Defining a mesocyclone, tornado cyclone, and tornado...

[Bill Tabor]

[MOD: Split from 4/22/2010 DISC thread]

Jeff, thanks for that link. Very nice!

As for tornado defs...this would likely make a good 'Weather and Chasing' category theme cause I'm sure everyone has their own feelings and reasons. I'd probably only call it a separate tornado if an initial tornado / rotation region occluded and then a new circulation such as to ne appeared and dropped a tube. If a single circulation put down, lifted, and then continued without a torn for 10 minutes and then dropped another tornado - is that a separate tornado? Good question.

Also, another good topic for W&C which Rich and Jeff bring up is regarding a meso circulation near the ground. I use the term the same way when describing a broad area of circulation in close proximity to the ground but not in contact - or perhaps at times possibly in contact as small vortices occasionally dipping down. Now for clarification for everyone meso or mesocyclone is primarily a radar term describing a feature on radar - no? Everyone uses meso out in the field in a visual context. Is this term now legitimately taking on a physical / visual definition alternatively to the original radar term? If not, what would we describe it as? Is is truly / more appropriately a wallcloud, or a tornado cyclone that is not in full contact with the ground? Perhaps the tornado cyclone is a better term - and actually that is probably the same as the radar term mesocyclone refers to - correct? Additionally it is interesting when people see a meso / wallcloud / tornado cyclone and it may have tendrils or vortices in contact with the ground as to me that must meet the classic defintion of a tornado assuming violent motion in contact with the ground and pendant from a cumuliform cloud. What are you guys thoughts on this, and how do you see it?

 

[Jeff Snyder]

Bill,

I'll just throw out my 2 cents since I don't have time to provide a more detailed response.

Mesocyclone need not refer only to the radar signature of rotation within a supercell AFAIK. From the AMS Glossary:

mesocyclone —A cyclonically rotating vortex, around 2–10 km in diameter, in a convective storm.

The term "tornado cyclone" is used to describe a scale between that of a tornado and that of a mesocyclone. From the AMS Glossary:

tornado cyclone —A term coined by Brooks (1949) to describe a surface low pressure area in a convective storm that, with its attendant winds, has a radius of about 8–16 km and is associated with, but is larger than, a tornado. Starting with Agee (1976), the tornado cyclone has been redefined as a distinct circulation with a scale larger than that of the tornado but smaller than that of the mesocyclone (although embedded within it and smaller in scale than the mesolow). The intermediate-scale tornado cyclone is sometimes inferred from high-resolution Doppler radar observations, but at other times it is not apparent

I think what some (myself included) sometimes call a "low-level mesocyclone" can also be labeled a "tornado cyclone". When I think of a tornado cyclone, though, a certain "type" of rotating lowering comes to mind -- one with a very rounded, condensed "curtain". I've seen several examples of these slowly dipping down to the ground, often with the "back" side the lowest, and sometimes associated with very large tornadoes. For example, the lowering from beginning stages of the Hallam tornado makes me think "tornado cyclone" (pic below). Note that the actual tornado cyclone may extend beyond the visual manifestation of condensation caused by the perturbation pressure minimum associated with the rotation in the tornado cyclone (just like how "the tornado" is likely bigger than the condensation funnel that we see).

Again, I've seen other cases that have featured something very similar, I just can't come up with other specific examples.

EDIT: There was a really good discussion on tornado cyclones back in late 2007 in the thread titled What is a "tornado cyclone". Incidentally, I use the same pic above as an example in my post in that thread.

 

[Bill Tabor]

Ok, appears maybe some of these definitions have changed a bit from what I remember last. I believe I remember mesocyclone was originally a radar term?

I think of Hallam also when I think of tornado cyclone or even mesocyclone in close proximity to the ground. Seems to me that as a tornado is often made of smaller vorticies - a tornado is similarly related to the tornado cyclone. I'm thinking / wondering that at some point with enough instability, shear, etc at some point under certain extreme circumstances the tornado cyclone can morph to a very large primary tornado - likely comprised of smaller tornado or tornado circulations. I'd call that a 'super tornado' - perhaps Greensburg was one of these? Perhaps Hallam was close to this as well?