Meso Icons On Non-Severe Storms

[Mike Krzywonski]

I've seen meso icons on GRL3 & Baron appear on storms that weren't even severe-warned. How accurate are these icons?

 

[Shane Adams]

I've seen meso icons on GRL3 & Baron appear on storms that weren't even severe-warned. How accurate are these icons?

Algorithms on most consumer-grade software are set up to trip on any type of significant shear; you will often see markers along squall lines where gust front shear is maximized. The little circles are fun, but you really need to know the storm's nature from either visual observations or warning statements.

The shear markers are more for visuals than anything, kind of a selling point for the makers. Perhaps the scariest thing about this is I've seen chief meteorologists in tornado-prone markets relying solely on the 'shear markers' while doing live break-ins and not really expounding on their own observations...makes me wonder if they're even capable.

 

[Robert Dewey]

I thought the mesocyclone data for GRL3 comes from the NWS? Isn't it encoded into the NCR product or something?

Either way, Shane makes a good point - you really need to know the environment and interrogate the storm via first-hand reports, or by using other radar products to get a better feel for the storm itself.

 

[Darrin Rasberry]

I've seen meso icons on GRL3 & Baron appear on storms that weren't even severe-warned. How accurate are these icons?

Isn't there some kind of scale for potency of mesos on the GRL3? When I was watching the outbreak I noticed tons of mesos but only ones colored like red or purple had TOR or SVR attached.

Remember the presence of a meso isn't enough to force a warning for either SVR or TOR no matter how "strong." The conditions specific to those warnings have to be met. If there's no wall cloud or rotation in a wall cloud (or, God forbid, a tornado on the ground), no 3/4 inch hail, and no 60+ gusts or the recognized radar prediction/detection thereof, there's probably no warranting of an SVR/TOR.

 

[Mike Krzywonski]

Remember the presence of a meso isn't enough to force a warning for either SVR or TOR no matter how "strong." The conditions specific to those warnings have to be met. If there's no wall cloud or rotation in a wall cloud (or, God forbid, a tornado on the ground), no 3/4 inch hail, and no 60+ gusts or the recognized radar prediction/detection thereof, there's probably no warranting of an SVR/TOR.

Correct, but what is the likelihood of a non-severe storm having rotation? Are these true mesos?

 

[Gene Moore]

Allow me to answer this in a different way. It has long been accepted that it takes a severe storm to produce a tornado. This old school concept is totally absurd. Unfortunately, when any storm produces a tornado, it defaults to a severe storm by definition....which confuses the true issue, was the parent storm severe?

That is....what's the requirement for a cell to contain (for example) large hail and winds over severe criteria etc.... before the tornadogenesis? If I may, the answer is, it has nothing to do with tornadogenesis.

Of course this is outside the realm of common radar discrepancies which have been addressed.

Gene Moore

 

[Darrin Rasberry]

Correct, but what is the likelihood of a non-severe storm having rotation? Are these true mesos?

I would wager so. As Dr. Moore stated, rotation does not a SVR make! Likewise, a storm could be severe without any rotation even showing up at all on the GRL3 - there was a few hours ago a TOR in LA that didn't even show rotation on the radar.

The percentages likely change given the situation - I'm not qualified enough to state any more than a few observations here, but from what I've seen the majority of rotation markers are not warned. One of the storms in advance of that huge line a week and a half ago had purple rotation with a TVS and wasn't warned (and was never warned before it was swallowed up by the line). Now I doubt seeing that is unlikely, but I'd say greater than half of rotation icons that I've seen in lines and even with supercells don't have a warning attached to them.

 

[Bobby Prentice]

I've seen meso icons on GRL3 & Baron appear on storms that weren't even severe-warned. How accurate are these icons?

Not very accurate. Here is the definition of supercell:

supercell—An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min).

Most rotating updrafts are characterized by cyclonic vorticity (see mesocyclone). The supercell typically has a very organized internal structure that enables it to propagate continuously. It may exist for several hours and usually forms in an environment with strong vertical wind shear. Supercells often propagate in a direction and with a speed other than indicated by the mean wind in the environment. Such storms sometimes evolve through a splitting process, which produces a cyclonic, right-moving (with respect to the mean wind), and anticyclonic, left-moving, pair of supercells. Severe weather often accompanies supercells, which are capable of producing high winds, large hail, and strong, long-lived tornadoes. See also convective storm, thunderstorm, splitting convective storm, cell, bulk Richardson number.

There are a few problems with using the WSR-88D weather radar mesocyclone detection algorithm (MDA) for supercell identification. Supercell mesocyclones must be vertically deep and persistent, but the MDA will trip for shallow (one elevation scan) rotation during only one volume scan. I won't get into all the details, but in order to be certain that your "meso" is associated with a bonafide supercell, watch to see if it has vertical and temporal continuity (lasts multiple volume scans).

 

[Greg Higgins]

As our local mets have preached now for several years, the radar data incorporates algorithms and, as indicated above, are oft times inaccurate due to the nature of how the algorithm was developed (and the "developers" are always working to improve these formulas!). These are only tools and it takes many years of radar interpretation and weather knowledge to accurately "read" the radar screen.

Greg Higgins

 

[Greg Stumpf]

Supercell mesocyclones must be vertically deep and persistent, but the MDA will trip for shallow (one elevation scan) rotation during only one volume scan.

Not true. The MDA requires two vertically-associated features from adjacent elevation angles, and that the depth is at least a) 3 km or b) 25% of the average depth of the 10 strongest SCIT detections. It also requires detections on two adjacent volume scans for persistence.

That being said, there are other issues with the MDA that are currently being investigated, 1) decreased data quality in the clear air since 2006 causing false shear detection, and 2) a small bug in the code for low-core detections.

 

[Bobby Prentice]

Not true. The MDA requires two vertically-associated features from adjacent elevation angles, and that the depth is at least a) 3 km or b) 25% of the average depth of the 10 strongest SCIT detections. It also requires detections on two adjacent volume scans for persistence.

That being said, there are other issues with the MDA that are currently being investigated, 1) decreased data quality in the clear air since 2006 causing false shear detection, and 2) a small bug in the code for low-core detections.

Thanks for the clarification of this and other issues Greg. Nothing like hearing about this from someone directly associated with the program.

A minimum of two elevation angles is needed for the 3D correlation of 2D features to identify a circulation or a mesocyclone.
<snip><snip>

</snip></snip>

 

[Dave Gallaher]

The NMD filtering now available on GR makes for a major reduction in those false icons.

 

[James Aydelott]

I've seen chief meteorologists in tornado-prone markets relying solely on the 'shear markers' while doing live break-ins and not really expounding on their own observations...makes me wonder if they're even capable.<!-- / message --><!-- sig -->

You can see this in Dallas, quite often. Ugh.