STORM TRACK: January 31, 1985 (Volume 8 Issue 2)

Under the preceding heading in the last issue, Storm Track listed and defined 41 commonly used meteorological terms associated with storms or severe weather. It also presented two illustrations of (1) a severe storm atmosphere over the southern plains and (2) a three quarter view of a tornadic thunderstorm. As already noted, many positive responses wers received, plus a few cautionary comments from some research meteorologists regarding the tornadic thunderstorm. These latter statements addressed differing views on the role of horizontal vorticity, which occurs at the interface of the small scale, temporary warm/ cold fronts generated by a severe thunderstorm. It should be noted that such vorticity does not begin at tornadic wind speeds but, rather, at a slower but organized rotational speed. As drawn up into the mesocyclone, it may accelerate into, or somehow foster, the tornado. Also, note that it is not strictly speaking a "vorticity loop", as defined in the last issue.

Before getting into the details of this discussion, the Editor wants to point out that Storm Track inadvertently touched on the cutting edge of current knowledge in this area. In fact, "informed speculation" might be a better phrase than "knowledge", since several meteorologists contend that the interface-vorticity theory is not yet generally accepted (nor, as I understand, is any other single explanation for initial tornadic rotation). The lay reader is privileged here to read a little of the interplay between such professionals, who sometimes take exception with their colleagues. However, far from being disagreeable, the expression of such differences is essential to intellectual growth and knowledge. "As I think you are discovering, there is still a lot of room for differences of opinion. This makes the job of synthesizing what we think we know into a schematic very challenging, and almost certain not to please everyone. This is fine and does not in any way detract from [Storm Track's] efforts. Perhaps you get some flavor of how science works from this experience -- I think science thrives on controversy, rather than agreement" (Doswell).

The idea for the vorticity interface came, initially, in a letter to the Editor from John Weaver. "...the micro-warm front ... may turn out, to be the most crucial part of a tornadic thunderstorm. Recent work by Klemp and Rotunno points very strongly to this region as the PRIMARY source of vorticity for tornadic spin-up. In case you haven't been following their developments, it goes like this. There is a circulation around a horizontal axis at the warm front/cold air interface...this is well known. Also, there is convergence along this boundary, and advection toward the updraft ... Klemp and Rotunno have shown (very convincingly, I think) that the vorticity along this boundary is drawn into the updraft, tilted into the vertical by the upward motion, and stretched to become the low level vorticity source for the vortex. In stronger tornado situations, I think this process becomes visible with the formation of the 'tail cloud.' As we both know, the tail cloud forms along the micro-warm front, and is composed of scud, moving rapidly toward the updraft region. Anyway, a very large percentage of the people I've talked with are accepting these results as showing the primary vorticity source."

After receiving the last ST and a separately mailed copy of John's comments, Dr. Doswell responded: "I think that the origins of tornadic vorticity are still sufficiently debatable, that one should not take quite so emphatic a stance, as I read John's statements to be. Perhaps you could profit from a discussion of this with Rich Rotunno. I am also disinclined to believe that this is the source of the tail cloud (or at least not all tail clouds). I think that many tail clouds have their origins totally within the outflow/precipitation region, as opposed to originating along the interface." Dr. Doswell also added, regarding my illustration of the mid-level wind flow around the meso-cyclone, "please give Dr. Rich Rotunno some credit for input, as well, since I solicited his comments, and it was his idea to use the figure from Time-Life's 'Storm' volume as a point of departure." (The Editor also takes this opportunity to thank Lou Wicker for his telephoned comments to the last ST article on this subject. This omission was an oversight in the last issue).

In response to Dr. Doswell's suggestion, I wrote to Dr. Rotunno with these particulars and reservations. He wrote back and kindly sent a detailed study of his on "Tornadoes and Tornadogenesis," May 1984/Revised November 1984 (hope to include some of this in the next ST). In his letter, he said: "In the vast majority of cases, the supercell-associated tornado forms after rain starts falling and cool outflow is produced. The late and legendary storm chaser, Neil Ward, observed this in 1960, and this scenario has been observed many times since. Now, one could argue that this outflow provides the added amount of convergence needed to spin up pre-existing vorticity. However, this convergence is really no stronger than in linear squall lines, which do not in general produce tornadoes. This datum led Joe Klemp and me to believe (Klemp and Rotunno, 1983) that the cool-air boundary is of a more fundamental significance. Our hypothesis is that due to the three-dimensional nature of the airflow, the updraft can draw upon the horizontal vorticity of that part of the outflow situated upstream from it (the micro-warm front).

Returning to the specifics, I think that one or two qualifiers (which I'm sure John would have included in a more formal letter) would make John's statement less 'emphatic'. The contention of Klemp and Rotunno (1983) is not that all atmospheric vortices, which one could reasonably call a tornado, form by this process, but rather that those tornadoes associated with the supercell do. Within the context of the general discussion in the article in question, I think this is understood by us technicians, but maybe not by the majority of readers. I agree with Chuck Doswell that the tail cloud does not necessarily lie along the micro warm front (see Fujita, 1959, where there are two tail clouds extend- ing from the wall cloud into the cool air, at different azimuths). But this issue is only of peripheral relevence to the vorticity-generation question."

There rests the discussion. Nothing was finally resolved, of course, but differences were closed. As you can see, though, some very basic things are going on "out there" in the plains and midwest, each spring, which we still don't understand -- even after more than 10 years of Doppler radar, satellite photography and meso-network observations. With complex or incomplete results from these sources, some scientists will occasionally draw different inferences and conclusions. This was one such case. ST regrets having been unable to include John Weaver's explanation on this aspect of the thunderstorm illustration in the last issue due to space limitations. Likewise, ST asks the reader's indulgence on such cases in the future. As a newsletter and not a scholarly journal, it can only touch on or suggest certain scientific knowledge and does not pretend to provide a full discussion. Properly written, the last issue of ST could have filled a book! Likewise, I appreciate the frustration of the professional, who frequently sees incomplete explanations for subjects worthy of long, loving and careful attention. Hopefully, ST covers just enough information to provide a basic conceptual framework for the non-professional reader, which will spur him or her to further study. If so, then one of ST's objectives is realized.