14 Dual-Polarization Products
Dual-polarization (“dual-pol”) radar transmits and receives both horizontally and vertically polarized pulses. Comparing the two reveals the shape, size distribution, and uniformity of the targets, which lets you distinguish rain from hail, snow from sleet, and weather from non-weather clutter.
14.1 Correlation Coefficient (CC / ρhv)
Correlation coefficient measures how consistently shaped the targets in a sample volume are, on a scale from 0 to 1.
- High CC (≈ 0.97–1.0): uniform targets. Pure rain, snow, or hail of consistent type. This is “clean” meteorological scatter.
- Lower CC (≈ 0.8–0.95): a mix of target types or shapes, such as rain mixed with hail, the melting layer, or tumbling irregular particles.
- Very low CC (< 0.8): chaotic, irregular targets. Usually non-meteorological returns (birds, insects, ground clutter, chaff), or, critically, tornado debris.
A small patch of very low CC co-located with a strong velocity couplet and a reflectivity hook is a Tornado Debris Signature (TDS): radar confirmation that a tornado is lofting debris, which means it is on the ground. This is one of the most important confirmations in modern radar interpretation. See Storm Signatures.
14.2 Differential Reflectivity (ZDR)
Differential reflectivity compares horizontal vs. vertical returned power, telling you about target shape:
- Positive ZDR: targets wider than they are tall, such as large raindrops (which flatten as they fall). Big positive ZDR marks regions of large drops.
- Near-zero ZDR: roughly spherical targets, such as small drops, or tumbling hail that averages out to no preferred orientation.
- ZDR columns: vertical plumes of enhanced ZDR mark where supercooled liquid is being lofted in a strong updraft, a useful nowcasting clue that a storm is intensifying.
14.3 Specific Differential Phase (KDP)
KDP is derived from how the phase difference between the two polarizations changes with range. It’s largely immune to beam blockage and calibration issues, and is excellent for estimating rainfall rate. High KDP indicates lots of liquid water (heavy rain) and helps flag flash-flood threats.
14.4 Putting dual-pol to work
A quick mental checklist when you suspect something is not ordinary rain:
| What you see | Likely interpretation |
|---|---|
| High dBZ, high CC, high ZDR | Heavy rain, large drops |
| High dBZ, lower CC, near-zero ZDR | Hail mixed in |
| Patch of very low CC under a couplet | Tornado debris (TDS) |
| Low CC, low dBZ, near radar | Birds, insects, or ground clutter rather than weather |
| Ring of lower CC + higher ZDR aloft | Melting layer (bright band) |
Dual-pol is what turns “there’s a red blob” into “that’s a hail core” or “that’s debris.” Combined with reflectivity and velocity, it completes the picture for reading Storm Signatures.