What is a "tornado cyclone"?

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I first ran across the term "tornado cyclone" as a kid reading an old Encyclopaedia Britannica article on tornadoes, so I know it's at least over forty years old. I've always thought it was just another name for a mesocyclone, but having seen it used several times recently, I get the impression that the two terms mean different things. Can someone explain how a tornado cyclone differs from a mesocyclone?
 
I don't really read of the "tornado cyclone" nearly as much as the mesocyclone or tornado, but it's usually used to describe a vortex that is larger than a tornado but smaller than a mesocyclone. I have to think that's it's sometimes very difficult to define "tornado cyclone" for individual supercells given that there may be a smooth continuum of rotation from storm-scale to tornado-scale. I think the picture of the Hallam tornado as it is near Daykin shows a nice tornado cyclone -- the cylindrical (barrel-shaped) vortex that has a base below ambient cloud base but it obviously above ground:

05220412_std.jpg


There are several publications about VORTEX-observed storms that mention and/or address tornado cyclones. For example:
"We define the TC as that region surrounding the tornado in which angular momentum generally increases with increasing radius and a certain degree of axisymmetry is maintained."
--> Rasmussen and Jerry M. Straka: Evolution of Low-Level Angular Momentum in the 2 June 1995 Dimmitt, Texas, Tornado Cyclone. Erik N. Journal of the Atmospheric Sciences
Volume 64, Issue 4 (April 2007) pp. 1365–1378
 
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I don't really read of the "tornado cyclone" nearly as much as the mesocyclone or tornado, but it's usually used to describe a vortex that is larger than a tornado but smaller than a mesocyclone. I have to think that's it's sometimes very difficult to define "tornado cyclone" for individual supercells given that there may be a smooth continuum of rotation from storm-scale to tornado-scale. I think the picture of the Hallam tornado as it is near Daykin shows a nice tornado cyclone -- the cylindrical (barrel-shaped) vortex that has a base below ambient cloud base but it obviously above ground:

Image deleted...........

There are several publications about VORTEX-observed storms that mention and/or address tornado cyclones. For example:
"We define the TC as that region surrounding the tornado in which angular momentum generally increases with increasing radius and a certain degree of axisymmetry is maintained."
--> Rasmussen and Jerry M. Straka: Evolution of Low-Level Angular Momentum in the 2 June 1995 Dimmitt, Texas, Tornado Cyclone. Erik N. Journal of the Atmospheric Sciences Volume 64, Issue 4 (April 2007) pp. 1365–1378

I like the word tornado cyclone, it's one of the meteorology terms that can simply convey the same mental image of what's going on to both technical people and the general public. That is, we know a cyclone is a low pressure system, but in my opinion it lost favor because it lacked a size distinction. Mesocylone does accomplish this task, although a bit too technical to the general public especially outside the tornado aware states. I won't tackle the word meso here, we can find meteorological definitions for that. My concern is for what's really going on in the so called tornado cyclone and how should we define it. In this discussion I agree with the papers cited, although I would like to take the discussion a bit further. That is, what we see visually vs what is actually taking place. This is important for chasers so we know the bounds of what we're dealing with in the field. The new breed of video chasers tend to drive as close to the tornado as possible with little concern of the storm scale motions associated with this system. It's important for both our safety and appreciation of the event to dig a bit deeper on this subject. A tornado may form in almost any area in a rotating supercell, but it is more likely within the bounds of the mesocyclone(s). The "s" is because many storms have more than one tornadic circulation, least we tend to forget the one's that occlude and rotate into the rain and out of sight. That certainly does not diminish their ability to produce tornadoes, even through they are surrounded by a precip downdraft.

My concern with the image shown as an example of the tornado cyclone is that it's too small. It characterizes the same problem inherent in mesocyclone=wall cloud. It is quite common for a mesocyclone or tornado cyclone to show up much larger on a radar display than visually in the field. That is, if we have enough rain/hail to reflect the whole circulation (not always possible in early stage LP supercells). So what is the wall cloud or in this case the tornado cyclone visually. It is where a strong updraft lifts and condenses air below the cloud base of the storm. Originally this was thought to be due to the pressure reduction in the low pressure center of the mesocyclone. Some of this still may be possible, but a much greater contributer is the cold air mixing from the downdraft or forward flank of the supercell. Often what we call an inflow band that stretches across the south face of the precip wall is actually a mesoscale warm front dividing the warm moist inflow from the cool nearly saturated downdraft. When this parcel of air is pulled back west into the mesocyclone and lifted it will condense much lower than the ambient air....giving (on occasion) that time lapse effect that drives storm chasers gonzo. Thus, what we're seeing visually in a wall cloud, tornado cyclone, collar cloud (Fujita term) or barrel cloud may better characterize the area of mixing and lifting of this more saturated air. Thus, I would not trust my visual senses to tell me the true bounds of this feature that is comprised of rotating air. The true circulation may or may not include cloud material, condensation or dust. Actually, the dust and dirt of a rotating dry LP may be better at showing the outer bounds of the circulation.

