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.