Supercell vs. "Super cell structures"; rhetorical or substantial difference?

[Jason Harris]

Looking at the convective outlook of SPC for Thurs., noted the following phrasing:
_

A FEW STORMS MAY ALSO DEVELOP
SUPERCELL STRUCTURES WITH POTENTIAL FOR LARGE HAIL AND PERHAPS A
TORNADO OR TWO...ESPECIALLY OVER A PART OF THE NERN STATES WHERE
STRONGER EFFECTIVE SHEAR WILL EXIST NEAR AND JUST S OF THE WARM
FRONT.

Does this "supercell structures" in fact mean something different than "a few storms" may become supercells? Does this refer to developing a mesocyclone that may not be quite deep and persistent enough to qualify 100% as a supercell? Is something intended with regard to a storm that is quasi-linear, some sort of hybrid? Is this just a matter of phrasing or something in particular intended that is a specific distinction?

 

[Bob Hartig]

Jason, I don't know how NWS meteorologists view this term, but I believe it's one that has snuck in via the back door. It's not part of the official nomenclature that I'm aware, but the fact that it crops up often shows that forecasters find it useful. When I see it, it's almost always in conjunction with some form of MCS and seems to be a way of recognizing mesocyclonic rotation within such a setting. To me, the term's noncommittal nature--i.e. it's not exactly a supercell, it's a "supercellular structure"--suggests that it applies to storms that fall on the fuzzy side of the storm spectrum, whether in depth or longevity or visual features or perhaps some other criterion. In other words, a supercellular structure is something more than just brief, shallow rotation but less than a clear-cut, discrete supercell with sustained, deep rotation that can last for hours. The vagueness of the term allows it to be used in a variety of ways, but to me it has always suggested a fleeting phenomenon, something that's here now and gone within a few radar scans. Yet within those scans, it's capable of doing the work of a supercell, including producing a wall cloud and perhaps a tornado.

That's my layman's take on the matter. Those more knowledgeable than I, feel free to correct me or add to what I've said.

 

[mikedeason]

Seems like a pretty good explanation. I think is applies quite a bit down here in the southern states during the late fall/winter severe weather episodes that include transient QLCS features which produce short lived tornadoes.

 

[Patrick Marsh]

As far as I know, this term grew out of numerical modeling. When using convection-permitting models, we see storm structures that resemble supercells, but actually don't have a bonified way to verify that they are actually supercells. At least in the Hazardous Weather Testbed, we tend to call these numerical storms "supercell-like structures". One major reason that we can't evaluate whether or not they are true supercells is that we don't output the entire vertical resolution so we can't investigate the numerical thunderstorms updraft. Instead we use proxies such as "updraft-helicity" integrated over a vertical depth to infer the presence of a mesocyclone.

I think the term jumped to observations because in many instances the same dilemmas occur. A thunderstorm could be too far from a radar site to allow for adequate sampling of the updraft to determine a mesocyclone. Another term that sort of relates to the "supercell structure" is this notion of a "transient supercell" or "transient supercell-like structure". The idea here is that a thunderstorm may briefly develop a mesocyclone, but is unable to sustain the mesocyclone. The term "supercell structure" could be being used by some forecasters to describe "transient supercells" because to him or her, a mesocyclone, albeit transient, is a supercell structure, but does not meet his or her definition of a supercell.

 

[Bob Hartig]

...Another term that sort of relates to the "supercell structure" is this notion of a "transient supercell" or "transient supercell-like structure". The idea here is that a thunderstorm may briefly develop a mesocyclone, but is unable to sustain the mesocyclone. The term "supercell structure" could be being used by some forecasters to describe "transient supercells" because to him or her, a mesocyclone, albeit transient, is a supercell structure, but does not meet his or her definition of a supercell.

Patrick, your entire response is most insightful. Thank you! The last part, which I've quoted above, brings out the time factor of defining a supercell--to wit: a supercell has a deep, persistent mesocyclone versus transient rotation. But what qualifies as "persistent"? The term seems subjective to me. Does "persistent" mean ten minutes? Twenty minutes? Half an hour? At least three radar scans, or four, or five, or ...?

In his paper "What Is a Supercell?" Chuck Doswell wrote, "By 'deep' I mean that the circulation meeting mesocyclone criteria is present and vertically connected through a significant (say, 1/3) fraction of the depth of the convective storm. By persistent' I mean in comparison to a convective time scale defined by the time it takes a parcel to rise from the base of the updraft to its top (on the order of 10-20 min)."

Obviously the storms don't care about any of that, though; they simply evolve as they please.

Given the cyclical nature of many supercells, maybe "persistent" means that a storm's overall tendency is to produce mesocyclones of varying durations.

I'm just pondering as I write. The more I think about it, the more it seems to me that the question leads toward the same fuzzy territory as, "What is a tornado?" In practical terms, though, the answer is probably simple: you know one when you see one.

 

[Zach Young]

This sort of reminds me of the "Definition of a Planet" debate. We have a collection of terminology to describe various phenomena, but the "boundaries" where the terminology and the phenomena intersect is a little fuzzy. For the most part I don't think we need to really nail it down, but if at some point a topic for research comes up that makes it necessary to firm up the boundaries we'll take care of it then.

 

[Jason Harris]

Thanks for the responses, much appreciated. I look forward to seeing more sustained supercellular structures in Ohio than Florida.