13 Velocity
If reflectivity tells you where the rain is, velocity tells you what the air is doing. It is the most important product for spotting rotation, and the key to identifying tornadoes, mesocyclones, and damaging winds.
13.1 What velocity measures (and what it doesn’t)
Doppler velocity measures only the component of motion along the radar beam, toward or away from the radar:
- One color family (commonly greens/blues) = motion toward the radar.
- The other (commonly reds/yellows) = motion away from the radar.
- Near-zero (gray/neutral) = no motion along the beam, or motion purely across the beam.
The radar cannot see motion perpendicular to its beam. A wind blowing straight across the beam reads as zero. Always interpret velocity with the radar’s location in mind. NeoWeatherwall shows you the active site so you know which way “toward” and “away” point.
13.2 The signature that matters: the velocity couplet
Rotation shows up as a velocity couplet, two adjacent areas of opposite colors right next to each other:
- Inbound (toward) directly beside outbound (away), with a tight boundary between them, means air is spinning.
- The tighter and stronger the couplet, the more intense the rotation.
When a couplet is small, intense, and sits beneath a supercell’s hook echo, it marks a mesocyclone. A very small, very intense couplet can indicate a tornadic vortex signature (TVS).
The cyclonic (counter-clockwise, in the Northern Hemisphere) sense is what you want for most tornadoes: with the radar behind you, inbound on the left, outbound on the right. Reverse that and you may be looking at anticyclonic rotation or simply divergent/convergent flow, so check the geometry.
13.3 Convergence, divergence, and outflow
Couplets also mark flow other than rotation:
- Convergence (inbound meeting outbound along the beam) marks boundaries, gust fronts, and updraft inflow.
- Divergence aloft over a storm top indicates a strong updraft; divergence near the surface marks a downburst / damaging wind core.
13.4 Range folding and aliasing
Velocity has two well-known problems, both covered in detail in Artifacts & Limitations:
- Velocity aliasing: when winds exceed the radar’s Nyquist velocity, fast inbound suddenly “wraps” to display as outbound (and vice-versa), producing abrupt, unrealistic color flips. NeoWeatherwall can dealias velocity to correct this so couplets read true.
- Range folding: returns from beyond the radar’s unambiguous range get misplaced and are often shown as a “purple”/no-data flag.
13.5 Storm-relative vs. base velocity
Base velocity is what the radar measures directly, and it includes the storm’s overall movement. Storm-relative velocity subtracts the storm’s motion so the rotation stands out, which can make a mesocyclone far easier to spot when a storm is moving fast. Know which one you’re viewing before judging rotation strength.
With reflectivity and velocity together you can do most operational radar interpretation. Dual-pol products add the finishing detail: what the targets actually are.