Velocity from phase
By comparing the phase of consecutive pulses bouncing off the same volume of rain, the radar measures radial velocity to within 1 m/s. Red shows motion away, green shows motion toward the radar. A neat red-next-to-green couplet in a storm means rotation — and rotation tight enough to trigger a tornado warning.
Dual polarisation
NEXRAD upgraded to dual-pol in 2013. It transmits both horizontally and vertically polarised pulses and compares the returns. Spherical raindrops return both equally; flat raindrops (typical of heavy rain) return more horizontal; tumbling hail looks chaotic; tornado debris is highly anisotropic. The polarisation signatures separate these classes automatically.
Tornado debris signature
When a tornado lofts pieces of houses, trees and cars into its rotation, the polarisation correlation drops dramatically — a TDS. Forecasters use this to confirm a tornado is on the ground, not just suspected. It turned tornado warnings from hopeful to evidence-based.
Limits of velocity
Doppler measures only radial velocity — the component along the beam. A tornado moving perpendicular to the beam shows as zero velocity. This is why a network of radars matters: a storm visible from two angles can be reconstructed in 3D. The MRMS system fuses NEXRAD data nationally every two minutes.