Gate-to-gate shear (GTG) is the radial-velocity difference between two immediately adjacent radar sample gates, used as a compact measure of how sharply the wind is changing across the smallest distance the radar can resolve.
What GTG is measuring
Every radar volume scan is a grid of small sample volumes, each one a gate. Adjacent gates in the same radial line, or in the same range ring across neighboring beams, sit only a few hundred meters apart at close range. When the wind changes direction sharply between two of those neighboring gates, the difference in their radial velocity is large. GTG reports that difference directly, without smoothing or fitting.
The tightest radial-velocity gradients in a convective storm sit inside tornado-scale vortices and inside the strongest core of a mesocyclone. GTG is designed to isolate those tightest gradients so a small vortex does not get averaged out against the slower surrounding rotation of a broader circulation.
How forecasters use it
Operationally, GTG feeds the Tornadic Vortex Signature (TVS) algorithm on the WSR-88D. When adjacent gates show a large enough velocity difference in the right geometric pattern, the algorithm flags a TVS icon at that location. The traditional operational rule of thumb requires strong inbound values next to strong outbound values, with the exact thresholds tuned to storm environment and to range.
GTG is also useful by eye. A tornado-warning meteorologist scanning a rotating storm looks for the tightest inbound-outbound pair the radar is showing, checks whether it persists across successive volume scans, and compares it against dual-polarization signatures and spotter reports. A strong GTG value that intensifies and descends is the kind of trend that supports upgrading a warning.
Important limits
Range is the largest source of confusion. At 30 nautical miles the gates are close together and the beam is narrow, so a real tornado-scale vortex fills only a couple of gates and produces a large GTG value. At 100 nautical miles the same vortex may fit inside one gate, or may be smeared across several beams, and the reported GTG value drops sharply even though the physical circulation is unchanged. Comparing GTG readings between storms at very different ranges is not meaningful without correcting for beam geometry.
GTG is also noisier than smoothed derived products like azimuthal shear. A single spurious gate from a dealiasing error, non-meteorological scatterer, or velocity fold can produce a large gate-to-gate difference that is not associated with any real circulation. Read GTG as a trigger for a closer look at the storm rather than as a standalone verdict, and always confirm its persistence over multiple volume scans before treating it as a real signal.