Finally, some tornadoes grow to the outer bounds of the wall cloud or visual tornado cyclone. One well documented event where this happened was Manchester SD, another Jarrell TX. The visual wall cloud or tornado cyclone quickly becomes the tornado. A recent event was Conway Springs KS (2004). In some of the better videos filaments of condensation can be see forming around the outside of the tornado cyclone then the tornado drastically widens to the width of the wall cloud or tornado cyclone. Question is, what really happened in these cases. It's possible that the strong circulation was already in place and condensation or dirt filled it the outer bounds. Red Rock was another fine example because it visual appearance went from cone to wedge as soon as it moved off wet grass into a plowed field. Certainly the dirt did not make the tornado double its size, that strong circulation was already there outside the bounds of the condensation funnel and visual wall cloud.

Some of my opinions here are in the literature, some are not.....as always my assessments of events are always open for further discussion.

Gene Moore
 
In the early 1980s I met Jim Leonard in Florida, and he later sent me an enlarged print of the Pampa tornado of May 19, 1982. This shot is well known among chasers as the one that looks over trucks at an intersection toward the storm.

http://www.cyclonejim.com/pampa16.jpg

I wish there was a better image of this online, as my framed copy offers much better detail. The tornado is just above the pickup in the center of the three trucks. The collar (?) seems to be indented surrounding the wall cloud, though it doesn't show too well here.

Perhaps Jim has a higher-res version that could be posted in CB&G where it would be safer from theft?

Structure on this storm has always intrigued me as it seemed to have a collar cloud and a wall cloud with a mature cone tornado beneath it. I'd be interested in reading Gene's and other opinions on this structure as it relates to this tc thread.

Thanks.
 
i was going to say that maybe it was an older book, or some bad terminology...

ive never heard of a "tornado cyclone" but apparently others on here have, so i dont know...
 
Finally, some tornadoes grow to the outer bounds of the wall cloud or visual tornado cyclone. A recent event was Conway Springs KS (2004). In some of the better videos filaments of condensation can be see forming around the outside of the tornado cyclone then the tornado drastically widens to the width of the wall cloud or tornado cyclone. Question is, what really happened in these cases. It's possible that the strong circulation was already in place and condensation or dirt filled it the outer bounds. Red Rock was another fine example because it visual appearance went from cone to wedge as soon as it moved off wet grass into a plowed field. Certainly the dirt did not make the tornado double its size, that strong circulation was already there outside the bounds of the condensation funnel and visual wall cloud.

I completely agree with that. Another example (IMO) is the debris field surrounding some violent tornadoes. Examples would be Pampa '95 or OKC '99. Both of these tornadoes plowed through industrial areas and carried a ton of heavy debris outside of their visible condensation. The famous Pampa video clip showing the vehicles being tossed and flying through the air is another thing. Yes, the core of the tornado flung the vehicles outward, but they flew a considerable distance out and away from the visible condensation funnel before free falling straight down. It's arguable, but I suspect tornadic winds outside the visible funnel carried those vehicles before they reached the edge of the true tornadic wind field.

Tornado circulations are there before we see them, visualized by pressure drops and subsequent condensation. Of course with this comes intensifying as well, as the pressure fall creates increased spin (the old ice skater philosophy). So what happens in an already-established tornadic funnel to increase its size? Is it experiencing additional "pumps" of RFD air descending in small doses after tornadogenesis has occurred, possibly driving the outer shell of tornado cyclone circulation towards the ground (similar to the 6-24-03 "shell" tornadoes that seemed to have several vertical layers)? Is it simply encountering a higher dewpoint, thus visualizing the outer core winds that are invisible but present? Or is it circulation so violent it cannot organize initially, appearing as writhing multiple vortices, that is somehow focused into one large funnel? All are possibilities.

I have no answers, but I like to throw ideas out here in hopes that someone more skilled can explain how they may or may not be feasible.
 
