Mesocyclone as a visible feature.

[Darren Addy]

I believe the Latin meso ("middle") refers to scale, not location within a thunderstorm. Cyclonic rotation ranges from the very large (i.e. wave cyclones) to the very small (tornadoes and even smaller suction vortices). Mesocyclones fall somewhere in the middle of that spectrum.

While you are not incorrect, Bob, the AMS would seem to point out that a mesocyclone is more than that:

(It should be noted that a mesocyclone is not just any cyclone on the mesoscale; it refers specifically to cyclones within convective storms.)

The AMS definition does shed more light, but also still highlights my main question. Again, correct me if I'm wrong, but I get the impression that stormchasers seem to think that a supercell (by definition) must contain a mesocyclone. But the AMS definition specifically says "Mesocyclones are frequently found in conjunction with updrafts in supercells." (emphasis mine) I'd also note that "in conjunction with updrafts" is not the same as "the entire updraft" (stated in the Scientific American quote from the OED, above). From this, I would tend to lean toward the position that the Scientific American definition (incorrectly?) broadened Fujita's original definition.

I also think that, as Glen pointed out, the AMA separation of the mesocyclone signature from the mesocyclone itself, is more precise. I believe that the NOAA Storm Spotter Glossary should be changed (and "mesocylone signature" added) to make the differentiation clear.

Darren Addy
Kearney, NE

 

[Darren Addy]

Problem is, not everyone agrees on what the definitions should be - so there is precendence for describing things in a number of different ways, with fairly little agreement. My perspective is whether the rotation features a radar senses are visible to the naked eye - and I would contend that in general they are not. Then there is the whole argument about the role of the mesocyclone in tornado processes - a good read on the topic (albeit a bit technical at times) is provided by Chuck Doswell here:

http://www.cimms.ou.edu/~doswell/Tornadostuff.html

It is the nature of science to try and classify things - problem is nature has no interest in such classifications so divisions are often fuzzy.

Glen, this was a VERY educational post and I'm quoting part just for brevity's sake.

From Doswell's paper, it certainly appears that it is helpful to agree on the terms being used. For example, he says:

Assuming that a supercell is a storm with a mesocyclone...

In his essay What is a tornado? he again alludes to the problem of agreeing upon even the definition of a supercell:

We must expand our data base to include information about the storm that produced the event: was it a supercell (using whatever definition we can arrive at as a consensus)?

So, I would guess that it is no surprise that we find similar disagreements and inconsistencies with regard to the term "mesocyclone". In Glen's Doswell link, Doswell defines mesocyclone:

mesocyclone can be defined as a vortex meeting some vorticity (or shear) magnitude and temporal/spatial continuity thresholds.

I may need to chew on that one for a bit. :?

Thanks to all who have contributed to the discussion thus far. I have learned a lot!

Darren Addy
Kearney, NE

 

[rdale]

Supercell Definition (AMS)

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..

 

[Doug Lee]

I believe part of the discontinuity in terminology stems from the fact that many chasers are "schooled" in meteorology, but they do not have to follow the official definitions, while NWS mets generally do. Sometimes NWS mets do get out of the office and chase, and I imagine they would use the same language in the field as they would in the office. Because they do not compose a large sector of the chasing community, their usage of "official" language does not influence the whole group.

Does this mean chasers are wrong when they say they see a meso? Not necessarily. They are only doing what most people do when they talk about anything - say something using the fewest words. Instead of saying "I see signs of a meso," they say "I see a meso."

To illustrate the simplification that normally takes place between an observation and the expression of that observation, I will use a character from Stranger in a Strange Land, by Robert Heinlein. She was a "Fair Witness," a member of a group of people trained in objective observation. One of the main characters pointed to a house and asked her what color it was. She said, "It is white on the side that I can see." She did not do what most people would have done - assume the house was white on all sides.

We often tend to summarize our observations, and the conclusions we reach by summarizing are not always correct. I've looked at drawings sent in by my client, and seen a large number of marks indicating the changes he wants made, and figured it would take me a long time to finish. Then 15 - 30 minutes later, I'm done.

 

[rdale]

"but they do not have to follow the official definitions, while NWS mets generally do"

However the "official" definition as included in the AMS Glossary says nothing about it not being visible, it's apparently just one person's website that included that information.

 

[Greg Stumpf]

Originally posted by rdale
\"but they do not have to follow the official definitions, while NWS mets generally do\"

However the \"official\" definition as included in the AMS Glossary says nothing about it not being visible, it's apparently just one person's website that included that information.

The original mesocyclone definition quote that started this thread was from the NOAA Storm Spotters Glossary. Is that not official?

Actually, if you talk to a lot of folks in the radar research community, they'll tell you that the mesocyclone is a vortex of a specific scale and strength associated with deep-moist convection, an updraft-downdraft pair, made "visible" by a remote sensor (radar). In some instances, there are observable cloud indications (e.g., a barberpole updraft). But in radar circles, the following convention is typically used:

radar signature <---> phenomena

mesocyclone <-------> rotating updraft-downdraft pair
TVS <------------------> intense smaller vortex (but not necessarily a tornado)

But what strength and dimensional criteria define the cutoff between a mesocyclone and "not a mesocyclone"? It's a rhetorical question I raise in the discussion of Stumpf et al (1998).

