STORM TRACK: March 31, 1985 (Volume 8 Issue 3)

John Weaver writes "When I made my comments on your tornadic thunderstorm diagram, it was late at night, and I had a lot of other projects on my mind. It didn't occur to me that my suggestion regarding the vorticity along the storm's micro-warm front would develop into anything as complex or formal as it became. Since the situation evolved as it did, I would like the opportunity to expand a bit. In particular, there are two points.

First, on re reading my note, I agree with Chuck Doswell that the emphasis on the horizontal circulation at the micro-warm front may have seemed too strong. My comment could easily be understood to mean that most meteorologists believe this circulation to be the primary vorticity source for tornado production. This is not the idea I wanted to convey. I personally do not believe there is any single vorticity source which accounts for all tornadoes. However, for large tornadoes -- in supercell situations -- the micro warm frontal region probably does supply the low level vorticity requisite to tornado formation, as shown by Klemp and Rotunno. But I also believe that to realize this vorticity in the form of a tornadic circulation, a great deal of 'stretching' of the updraft is needed. This final ingredient (I think) is supplied with the onset of the rear-flank downdraft. Regardless of the final solution of the problem (i.e. the exact nature/order of the mechanisms of tornado production) though, my intent, was to convey that I felt it very important to at least show the feature in your figure.

Second point. In my letter, I said 'in stronger situations, I think this process becomes visible with the formation of the tail cloud.' Most readers apparently took this to mean that I think the tail cloud results from the warm frontal vorticity. Not so. What I meant was that I have actually seen the horizontal circulation in the scud along the micro-warm front in a few storms, which happened to have a tail cloud at that interface. Further, I'm sure we've all seen the larger cloud elements advecting into the updraft from the east. In this manner, the Klemp-Rotunno scenario is observed to occur in the real-life laboratory.

...even though my first remarks were written pretty hastily, I still think the discussion in the last Storm Track ... is an extremely interesting and informative exchange. The abstract does represent a look at the present 'frontier' of tornado research. Take care."