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?