 

[rdale]

"The original mesocyclone definition quote that started this thread was from the NOAA Storm Spotters Glossary. Is that not official? "

I don't know, I guess I'd put the AMS at the top of the "official" list?

In any case, the Storm Spotters Glossary was compiled by one individual (with assistance) and I'll point out some parts of his introduction:

>
I have written the definitions in what hopefully passes as "layman's terms." They are written to be easily understood by the storm spotter, regardless of his or her meteorological background. At times I have sacrificed technical purity for simplicity, and the result may prompt a few moans from the technical purists. So be it; this glossary wasn't written for them.
===
The question arises as to just how far one should go into the technical realm of operational meteorology when compiling a glossary like this for storm spotters. The dilemma is thus: The spotters' thirst for knowledge is admirable, but how much of the technical jargon really needs to be understood by spotters in the field?
<

Since purity was sacrificed for readability, I'd have to vote that the AMS definition is the official meteorological definition.

- Rob

 

[Greg Stumpf]

Originally posted by rdale
...I'd have to vote that the AMS definition is the official meteorological definition.

If you read the list of contributors to the NOAA Glossary, I'd argue that it is quite official.

But also bear in mind that definitions are "living" entities, and are subject to re-definition, and debate (even among experts), as phenomena are better understood. Case in point, do you want to accept the official definition of a tornado as defnied by either of these two glossaries, or do you want to open your mind about other possibilities? Like what was done here:

http://www.cimms.ou.edu/~doswell/atornado.html

 

[Darren Addy]

The spotters' thirst for knowledge is admirable, but how much of the technical jargon really needs to be understood by spotters in the field?

Assuming that we are replacing "spotters" with "chasers", I contend that there are two fields of "jargon" that really needs to be understood:

1) The official terminology
2) The chaser terminology

They don't have to be the identical but one would have to understand which version of the term was being used in the information that one was currently reading. (I'm coming at this from the perspective of a newbie interested in learning the forecasting necessary to put one in the right place at the right time.) Misunderstanding a use of a term is not going to contribute to a proper understanding of the whole.

I understand that "shorthand" language can be used, particularly among people who share the same level of knowledge. But a newcomer is going to need to understand the shorthand, in addition to the fuller picture of what is behind the shorthand. This is what led to my original question. I was led to believe that the "meso" was one thing (listening to the chaser shorthand) and this did not jive with what appeared to be the "official" definition.

If a newbie chaser is simply interested monitoring where the knowledgable stormchasers are going and following them, they don't need to know anything. However (IMHO) if a newbie chaser is interested in becoming a knowledgable storm chaser, then they very much need to know the terminology. It is the only way to understand what the books, articles and online information is saying.

Darren Addy
Kearney, NE

 

[Glen Romine]

Well, it's a nice thought Darren, but odds are you couldn't even find a group of mets to agree on the definition of a mesocyclone - so finding common ground between mets and chasers is going to be nearly impossible.

From the met side, the official AMS definition of a mesocyclone is problematic in that it assigns specific criteria to be met in order to call it a mesocyclone - which cannot be determined visually and often would not even be remotely sensed if it is a borderline case (if the storm was very close to the radar the better sampling there may confirm it to be a mesocyclone, whereas if it is further away it might not appear to meet the criteria because of beam spreading smearing the radar signature - so would it then NOT be a mesocyclone?). I didn't check, but this is probably discussed in the paper Greg referred to earlier.

From the chaser side, I think when chasers see the cloud associated with a supercell thunderstorm updraft - the general tendency is to refer to this as a mesocyclone. I think this comes from the most basic of explanations of the structure of a supercell - which refers to a single long-lived rotating updraft - and so the assumption is that that radar reference to a mesocyclone must be from sampling of this single rotating updraft associated with the supercell. Regions of rotation qualifying as a mesocyclone can exist at mid-levels and sometimes even low-levels of a storm - and these may well be completely independent circulations from each other. This doesn't help in clearing up the potential confusion. Some detailed studies have shown the mesocyclone signature generally follows the development of a clear slot occlusion - which obviously is a visible feature often seen by chasers. So, a chaser reporting seeing a mesocyclone who is instead seeing a clearly occluded updraft is probably right about the storm indeed have a mesocyclone at that time that would potentially meet the radar-based definition of one.

Glen

 

[Darren Addy]

I'm fine with that, Glen. The goal of my question was not to force everyone to agree on some arbitrary definition, but to learn the various ways it might be possible to understand (or use) the term. Once one knows that, it should be fairly easy to know which definition is appropriate from the context (article or discussion). This thread has helped me to do just that (at least with one term)!

You guys may now take it for granted, but you speak a completely different "dialect" of English!

Here's an example from a NOW discussion in late March:

The Tornado Warning continues for the central IL supercell as of 3:33 CST. The inflow environment of the storm is becoming contaminated but it is heading into a ~1500 CAPE environment near Chicago per the 21z SPC Hourly Mesoscale Analysis page. However, the 0-1 km shear is not nearly as favorable ~15 knots and LCL's are much higher that you would like for tornadic development ~1600 m. If this gets any closer to the Chicago area that may be for the best!

LOT was 79 over 52 as of 21z so low-level moisture appears to be lacking still.

This newbie to the forum realized that he had a lot to learn after attempting to parse that post! :cry:

Darren Addy
Kearney, NE