Mesocyclone as a visible feature.

[Darren Addy]

NOAA Storm Spotters Glossary says this under Mesocyclone:

Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone)."

Yet, storm chasers tend to use this term as a visually-observable thing. Is the definition by NOAA too narrow? Is the definiton changing? Or are stormchasers just using a technically imprecise term for a visible feature?

Darren Addy
Kearney, NE

 

[rdale]

Here's the official definition... I don't see any reason why it wouldn't be visible.

mesocyclone—A cyclonically rotating vortex, around 2–10 km in diameter, in a convective storm.
The vorticity associated with a mesocyclone is often on the order of 10-2 s-1 or greater. (It should be noted that a mesocyclone is not just any cyclone on the mesoscale; it refers specifically to cyclones within convective storms.) Mesocyclones are frequently found in conjunction with updrafts in supercells. Tornadoes sometimes form in mesocyclones. Persistent mesocyclones that have significant vertical extent are detected by Doppler radar as mesocyclone signatures. Tornado warnings may be issued when a mesocyclone signature is detected.

 

[Glen Romine]

The definition as you found it is correct - a mesocyclone is a radar signature of a circulation - not a visible feature. Many chasers who observe an updraft with apparent rotation either visible via time lapse or inferred from features such as striations often refer to these as mesocyclones but it is really the precipitation motions inside the storm that gives the true mesocyclone signature. By example, here is a striated updraft image. Not hard to convince yourself it is rotating:

http://216.241.44.49/2003-0914/Barrel%20up...20and%20DOW.jpg


The mesocyclone signature would appear on radar to be on the right half of the visible updraft. If you were seeing the radar display from the DOW in the image above, you would see strong inbounds in the clear slot occlusion region (tough to see in the image - but roughly in the middle of the updraft), and outbounds along the right side of the visible updraft. The left side is composed almost entirely of cloud only - so has almost no radar signature at all except with the most sensitive of radars.

Glen

 

[rdale]

I understand why every meso may not be visible, but what in the official definition says you can't see one? The official definition doesn't even use "radar" except when referring to the algorithm. Why (even using the stormspotter definition) would a meso not be visible?

 

[Darren Addy]

I'm curious rdale, where did you get your definition? I'm always looking for good source material. Thanks in advance!

Darren Addy
Kearney, NE

 

[Jeff Snyder]

I'm curious rdale, where did you get your definition? I'm always looking for good source material. Thanks in advance!

Darren Addy
Kearney, NE

Not to answer for RDale, but that definition is from the American Meteorological Society (AMS). From the given AMS definition, the mesocyclone certainly can be a visual feature. It seems to me that saying that striations (or other such visual features) are merely the visual representation or effects of the mesocyclone is like saying that we really don't see the tornado, we just see evidence of the tornado (e.g. condensation funnel, debris, dust, etc).

 

[Glen Romine]

From the given AMS definition, the mesocyclone certainly can be a visual feature. It seems to me that saying that striations (or other such visual features) are merely the visual representation or effects of the mesocyclone is like saying that we really don't see the tornado, we just see evidence of the tornado (e.g. condensation funnel, debris, dust, etc).

As someone who was taken radar analysis - this is the definition caveat as I've always been taught it (from TAMU glossary)

"Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone)."

The problem with the AMS definition - and as you are choosing to interpret it here - is that you can't possibly know by visually examining a storm updraft if the cell has "10-2 s-1 or greater" on the order of 2-10 km in diameter. Instead, the AMS seems to be pandering this off to mesocyclone signature:

http://amsglossary.allenpress.com/glossary...lone-signature1

A lot of storms you may be able to make a good guess as to whether it has a mesocyclone or not based on it's visual appearance - but there is really no way to be sure just by looking. The tornado definition is as easily confusing, as you noted Jeff, since if you can't visually see the connection between the surface circulation and the circulation at cloud base - then you can't really know that it is a tornado.

Oh well. I don't define this stuff - I just go with the definitions that suite my agenda.

Glen

 

[Robert Dewey]

From the given AMS definition, the mesocyclone certainly can be a visual feature. It seems to me that saying that striations (or other such visual features) are merely the visual representation or effects of the mesocyclone is like saying that we really don't see the tornado, we just see evidence of the tornado (e.g. condensation funnel, debris, dust, etc).

Ahhh, very interesting. I could certainly see this being the case... Since a mesocylcone is simply a cyclonic rotation of wind, and obviously you can't see the wind, you really can't see a mesocyclone - BUT, you can see the EFFECTS of the mesocyclone.

The same could be true about a tornado... You're not really seeing the tornadic winds, just the effects exhibited by them.

 

[Bob Hartig]

Bringing it all to street level, my question is this: I've heard language used such as, "There's a new mesocyclone forming directly overhead." But how can a person tell that if the rotation is so broadscale? Are you in fact referring to a wall cloud in formation? At what point, visually, do a mesocyclone and a wall cloud become indistinguishable? I have an idea, but I'm curious what those who have more experience than I have got to say.

 

[Darren Addy]

It seems to me that saying that striations (or other such visual features) are merely the visual representation or effects of the mesocyclone is like saying that we really don't see the tornado, we just see evidence of the tornado (e.g. condensation funnel, debris, dust, etc).

