Dual-polarization

Dual-polarization radar, explained

Correlation coefficient, differential reflectivity and KDP tell rain from hail, debris, birds and insects.

AleruaOctober 25, 20262 min read

The El Reno hook echo with the ground clutter filter off, showing no gaps — The El Reno hook, May 31, 2013. Reflectivity shows its shape but not what it is made of.

Correlation coefficient radar image: mostly light blue rain with a patch of orange and red where debris lowers the value — Rain reads near 1 (light blue). The orange and red patch at the tip of the hook is about 0.6.

Differential reflectivity radar image: rain in green and yellow with a curved band of gray-blue near zero at the hook — Rain reads high (green to yellow). Gray and blue, near zero, is hail or debris tumbling at random.

Since the early 2010s the US radars have sent pulses polarized both horizontally and vertically and compared the two echoes. The comparison says something about the shape and mix of whatever is in the beam. It answers what the color scale can’t: whether a red core is hail or heavy rain, whether an echo is birds, whether a curl is debris.

The ten views in Alerua

On US radars, Alerua has ten views of each scan, picked from the Product menu.

Correlation coefficient

Correlation coefficient asks whether the horizontal and vertical echoes agree. Rain and snow are uniform, so they agree and the value sits near 1 (light blue). Anything mixed up scrambles it: debris, insects, birds, the ground, or hail melting unevenly. Under 0.8 is usually not rain.

Differential reflectivity

Differential reflectivity compares how strongly the echo returns horizontally and vertically. Raindrops flatten as they fall, so they reflect more horizontally than vertically and read positive, and the bigger the drops, the higher the value. A tumbling hailstone looks round on average and reads near zero. A red core in reflectivity with near-zero differential reflectivity is a good sign of hail.

KDP and the volume views

KDP measures how much one polarization falls behind the other as the pulse passes through rain. Heavier rain, with more and larger drops, lags it more, which makes KDP a clean gauge of how hard it is raining, one that hail barely changes. Composite reflectivity, echo tops and VIL are built from the whole volume scan instead of one tilt, so they show what the lowest beam can’t: a core aloft, how tall a storm is, and an estimate of the water in it.

How rain, hail, debris and birds differ

How each one tends to look
What it is	How it shows up
Rain	Reflectivity anywhere from blue to orange. Correlation coefficient near 1. Differential reflectivity positive, and higher for bigger drops.
Hail	A red or purple core in reflectivity. Differential reflectivity near zero. Correlation coefficient lowered where the hail melts unevenly.
Debris	Gaps in the echo, or a debris ball when the filter is off. Correlation coefficient under 0.8. Differential reflectivity near zero.
Birds and insects	A faint, round bloom, usually under 25 dBZ. Correlation coefficient under 0.8.

No single view settles it: read them together, at the same spot and time.

Echoes that aren’t weather

A radar reports anything that sends energy back, and on a clear night that can look like a storm. This scan is from Dodge City, Kansas, with no rain nearby.

Radar reflectivity at Dodge City, Kansas, on a clear night: a faint round bloom of blue and green centered on the radar, most likely birds and insects

The same Dodge City radar scan with Filter ground clutter on: nearly empty

The faint round bloom is most likely birds and insects, the usual look of a clear night in migration season: a median echo of 14 dBZ and a correlation coefficient around 0.65. Rain drifts with the wind from frame to frame, takes the shapes storms take, and reads near 1. Birds, insects and most clutter read low, and that is what Alerua’s filter uses: it hides gates under 0.8. Why radar shows rain when it isn’t raining shows the other non-weather echoes.

In Alerua the switch is Settings, then Filter ground clutter. It is on by default; switch it off to see everything the radar returned.