I never anticipated so much discussion over this question. The contributions are fascinating, but I still find myself wondering about the actual meaning and useage of the term.

Jeff Snyder has provided a good example of what he considers to be a tornado cyclone, and Dave Gallaher has offered Jim Leonard's older photo. I can also think of a striking example from one of Mike Hollingshead's videos that shows a similar, well-defined structure, and other videos come to mind as well. Since nothing in my relative handful of successful chases compares to them, so far I have only photos and videos to go by. But then, I'm a bit of a junkie when it comes to those, and from I've seen, I find myself wondering whether a tornado cyclone isn't just a tightly focused, rotating wall cloud. Granted, the carousel-like appearance of the examples is dramatic, but when all is said and done, what really is the difference other than perhaps intensity?

Following the discussions here, which have included the lowering of the wall cloud and/or meso to ground level, the rapid widening of the condensation funnel, and a broader scale of circulation than is apparent to the eye, it seems there's a continuum involved that makes the term tornado cyclone hard to pin down. I first encountered it, as I had mentioned, in an old Encyclopaedia Britannica article. I wish I had access to that article right now so I could quote the exact wording for you.

Just for fun, though, here's what the contemporary Britannica Online has to say:

The extension of a concentrated swirling core to the surface—in other words, the actual formation of a tornado—can occur once the mesocyclone is established. Most mesocyclones do not generate tornadoes. In the ones that do, a small region of increased convergence and stretching that is typically no more than one kilometre in diameter develops in the mesocyclone for reasons that have so far eluded storm researchers. This usually occurs at the interface between the thunderstorm's updraft and downdraft. Enhanced spin begins several kilometres above the ground, then quickly builds downward. Around such a small volume, rotation is strong enough for a smaller dynamic pipe to form and extend to within several tens of metres of the surface. This dynamic pipe is called the tornado core. Once it forms, the parent mesocyclone is reclassified as a tornado cyclone.


The description of how and where tornado formation initiates (i.e. within the mesocyclone vs. ground level) makes me wonder how dated the article is; nevertheless, it is interesting and informative. Now here's still more from the same article a bit earlier in:

The period of storm maturity during which a tornado is most likely to form may last only a few tens of minutes. However, on rare occasions a storm may produce a tornado cyclone (a core of concentrated rotation within the storm from which tornadoes are spawned) that is stable and long-lived. The strength of the tornado cyclone usually pulsates, creating a sequence of tornadoes. This gives rise to what is known as a tornado family.

Does anyone besides me get the sense that we've got two definitions for one term in the same article? It would be helpful to see how tornado cyclone was originally used. I wonder whether there's any written material on it by Miller and/or Fawbush. I'm guessing that one of them coined the term. Anyone know?
 
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http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0493(1976)104%3C0552%3AMVFITT%3E2.0.CO%3B2&ct=1

Here is an interesting article excerpt with interesting facts about tornado cyclones. From the various sources I've seen and looked up, it appears that the "tornado cyclone" is a fairly informal description referring to a much larger area of tornadic related rotation within a parent supercell which may or may not be completely visible as the tornado or a superset of the tornado. I believe it is similar to storm scale mesocyclone in concept, but possibly related more specifically in reference related to an existent tornado as opposed to a mesocyclone which may not have spawned a tornado.

As the rest of you mentioned, it typically is not fully visible, but in some situations a tornado may extend almost to the full tornado cyclone in scale. Perhaps Hallam may be a good example. Gene's Red Rock tornado I assume indicates the dust from the field helped make the larger tornado cyclone visible as a 'tornado' by entraining dust thereby making the increase in visible size possible. However this was not an 'actual' increase in size because likely the tornado (visible and invisible) remained the same size it just encountered geographic features (in this case) that made more of it visible.

Shane I would think the reasons for a tornado funnel to change in size are many, and varied based on each case but may have to do with the strength of the tornado circulation within it's cyclic phase, strength of the supercell including what it is currently feeding on or ingesting, tornadic cyclone mergers between cells, local environment features (atmosphere and ground), as well as visibility features such as described in the Red Rock case. These just to throw out a few general factors, and I am sure there are many, many large and small scale features which come into play during the lifetime of a tornadic funnel which influence this.
 