If a newbie may disagree with a forum monitor :shock: I don't think that it is saying that we can't see it like we can't see the wind. (Correct me if I'm wrong, but, as far as tornados are concerned, a non-visible tornado -in it's earliest stages- is on the ground when you see the effects of it on the ground, but that doesn't mean that a visible tornado is not a tornado and that all you are seeing is debris.) The condensation funnel that you see (if it reaches the ground) is still the tornado. (Right?)

Tornado - A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

I found another scrap of relevant info:

The prefix "meso" comes from the Greek mesos, meaning ‘‘intermediate’’ or ‘‘in the middle.’’

Now does "middle" mean (from top to bottom) the middle section? That seems to be the way that it is used with many stormchasers. (The anvil is the top, the wall cloud is the bottom, and the meso is the middle). Or does "middle" mean "center" as in inside. That seems to be what the NOAA glossary quote is indicating.

Assuming that the word was coined for an etymological reason, seems to me that what the NOAA glossary quote (in original post) is saying is that since the mesocyclone is inside the storm, it (by definition) would be obscured by cloud - (although one could see structure and evidence that might -strongly?- indicate one inside?).

If saying "there's the meso" or "I can make out the meso" would be imprecise (to be charitable) then what is the more accurate term for what stormchasers are seeing when they say "I see the meso"? Or, if we like the term, can we say that the stormchaser use of the term "meso" is different from the meteorological usage?

Darren Addy
Kearney, NE

 

[Darren Addy]

I'm not sure the conclusions that I was reaching toward in the above post are necessarily correct. The Oxford English Dictionary gives the following historic usage of the term (beginning with Fujita):

1963 T. FUJITA in Meteorol. Monogr. Sept. 85/2 A mesolow which is found to accompany a definite circulation pattern is called a ‘mesocyclone’. 1963 T. FUJITA in Meteorol. Monogr. Sept. 89/2 The existence of a mesocyclone is first seen by radar as a pendant echo which soon curls around the circulation center. 1984 Sci. Amer. Apr. 60/3 First the entire thunderstorm updraft begins to rotate; the spinning column of rising air, 10 to 20 kilometers in diameter, is called a mesocyclone. (If it goes on to generate a tornado, which the majority of mesocyclones do not, it is called a tornado cyclone.)

(emphasis mine)

The Scientific American usage seems to expand on Fujita's and if we go by it the stormchaser's usage of the term may be accurate. I don't mean to be anal about this, but words do mean things.

Darren Addy
Kearney, NE

 

[Bob Hartig]

Now does "middle" mean (from top to bottom) the middle section? That seems to be the way that it is used with many stormchasers. (The anvil is the top, the wall cloud is the bottom, and the meso is the middle). Or does "middle" mean "center" as in inside. That seems to be what the NOAA glossary quote is indicating.

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.

 

[Glen Romine]

Well Darren, you are certainly doing a good job at digging out some of the gremlins in our field. 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. What seems well agreed upon by most is that the mesocyclone results from the titlting of environment shear - which provides a clean explanation of how rotation appears well above the surface (say 3-5 km above ground). More often in videos I've seen, some chasers see rotation at cloud base - which you sure hope isn't 3 km off the ground - and might say they are seeing a new mesocyclone developing. Problem is, this rotation appears to be too low to be developed by the same mechanism that leads to the rotation aloft - and more recent research suggests these may be completely different vortices that may or may not interact with each other. It gets very confsing the more closely you look at the problem.

Also, above it was brought up what the meaning of the meso part of the mesocyclone was - and I think here the point was just in reference to scale. It is intermediate in size - I guess relative to the size of the entire storm, but I'm not sure about that. Typically - the mesocyclone might be say 10 km in diameter. A tornado cyclone - which might appear as a rotating wall cloud at cloud base and may extend as a circulation all the way to the surface, can be embedded along the edge of the larger mesocyclone circulation, and is about 1/10 the size of the mesocyclone. Then, a tornado may develop within the tornado cyclone (or the tornado cyclone may contract and eventually lead to tornadic strength winds), and is most often about 1/20 the size of the tornado cyclone.

Hope this helps.

Glen

 

[rdale]

Fortunately (or un- in this case) the AMS definition is considered "official" - and it doesn't really answer the question technically but if you tell someone you see a meso overhead I think it's going to be alright.

 

[Robert Dewey]

(Correct me if I'm wrong, but, as far as tornados are concerned, a non-visible tornado -in it's earliest stages- is on the ground when you see the effects of it on the ground, but that doesn't mean that a visible tornado is not a tornado and that all you are seeing is debris.) The condensation funnel that you see (if it reaches the ground) is still the tornado. (Right?)

As long as there are tornadic winds touching the ground, it's a tornado - It doesn't matter whether you see debris or a condensation funnel.

I was just trying to say that you technically can't see a tornado unless there is some kind of debris or a condensation funnel - But, just because you can't see it, doesn't mean it's not there.

A tornado is a violently rotating column of air. Since you can't see air, you would have to rely on either a condensation funnel and/or debris to make the violently rotating column of air (a.k.a. tornado) visible.

As RDale just said... I don't think it really matters, if you said you seen a mesocyclone, or a tornado, people get the picture.

 

[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