Single Radar AzShear compared to Merged AzShear

The Fort Rucker, AL (KEOX) radar, well south of the storm of interest, shows strong azimuthal shear values up to 0.028 S-1.  The maximum values are slightly offset to the north of the surveyed tornado track (pink line) likely due to distance from the radar.  A mature hook echo is apparent in the reflectivity with a corresponding tight couplet in the 0.5 degree SRM.

The Maxwell, AL (KMXX) radar, located closer and to the west the tornadic supercell, was also detecting strong azimuthal shear up to 0.021 S-1.  An area of negative shear was located immediately to the south of the SRM couplet, along a weaker anticyclonic inbound-outbound interface.  This corridor of blue shades to bright white was a bit distracting from the more important red to white positive shear associated with the cyclonic rotation. The maximum positive shear values were located exactly on the track of the surveyed tornado.  Reflectivity and SRM KMXX data were similar to KEOX.

Merged 0-2 km azimuthal shear provided an accurate location of the meso/tornado and washed out the negative shear signal from the KMXX data. The shear values are lower than the single radar data.  Merged 3-6 km azimuthal shear is displaced to the north of the tornado path (pink line).

The merged 0-2 km rotation track has a very nice color scale (yellow to red to cyan) — key features really stand out.

CC plume or TDS were present on both radars. – Roy



AzShear picks up circulation along QLCS

The AzShear product did a nice job of highlighting and pinpointing a developing circulation along the convective line, as shown in the 2 images below from and 2144z and 2146z from kmxx. Zooming out, it’s a great indicator of where to focus on the line for a potential spin-up (bottom image from 2150z)


More Observations of AzShear

It seems to me the merged product, particularly the 0-2km, highlights nearly every updraft whereas the AzShear seems to do a good job discriminating, and highlights the cell most likely to have strong rotation. In this case the couplet shows up in the western side of the AzShear display and is associated with an embedded rotating updraft, while the AzShear does not highlight the cells further southeast which show weaker rotation. However, the merged product (bottom right) shows bullseyes on all these cells which would make it difficult to triage which storms need attention.


Initial impressions of AzShear

Two things jumped out at me right off the bat regarding AzShear. First, the couplet in AzShear really jumps out and your attention becomes focused on this area for further interrogation. It’s a great tool to further investigate the rotation couplet and storm structure in the base data (top picture).  Second, The couplet really jumps out in the AzShear product much better when compared to the merged products (bottom figure). Very impressive!


MRMS AzShear and SAILS scans

The MRMS AzShear product produces multiple areas of rotation as it tries to ingest 0.5 degree SAILS scans. The animated gif above is from WFO TOP late Tuesday afternoon as a tornadic circulation moved north of I-70. The AzShear algorithm easily identifies the area of rotation, however during SAILS cuts it breaks the rotation into multiple areas rather than moving the entire circulation forward with each time step. This may be due to only the lowest scan being available for AzShear during SAILS cuts, but it makes the product look disjointed and unrealistic. While this may be easy to ignore in discrete supercell mode, the AzShear output could be confusing if SAILS were deployed in situations that may have numerous areas of weaker rotation and/or shear zones, such as a QLCS/MCS.

Dave Grohl

AzShear Depicting Highest Tornado Threat

The 0-2km AzShear product is depicting the highest tornado threat along the convective line. There is a maxed-out area of 0-2km AzShear along this line.

This makes sense given extreme low-level shear values in this area with easterly winds at the surface lengthening the hodograph. According to the RAP, 0-1km SRH values are in excess of 400 m here.

Ron Dayne

Azimuthal Shear Noise Behind Convective Line/ProbSevere Analysis

Dealiasing problems in northwest Arkansas and southwest Missouri carried over to the operational version of the azimuthal shear rotation tracks.  The azimuthal shear in the lower left panel uses a new dealiasing suppression algorithm, which effectively eliminated the noise. Fortunately, the noise with the current operational version of azimuthal shear was in a stratiform area of precipitation well behind the convective line, making it easy to ignore this data.  Sampled winds in the region of the erroneous rotation tracks were very high — up to 80+ kts.

By focusing my attention on the azimuthal shear associated with the leading edge of the QLCS, and analyzing Probsevere data and trends, I was able to anticipate a strengthening of the line.  The Probwind component was only 32% at 1934Z, then steadily increased, reaching 89% at 2012Z.  At 2030Z there was a LSR of trees and power lines down in Mountain Home,  Arkansas. Probtor increased to 65% at 2014Z, and as this storm shifted north into southern Missouri, there was a report of a blown out store front in Hartville, MO.  It has not been confirmed whether or not there was a tornado. However, the increasing Probtor values increased my confidence for severe straight line winds that are often associated with strong mesovorticies.  Depending on the event, Probtor trends and values could be used as a confidence builder to increase my warning wind speeds from 60 to 70 mph or 70 to 80 mph. Roy



Azimuthal Shear in SGF CWA

The mid level azimuthal shear product is doing a good job pinpointing the strongest updrafts along the line in southern Missouri, while the low level azimuthal shear product shows a lot of elongated shear zones, which one would expect. One issue i noticed is the low level azimuthal shear product seems to leave certain features in place over a period of several minutes while at the same time propagating these features downstream, thus resulting in the same feature being plotted twice in two different places. See the loop at the bottom for an example


AzShear and QLCS

A little later in the day on March 3, 2019, a complex QLCS was seen on KMXX.

The base reflectivity at 0.5 degrees (left) shows several different features along the complex that need investigation given the favorable environment for tornadoes. The corresponding base velocity product (upper right) show several locations where shear is present. However, the single radar AzShear product highlights the portions along the line that look the most threatening.


ProbTor during Tornado Warning

The NWS office in Little Rock started issuing tornado warnings on storms along the line so I pulled up the ProbTor product to see how it performed. For the storm near Knoxville, I can’t figure out what’s going on but I’ll attempt to document it here. For each time, I plot CPTI top left, LZK SRM top right, low-level AzShear bottom left, and spectrum width bottom right.

At 1902, the algorithm has a 60% ProbTor based on high LLAzShear (0.021 /s). I don’t see any high values on the low-level AzShear product, but perhaps I’m missing something.

At 1904, a bullseye of high LLAzShear pops up just east of Knoxville. ProbTor is still 60%, which now makes sense to me. This identified shear region is not in the right place for a tornado and is just convergence along the line, but the ProbTor uses what it has and seems to generate an understandable ProbTor.

At 1906 the LLAzShear bullseye east of Knoxville persists,  but now the ProbTor drops to 19% with a LLAzShear max value of (0.009 /s). I still sample 0.020 /s in the bullseye. Spectrum width is not horrible (~7 kts) in the area of the AzShear bullseye. The AzShear detection is obviously misleading for a tornado, but the ProbTor product does not seem to be performing as we’d expect it.

At 1908 the bullseye in AzShear goes away and the ProbTor drops even further to 6% (as expected). The persistence of the bullseye in AzShear with an associated significant drop in ProbTor is perplexing for this case! -Atlanta Braves.