Radar Signatures & Dual-Pol Terms

This is the page for what storm chasers and warning forecasters actually look for on radar: velocity couplets, debris signatures, ZDR columns, three-body scatter spikes. Each signature is a physical process you can learn to spot. Members break these down on live radar every event in the Xtreme Weather Discord (XWD); the basics behind the products are on Radar Fundamentals.

What is a Hook Echo? #

A hook echo (sometimes just called a 'hook' or 'tornado hook') is a hook-shaped appendage on radar reflectivity at the back-right side of a supercell, usually the southwest flank. It marks where precipitation is being wrapped around the mesocyclone by the rear-flank downdraft — meaning the storm has a tight, organized rotation. A well-defined hook with a tight tip is the classic radar signature of a tornadic supercell, and chasers often watch for the tip of the hook to confirm a tornado is on the ground.

What is a Velocity Couplet? #

A velocity couplet is adjacent inbound and outbound velocity values on radar, indicating rotation. Strong, tight, low-level couplets are the radar signature of a tornado.

What is a Gate-to-Gate Couplet? #

A gate-to-gate (G2G) couplet is when adjacent radar gates show extreme opposing velocities — say, +60 kt inbound right next to -60 kt outbound. The radar can't get a tighter rotation signature than that. G2G couplets are one of the highest-confidence radar indicators of a tornado, especially when paired with a TDS.

What is VROT? #

VROT (Rotational Velocity) is the maximum gate-to-gate velocity difference on the lowest available radar tilt — a direct measure of circulation intensity at low levels. NWS operational guidelines use VROT thresholds: ≥ 40 kt supports tornado potential; ≥ 70 kt is one of the thresholds used to consider a Tornado Emergency.

What is a TVS? #

A Tornado Vortex Signature (TVS) is an algorithm-detected, intense gate-to-gate velocity couplet — radar's automated flag for likely tornado-scale rotation.

What is AzShear? #

AzShear (Azimuthal Shear) is an MRMS-derived product that measures rotation across adjacent radar rays — essentially an automated rotation detector. High low-level AzShear values flag tornadic or near-tornadic circulations even when the storm is out of range for a good velocity couplet on a single radar. Used heavily in operational warning operations.

What is Correlation Coefficient? #

Correlation Coefficient (CC) measures how uniform a radar volume's targets are. CC near 1.0 = uniform (rain). Low CC inside a hook echo = mixed debris = a tornado debris signature.

What is a CC Drop? #

A CC drop (correlation coefficient drop) occurs when values fall below ~0.95 inside a storm. This indicates non-meteorological scatterers: tornado debris (TDS), large/irregular hail, biological targets, or ground clutter. A CC drop co-located with a velocity couplet is strong evidence of a tornado lofting debris.

What is a TDS? #

A Tornado Debris Signature (TDS) is a dual-pol radar signature where Correlation Coefficient (CC) drops sharply inside a hook echo, indicating non-meteorological debris is being lofted. Strong evidence of a tornado on the ground.

What is a Debris Ball? #

A debris ball is a small, intense reflectivity blob inside a hook echo, caused by tornado-lofted debris. Visual confirmation that a tornado is on the ground and producing damage.

What is a ZDR Column? #

A ZDR column is an elevated plume of high differential reflectivity (ZDR) extending above the environmental 0°C level inside a thunderstorm updraft. It indicates large, oblate raindrops being lofted by a strong updraft — a reliable signature of a significant rotating updraft and potential for large hail.

What is a Dual-Pol Signature? #

Dual-pol signatures are patterns in ZDR, CC, and KDP that reveal precipitation type and storm structure. Key signatures: ZDR column (updraft), CC drop (debris/hail/mixed phase), KDP column (heavy rain core), ZDR arc (size-sorting of large drops at storm's leading edge).

What is a BWER? #

A Bounded Weak Echo Region (BWER) is a notch of low reflectivity at low levels on a storm's inflow side, overhung by high reflectivity aloft. It indicates an extremely strong updraft ingesting precipitation-free air. BWERs are associated with violent supercells and large hail.

What is a Three-Body Scatter Spike? #

A Three-Body Scatter Spike (TBSS) is a radar artifact appearing as a faint spike behind a strong reflectivity core. Caused by radar energy scattering off large hailstones — a near-certain indicator of large hail aloft.

What is a TBSS? #

A Three-Body Scatter Spike (TBSS) is a radar artifact appearing as a radial spike of weak reflectivity extending below a hail core. It's caused by energy reflecting off large hail to the ground and back to the radar. A TBSS is a reliable indicator of 2"+ hail (golf ball or larger).

What is a Side Lobe? #

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