Dual-Polarization: ZDR, CC And KDP

Between 2011 and 2013, every WSR-88D in the United States was upgraded to Dual-Polarization. Instead of sending only horizontally oriented pulses, the radar now sends and receives both horizontal and vertical pulses at the same time. By comparing the two, the radar learns about the shape, size, variety and orientation of what it is seeing, not just how much energy comes back. That is how the radar can tell rain from hail, snow from rain, and weather from tornado debris.

Why Shape Matters

Small raindrops are nearly spherical. As drops grow past about 1 to 2 millimeters, air resistance flattens their bottoms, so large drops look like hamburger buns: wider than they are tall. Hail tumbles, so on average it looks round. Snowflakes are flat but have low density. Debris is random and irregular. Each of these gives a different "fingerprint" in the horizontal and vertical returns.

What Is ZDR On Radar?

Differential Reflectivity (ZDR) compares the horizontal and vertical reflectivity: ZDR = Zh - Zv, in decibels.

• ZDR near 0 dB: targets are, on average, round. Examples: drizzle, small drops, tumbling hail, dry snow aggregates.
• Positive ZDR (1 to 4+ dB): targets are wider than tall. Examples: large raindrops, melting hail with a water torus.
• Negative ZDR: targets are taller than wide, such as vertically aligned ice crystals in strong electric fields near storm tops.

Useful patterns:

• A ZDR column is a tower of positive ZDR rising above the freezing level in a storm updraft. It marks large drops being lofted by a strong updraft and often appears before a storm intensifies or produces hail.
• A hail core often shows very high reflectivity with ZDR near zero.
• Along the forward flank of supercells, a ZDR arc of high values marks size sorting of big drops by strong low-level shear.

What Is Correlation Coefficient On Radar?

Correlation Coefficient (CC, sometimes called rho-hv) measures how similarly the horizontal and vertical pulses behave from pulse to pulse within a gate. It ranges from 0 to 1.

• 0.97 and above: uniform targets, almost certainly pure rain or pure snow.
• About 0.85 to 0.95: a mix of types, such as rain and hail, melting snow, or wet snow in the melting layer.
• Below about 0.8: non-meteorological targets: birds, insects, ground clutter, smoke, chaff, or tornado debris.

CC does not tell you what the targets are, only how diverse they are. Pair it with reflectivity and ZDR.

The Tornado Debris Signature

When a tornado lofts leaves, insulation, wood and dirt, those irregular objects produce a compact area of low CC (often below 0.8), collocated with a strong velocity couplet and moderate to high reflectivity. This Tornado Debris Signature (TDS) confirms that a damaging tornado is on the ground, even at night or when it is wrapped in rain. It does not predict tornadoes; it confirms them after debris is lofted.

What Is KDP On Radar?

As a pulse travels through heavy rain, the horizontal wave slows slightly more than the vertical one, because flattened drops present more water in the horizontal. The difference in phase accumulates with distance; this is the differential phase (PhiDP). Its rate of change with range is the Specific Differential Phase (KDP), in degrees per kilometer.

• KDP is closely tied to the liquid water content, so it is an excellent heavy-rain indicator.
• It is not affected by hail (tumbling hail is nearly round), by partial beam blockage, or by radar calibration errors.
• High KDP (above about 2 degrees per km) signals very heavy rain and flash flood potential.
• KDP is noisy in light rain, so it is most useful in heavy precipitation.

How Does Radar Show The Melting Layer?

Where snow melts into rain, the radar sees a band of wet snowflakes: high reflectivity (the bright band), elevated ZDR, and lowered CC (about 0.90 to 0.95). Dual-pol makes the Melting Layer easy to locate. It appears on lower tilts as a ring around the radar, whose distance tells you the height of the melting level. In winter, watching that ring shrink toward the radar shows cold air arriving and a change from rain to snow.

Putting It Together

• Heavy rain: high Z, high ZDR, high CC, high KDP.
• Hail: very high Z, low ZDR (near 0), slightly lowered CC, modest KDP.
• Dry snow: low to moderate Z, low ZDR, high CC.
• Melting layer: enhanced Z, higher ZDR, lowered CC.
• Tornado debris: low CC and low or noisy ZDR under a velocity couplet.
• Birds and insects: low CC, very high or noisy ZDR, low Z.