Erik Rasmussen's Denver presentation....any additional info?

George Tincher

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We had a discussion about this a few nights ago in chat. Apparently, Erik has brought forth a new theory concerning tornadogenesis. Can anyone who was present elaborate on what he said? Is there any papers online outlining this new theory?

What I heard about sounded interesting and different, though I just caught bits and pieces of what he said. I figured any new revelations or theories concerning such a topic would make for an interesting discussion here.

-George
 
I'm sure Aaron Kennedy can comment on it, since he's heavily working on this research... Gabe Garfield and I spent quite a bit of time on the long drive from DEN/DIA to OUN talking about it, and, from an anecdotal since, it seems plausible and quite interesting...
 
That is does Jeff. This seems to be the first really new idea to come along in a long time regarding tornadogenesis. Being Erik has spent so much time in the field studying this, recording data and analyzing said data, it makes his thoughts on the subject even more interesting.

I'm sure we'll be hearing much discussion on this matter over the coming months. And even though I'm sure there will be differing opinions (as there always is), I for one will try to keep an open mind. Afterall, that's how we learn.

-George
 
The theory George is referring to is what may be called "Blob Theory" (Erik R. used this term). Essentially, in many tornadic supercells, a slightly higher reflectivity signature appears in 88-D data that starts at higher elevations and descends toward the ground just before tornadogenesis (the scientific name for it is a "descending reflectivity core").

Visually, it can appear as a rain band directly behind the low-level meso which has no apparent connection with the main precipitation core.

After thinking about it for a while, I can think of at least a few events that have had a small rain band that was located just to the west of the tornado.

Belle Plaine, KS tornado of June 12, 2004.
Mulvane, KS tornaod of the same day.
Atlanta, KS tornado, also of the same day.
Harper, KS tornado of May 12, 2004.

The most notable of these is probably the Atlanta tornado. Just prior to tornado touchdown, a thin, compact rain band obscured the funnel from sight (as viewed from the west). Shortly after this began to clear, the tornado touched down.

Gabe

Gabe
 
A few images from Brian Morganti's page which show this feature quite well:

May 24, 2004 in Chester, NE.
http://www.stormeffects.com/images/52404-C...hesterNE-D4.jpg

May 24, 2004 near Belleville, KS.
Before
http://www.stormeffects.com/images/52404-B...llevilleSC1.jpg
After
http://www.stormeffects.com/images/52404-C...hesterNE-D3.jpg

May 29, 2004 in Sumner County, KS.
http://www.stormeffects.com/images/52904-S...mnerCoWedge.jpg

Notice in this image that the "wrapping rain curtains" do not actually originate from the main precipitation core. This "blob" is only associated with the localized tornadic circulation.

Gabe
 
Gabe, fascinating stuff! Thanks for sharing.

I'll be sure to keep a close eye on the radar images this Spring and see if I can spot some of the tell-tale signatures.
 
That is interesting. I chased the Chester/Belleville storm. I have video of the storm in the photos posted above. In fact I unfortunately, HAIL, was the last person to leave the area near where those photos were taken. At first there was just a beautiful Meso and wall cloud then an intenese precip core formed right in the middle of the Meso. After that precip dissipated the attendant tornado formed in the exact location where the precip core had been. I identified that as the RFD precip. Is the descending reflectivity core the samething as the RFD?
 
I guess my next question will focus more specifically on the the processes involved with tornadogenesis.

Has Erik stated what effect he thinks this "Blob Theory" plays in getting the tornado to the ground? Does it happen in conjunction with the meso? Or it it independent of the meso, which his theory seems to hint at?

It also seem the vorticity tube Erik thinks stretches over the RFD may play a greater role in this theory than the meso itself, when it comes to tornado production. But this is the part where I kind of get lost, so that's why I'm asking here.

BTW, great discussion so far folks. Thanks for keeping this going.

-George
 
I would think that the descending reflectivity core (DRC, or "the blob"), when descending in (?) the RFD, would enhance convergence along the leading edge of it when it hits the ground and spreads out. As the blob hits the ground and spreads out downwind, there is enhanced convergence along the leading edge, which may help 'tight'/focus the rotation. In addition, the baroclinicity involved would enhance vorticity, which, when ingested into the updraft, may increase rotation.

