A velocity couplet is a pair of adjacent inbound and outbound radial velocity returns on a radar image, indicating a localized change in wind direction across a short distance that is most often produced by rotation or strong shear.
What the pattern is showing
Radial velocity assigns a color to each gate that reflects motion toward or away from the radar. In steady flow, adjacent gates look similar. In a rotating vortex the wind on one side of the axis is moving one way and the wind on the other side is moving the other way, so two neighboring gates end up with sharply different colors, one inbound and one outbound. That paired inbound-outbound signature is the couplet.
The strength of the couplet is usually described by the sum of the peak inbound and peak outbound values, and its tightness by the distance between them. A tight, strong couplet with a large velocity difference across only a few gates indicates a concentrated rotation. A broad, weak couplet spread over many gates indicates diffuse shear or a wider circulation.
How forecasters read it
The most common use is mesocyclone identification. A persistent, well-defined couplet in the mid-levels of a supercell marks the parent mesocyclone that anchors the storm's rotation. A couplet that tightens, deepens, and descends toward the low levels through several volume scans is the classic radar evolution of a rotating storm organizing toward possible tornadogenesis.
Warning decisions also lean on where the couplet sits. A couplet centered on the low-level inflow, embedded within a hook echo, and paired with a correlation coefficient drop on dual-polarization data is the signature that supports the highest-confidence warning language. A couplet high in a storm without a companion low-level circulation warrants closer watching but does not by itself justify the same call.
Important limits
The couplet's apparent strength depends on range. Close to the radar, the beam is narrow and the gates are small, so a real rotation of a given size is resolved cleanly and looks strong. Far from the radar, the beam is wide and the gates are large, so the same true rotation is smeared across a broader volume and looks weaker or may not be resolved at all. Two couplets of the same reported velocity difference at different ranges do not represent the same physical vortex.
The couplet also only shows the beam-parallel component of the rotation. A vortex with an axis oriented along the beam is easier to see than one oriented across it. Non-meteorological signatures, misaligned dealiasing, and storm-motion errors on storm-relative velocity displays can all produce apparent couplets that do not correspond to real circulations. Read the couplet in context with the reflectivity structure, dual-polarization data, and spotter reports before treating it as a hazard call.