Strengths

Unlike the Storm Cell Identification and Tracking (SCIT) and Mesocyclone Detection Algorithm (MDA) from the WSR-88D Radar Product Generator (RPG), the Linear Least Squares Derivative (LLSD) algorithm that computes Azimuthal Shear is not centroid based. Instead, it is able to provide the forecaster a single image showing both the location history and the intensity trend of rotation.

Like all MRMS products, the use of multiple radars is more robust than single-site radar alone. It provides faster updates and helps the forecaster integrate data from multiple radars. It also compensates for cone-of-silence, beam broadening at far ranges, and terrain blockage.

Limitations

Output is only as good as the Azimuthal Shear data that went into it.

Data gaps (i.e., “heartbeat effect” or “strobing effect”) in Rotation Tracks are observed between product scan times (e.g., Figs. 1 and 2). This effect is most (least) noticeable for fast (slow) moving storms and when only one radar samples the storm. Some forecasters may consider this a strength because it can provide a sense of the speed of motion of the event.

Training storms may mask overlapping tracks. In these cases, shorter duration accumulation times are recommended.

Mesocyclone and tornadoes tracks (particularly the weaker ones) can be obscured within the noise generated by gust fronts, wind shear zones, the heartbeat effect, and other limitations noted here.

Quality Control

See the Azimuthal Shear product for quality control information.

Applications

Provides a history of the intensity and spatial coverage of strong storm circulations that may be associated with mesocyclones, tornadoes, and/or damaging winds.

Used to determine if a storm has intensified or decayed over time.

0–2-km Azimuthal Shear Tracks have shown enormous utility after events for guidance in immediately directing damage survey ground teams and aircraft, the Red Cross, and other first responders to areas most likely affected by tornadoes.

Example Images

For a Supercell Thunderstorm

Fig. 1: (a) 120-min Low-Level (0–2-km AGL) and (b) Mid-Level (3–6-km AGL) Rotation Tracks ending at
2345Z on 14 April 2012 for multiple tornadic supercell thunderstorms.

For a Mesoscale Convective System

Fig. 2: 120-minute 0–2-km Rotation Tracks ending at 0212Z on 22 July 2014 for a MCS over North
Dakota and Minnesota. The white ovals illustrate tracks associated with tornado reports, while the blue
oval highlights smaller-scale circulations along the leading edge of the bowing segment. Artificially-large
values of Azimuthal Shear surrounding the radar sites are not associated with the storm system.

Fig. 3: Same as in Fig. 2, except for 120-minute 3–6-km Rotation Tracks ending at 0214Z.

Fig. 4: Azimuthal Shear associated with a MCS over North Dakota and Minnesota at 0202Z on 22 July
2014. Instantaneous Azimuthal Shear for (a) 0–2 km and (b) 3–6 km and 60-minute Rotation Tracks for
(c) 0–2 km and (d) 3–6 km are shown.