Improving NSSL Azimuthal Shear Calculations Using an Updated Derivation and Range-Based Corrections

Azimuthal shear is a widely used radar velocity-derived product produced by the National Severe Storms Laboratory (NSSL).  In addition to being used operationally by National Weather Service forecasters as part of the product suite available from the NSSL Multi-Radar Multi-Sensor (MRMS) system, this algorithm serves as the basis for the popular rotation track product used to track the progression of areas of rotation over space and time. While azimuthal shear produces values fairly similar to theoretical calculations, it has historically suffered from artificially inflated values at short range from the radar and degrading accuracy with increasing range from the radar. Recent work has improved the linear least-squares derivative (LLSD) method that derives azimuthal shear from Doppler velocity fields, producing a more physically-consistent calculation near the radar. Additionally, to improve medium- and far-range calculations, a range-based correction was derived from a model that produces azimuthal shear from simulated velocity fields of Rankine vortices of known strength, size, and distance from the radar. Combined, these updates produce a more complete and accurate portrayal of low- and mid-level rotation.