Wow, this is the first weather-related post on this Blog. Anyway, I have a Theory.
Well, more like a theory that’s already been explored (informally or in the literature?), but I’ll explain it here, for posterity I guess.
This year has been the year of the tornado, just as 2005 was for hurricanes, and well, social media has advanced well enough such that tornado videos are rather commonplace nowadays. Specifically, as I was watching YouTube videos from the 5/24 outbreak in OK, and in reading discussions of the infamous Joplin tornado, I discovered that there were two fundamentally different formation modalities. I will explain below.
I. THE REVERSIBLE TORNADO
This is the canonical tornado. A funnel drops from the sky, kicking up debris on the ground. The tornado slowly gains strength and size, until it reaches a maximum radius. Then it begins to shrink and shrivel up as the RFD overtakes to parent mesocyclone. Finally, the vortex dissipates.
The key to the realizing the tornado’s reversibility is that at any stage, the tornado could max out and go back to where it came from, a thin funnel. It could never go past the rope stage and dissipate very quickly. On the other hand, it could widen all the way to a “wedge” tornado before weakening. Reversible tornadoes vary widely in maximum strength, ranging from the weak 1-min tornado to the EF5. But in each case, it goes back through the same path as it formed.
Here’s a schematic of the typical REVERSIBLE tornado:
The first tornado that was documented to go through these distinct stages was the Union City tornado (F4) on May 24, 1973. 28 years later to the day, the Chickasha (EF4), Canton (EF3), Fairview (unrated as of this post, but probably EF0/1), and Goldsby (EF4) tornadoes would show the same type of morphology.
II. THE IRREVERSIBLE TORNADO
These tornadoes are comparatively rare, accounting for, as I estimate, less than 1% of all tornadoes — but perhaps maybe as much as 25-50% of all violent tornadoes. They dissipate the same way as reversible tornadoes do, but they form in a completely different manner. Unlike reversible tornadoes, rotation doesn’t pre-concentrate in a funnel first before dropping down.
The tornado simply drops.
Let me explain further. In these tornadoes, the maturation process doesn’t go through distinct stages. A large, mature tornado simply drops out of the parent mesocyclone. The formation process is irreversible because the tornado doesn’t simply go back up into the clouds… it slowly shrinks and then lifts back up. It’s mostly one directional (aside from oscillations in width all tornadoes go through).
IMO, almost all irreversible tornadoes are strong to violent, but because of how EF ratings work, they will be rated on how much damage they cause.
Typical schematic of an IRREVERSIBLE tornado:
Examples of irreversible formation (note how in each of these videos, the wedge look condenses very rapidly):
Lookeba, OK – May 24, 2011 (go to 00:38)
Joplin, MO – May 22, 2011 (the initial rope-like funnel is actually one of the subvortices within the at-the-time invisible funnel)
Manchester, SD – June 24, 2003 (go to 00:40)
(As I find more definitive videos, I will add to this list.)
Of course, tornadoes don’t always fit into man-made categories; they occupy a continuous spectrum. So some will inevitably fit in between the two categories where a large funnel drops from the sky, looking like the mesocyclone extended to the ground, or the extension exists, but the funnel looks narrow. But nevertheless, we can start looking at the differences between initiation mechanisms and intensities between ‘reversible’ and ‘irreversible’ tornadoes, and start unraveling more mysteries about these phenomena.