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Great discussion on one of the mysteries of storms. On a fully developed supercell, I've generally thought of the tornado cyclone as perhaps 1 km across, usually not defined by a wall cloud (which is usually much smaller in diameter), and on rare occasions it reaches the ground as a large wedge. Here's a photo that seems to me to show the entire spectrum (the 4-7-02 Throckmorton tornado captured by Doswell):
http://www.flame.org/~cdoswell/chasesums/Throckmorton09.JPG

Perhaps it is my (irrational?) fear of the unseen tornado cyclone that keeps me at a healthy distance from tornadoes. I've never quite understood how much risk is involved when a strong tornado is a couple of hundred yards away. What's the time frame in which the tornado can expand to the full width of the TC? Are there really sufficient signs of that expansion before it's too late to back off from a close encounter?
 
Perhaps it is my (irrational?) fear of the unseen tornado cyclone that keeps me at a healthy distance from tornadoes. I've never quite understood how much risk is involved when a strong tornado is a couple of hundred yards away. What's the time frame in which the tornado can expand to the full width of the TC?

I remember watching some of Gene Moore's video of (I believe) the Harper, KS tornado a few years ago which rapidly morphed from one type and size to another within literally seconds as I recall. My guess is a lot of that was just a change in the visibility of the always present tornado cyclone although it seems there was a likely continual evolution of the tornado dynamics at the same time but at a slower pace. Gene probably has more insights on that tornado(s).

Are there really sufficient signs of that expansion before it's too late to back off from a close encounter?

In my opinion and experience usually but not always. Since tornadic size and intensity is based on a much larger environment in the storm itself and the tornado is a 'feature' of those dynamic forces at work it is possible for dramatic forces to rapidly build or change unseen in the supercell or it may react that way in response to much favorable tornadic conditions the storm may encounter as it moves and propagates. That's why it really isn't a good idea to get right under or near a tornado, or tornadic lowering / updraft. People do it and usually get away with it, but sometimes folks get 'caught' and hopefully when that happens they aren't caught in 2 mile wide F5 suddenly when it was only a 1/4 mile F2 a moment ago.

Hmm...as I recall the Tulia tornado this last season from a distance appeared as a very large wedge - perhaps 2 miles or so, but from what I have heard / seen from photos the tornado itself extending below was fairly small. Imagine if that large 'wedge' were the tornado cyclone or part of it (perhaps it was), and had instead strengthened and the whole thing touched down. Then we would be talking about the Tulia wedge. Amos/Eric would have been under the whole thing? No?
 
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For what it is worth here is my take on the Tornado Cyclone (TC). The term as I understand it is used to describe the area or areas within a mesocyclone that have enhanced rotation to the surface on an order stronger than the mesocyclone itself. The TC may or may not contain a tornado and it may only be visible as an area of enhanced cloud based rotation. A TC is contained by the larger mesocyclone and is typically 1/4 to 1.5 miles wide. Mesocyclones do not always have a TC but they can have several at the same time. Often chasers (myself included) have seen large rotating bases and describe them as mesocyclones when in reality they are TCs. The mesocyclone is much larger than the updraft base and it encompasses both updraft and downdraft portions of the supercell. There is a good discussion of this in the last episode (Dec 5th) of Gene Rhoden’s “High Instability” radio show.
 
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You might need to go back at least as far as Brooks, E.M., 1949: The tornado cyclone. Weatherwise, 2, 32-33 in order to get the original definition of the term tornado cyclone. The AMS glossary tends to agree this is the first use:

http://amsglossary.allenpress.com/glossary/search?p=1&query=tornado+cyclone&submit=Search

Prior to the Agee reference previously noted, the tornado cyclone was basically a tornadic mesocyclone. The Agee reference then is using tornado cyclone to describe what we would probably just call a multi-vortex tornado these days. Very different scales. These works predate the use of the term mesocyclone in storms, and interpretation of the tornado cyclone terms changed after the mesocyclone term came about and saw much wider acceptance. This came though with it's own controversy as then there was a question over a remotely sensed representation of a circulation vs. what was actually in storms (the resolution of the observations scales the smallest resolvable circulation). Almost certainly there must be occasions when distinguishing between circulation scales becomes rather blurred. I'm not sure who first defined the tornado cyclone in it's currently accepted form.

Scott's definition above well reflects the currently accepted use of the term as I've seen in recent literature. Perhaps because of the questionable historic definition of the term some researchers seem to make reference to a tornado cyclone type circulation, without ever using that term (instead just referring to the circulation). The latest Rasmussen and Straka (2007) article gives the following definition: "We define the TC as that region surrounding the tornado in which angular momentum generally increases with increasing radius and a certain degree of axisymmetry is maintained."

Glen
 
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