Azimuthal shear (AzShear) is a derived radar field that estimates the rate at which radial velocity changes across neighboring beams at the same range, giving a smoother, distance-corrected measure of local rotation than a raw gate-to-gate reading provides.
What the algorithm is doing
A raw velocity image is noisy, and gate-to-gate differences are strongly range-dependent. Azimuthal shear addresses both problems by fitting a small linear model to the radial velocity values inside a local neighborhood, usually a small window centered on each grid point, and reporting the estimated local gradient in the across-beam direction. The result is a field with units of inverse seconds, reflecting how rapidly velocity is changing per unit arc distance.
Because the fit uses several neighboring gates rather than a single pair, the resulting AzShear value is less sensitive to a single noisy gate and is easier to compare across ranges after remapping to a common ground grid. The National Severe Storms Laboratory uses this approach inside the Multi-Radar Multi-Sensor system to produce a rotation field that can be tracked in time and merged across overlapping radars.
How forecasters use it
The most familiar application is the MRMS rotation-track product. AzShear values are computed for the low-level and mid-level layers, then accumulated over a rolling window to draw a swath of peak rotation across the ground. That swath highlights where a rotating storm has been most intense and is one of the fastest ways to see a possible tornado track in near real time.
In a warning context, a persistent AzShear maximum that descends from the mid-levels to the low-level layer and tightens in area is the kind of trend that supports elevated concern for tornado development. Storm-scale operators use it alongside the base and storm-relative velocity images, dual-polarization signatures, and spotter reports rather than as a substitute for any of them.
Important limits
AzShear inherits every limitation of the underlying radial-velocity field. It cannot see motion perpendicular to the beam. It is affected by dealiasing errors, range folding, and non-meteorological targets. It is not a physical vorticity measurement, only an estimate of the along-beam projection of local rotation.
AzShear also can flag rotation that never reaches the ground. Elevated mesocyclones, mid-level circulations that never descend, and shallow shear layers can all produce meaningful AzShear signatures without ever producing a tornado. Rotation-track swaths are best read as evidence of where the parent storm was rotating strongly, not as a confirmed path of surface damage; that confirmation still requires ground truth from spotters, from damage surveys, or from a dual-polarization tornado debris signature.
Where it fits in the product stack
Azimuthal shear sits between the raw velocity products and the higher-level algorithms. Radial velocity and storm-relative velocity are the operator's primary hand-read images. Gate-to-gate shear is a compact single-pair number that drives TVS-style detectors. AzShear is the smoothed, range-corrected derived field that MRMS rotation-track products are built from, and that many research studies use to characterize rotation intensity in a way that can be compared across storms and across radars.
Treat AzShear as complementary to those other products rather than as a replacement. A well-organized warning workflow reads all of them together, along with the reflectivity structure and dual-polarization data, and reaches a judgment that no single field could support on its own.