I've always just thought of the precip behind/south/near the tornado as just 'wrap-around' precipitation. But the more I think about it, the more I realize that I've seen, on my chases, times when there is a very intense precip core ("The blob" perhaps) just to the south of the tornado. I find it very interesting, since I've never really thought of enhanced precipitation descending from the backside of the storm, rather than "wrapping around" the back side of the meso. Yes, precipitation falls towards the ground, but I'm talking about a particularly intense area of precipitation.

EDIT: Check out a presentation by Dr. Rasmussen at http://recuv.colorado.edu/website/Events/R...004/UAV2004.ppt , a powerpoint presentation regarding "The Blob" and the supercell that produced the Dimmitt, TX, tornado...
 
First, the blob (descending reflectivity core) and the RFD are not the same thing - in fact the blob is not the occlusion downdraft either, but does appear close to it. Observations have indicated that the blob can be caused by a few very large raindrops - which can be nearly invisible to the naked eye. The clear slot occlusion is basically not really clear, but instead has a few big raindrops in it, particularly along the southern wall of the clear slot in the Dimmit case most carefully studied. The massive wet microbursts shown in the images above may be something different entirely.

The relation between the blob and tornadogenesis is seemingly still unclear - mostly because there is a void in available data as to what happens between the ground (where the mobile mesonet are) and the lowest radar data (about 500 m above the ground). Without any information here - the vortex line analysis can't be completed to tie the whole thing together. The overarching problem is getting strong horizontal shear to the ground. There is a lot of vertical wind shear near the surface - the vortex lines are then parallel with the ground - and you need them perpindicular with the ground to get a tornado. You can tilt the horizontal lines to get the vortex lines vertically oriented - but you then somehow have to get this vertical circulation back to the surface to get a tornado - and this blob may play a role in that process. At least, that is what is being looked at as far as I'm aware.

Glen
 
I should add that the images I posted aren't necessarily physical manifestations of "the blob", but nevertheless may be related.

Gabe
 
Originally posted by Jeff Wear
I think I got this \"blob\" on video during the first tornado of the series near Mulvane KS on 6/12/2004....look closely about 10-12 seconds into the video to see a burst of precip descend west and south of the meso then wrap around it immediately before the funnel forms.......
Click to expand...

Great Video Jeff. There is clearly precipitation from the clear slot just prior to the funnel development. It can be seen best when stepping through the video backwards. From my understanding of the presentation, this is the area his studies were focused on. It was also mentioned that the DRC or “Blobâ€￾ would return at about 35 – 40 dbz, somewhat less than the main precipitation core. I think it was also noted that this DRC needed to be “warmâ€￾ so that when it fed into the meso circulation it might provide a boost to tornadogenesis. The other significant thing Dr. Rasmussen talked about was the idea that the cyclonic rotation located in the right leg of the horseshoe was actually connected, via an updraft arch to a anti-cyclonic rotation located on the other side of the DRC, or in the left leg of the horseshoe. In Jeff’s video, the cyclonic rotation is clearly visible on the right side, and it appears the low clouds in the foreground may be feeding into the meso right to left, or in an anti-cyclonic direction. Also at 7 sec. into the video it appears that a small hole opens up in the left leg of the horseshoe. What does all this mean? I don’t know, but I do find it very interesting that the posted pictures and video would seem to acknowledge at least some of points of his presentation. Hopefully Aaron can maybe share some more information with us. I would really like to see some additional printed material to review this subject again.
 
Originally posted by Kurt Silvey

I think it was also noted that this DRC needed to be “warmâ€￾ so that when it fed into the meso circulation it might provide a boost to tornadogenesis.
Click to expand...

As Glen brought up, the Blob (DRC) and RFD are not one in the same. There has been discussion and studies in the past few years examining the relationship between RFD thermodynamic characteristics (theta-e, theta-v, etc... or 'warm' vs. 'cold') of tornadic and nontornadic supercells. The Blob theory seems to be seperate, yet I can't imagine to be entirely unrelated, from the RFD... Correct me if I'm wrong, Aaron or Glen...
 
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