornadoes may never produce one. Distance from the radar, terrain, and beam height all affect whether a real tornado shows a TDS at all.
Where CC misleads
CC is noisy in weak signal, so isolated low pixels at the edge of a storm can be sampling artifacts rather than real changes in the target. It also grows less reliable at long range, where beam broadening blends dissimilar targets into the same volume and pulls the value down.
Interpret CC with reflectivity and velocity in view. Rain surrounded by a clean, high CC field, a debris ball with a tight velocity couplet in low-level scans, and a bright band of reduced CC across a stratiform region each tell a different story, and none of them can be read from CC alone.