The "tornado recipe"...ingredients that make for a tornado

[Jeff Hawkins]

Another thread posted recently about an article suggesting building a "Great Wall" in the USA to reduce the dynamics at play that contribute to strong tornadoes in the central USA. (Here is the link: http://http://www.stormtrack.org/forum/showthread.php?30279-The-Great-Wall-of-Tornadoes-How-to-Protect-the-U-S-! ). In reading the article and the subsequent criticism about the author's lack of accuracy in what causes a tornado, I began to say to myself "ok...what DOES cause a tornado?" So I thought I'd dig around a bit for a primer on the ingredients that contribute to tornado genesis (in the interest of those who want to know the facts behind the criticism). I found a NWS presentation by Ted Funk from 2002 on the subject. I would expect there has been some advance and expansion of knowledge in understanding of the processes involved since this presentation was made, but the basics to still be valid (for example, it talks about the "tornado cyclone", which I interpret to be what is now called the mesocyclone...although the author does also mention mesocyclone but doesn't differentiate the two).

The basic ingredients the presentation mentions are: "1. Development of a persistent rotating updraft; 2. Development of a “special” rear flank downdraft that contains rotation to aid in the development of rotation to the ground; and 3. Focusing of that low-level rotation through convergence and upward spin- up into the updraft". Additionally, the author points out contributing factors for supercell tornadoes, but clarifies these are not necessarily ingredients needed for all tornado genesis. The factors identified in the presentation include: "1. Ample low-level moisture; 2. Sufficient CAPE (instability); 3. Deep-layered vertical wind shear; 4. Significant forcing mechanism". What is obvious in the presentation is that very special ingredients that don't commonly occur in weather must occur together, in adequate amounts, at the right time, in the right way, to make a tornado.

I was initially going to make this a reply in the previous thread, but then thought it would be useful for those just learning basics, so I made it a separate thread. (Mods if you feel it should be part of the other thread feel free to move it there).

Here is the link to the powerpoint document: http://www.crh.noaa.gov/lmk/soo/presentations/tornadogenesis.pdf

 

[Matt Hunt]

Yes, those are the 4 main ingredients we always look for in order for storms to have the potential to form a tornado. Of course, some numbers might be helpful! Somewhere years ago I saw a plot that showed CAPE vs. 0-1 km SRH or something like that. It was a good look at how a perceived lack of one ingredient could be made up for with an abundance of another. I just did a quick search, but I can't find it. I thought it was by Jon Davies. I did find this by Jon Davies: http://www.jondavies.net/tornado_fcsting/sprcll_tors.htm

That at least has some numbers. In a more general sense, as the physicist incorrectly (for the most part) was trying to explain, it's the convergence of 2 different air masses that typically initiates storms. In the plains, it is not usually a cold front, as he was describing, but rather the dryline. This is where warm moist air coming up from the Gulf of Mexico collides with warm dry air coming from out west. But all across the country, tornadic storms could form from cold fronts, warm fronts, a shortwave, or any other kind of boundary (these would be the forcing mechanisms). Of course they can also happen without a boundary, but it's not as predictable.

 

[Mike Johnston]

Yes, those are the 4 main ingredients we always look for in order for storms to have the potential to form a tornado. Of course, some numbers might be helpful! Somewhere years ago I saw a plot that showed CAPE vs. 0-1 km SRH or something like that. It was a good look at how a perceived lack of one ingredient could be made up for with an abundance of another. I just did a quick search, but I can't find it. I thought it was by Jon Davies. I did find this by Jon Davies: http://www.jondavies.net/tornado_fcsting/sprcll_tors.htm

That at least has some numbers. In a more general sense, as the physicist incorrectly (for the most part) was trying to explain, it's the convergence of 2 different air masses that typically initiates storms. In the plains, it is not usually a cold front, as he was describing, but rather the dryline. This is where warm moist air coming up from the Gulf of Mexico collides with warm dry air coming from out west. But all across the country, tornadic storms could form from cold fronts, warm fronts, a shortwave, or any other kind of boundary (these would be the forcing mechanisms). Of course they can also happen without a boundary, but it's not as predictable.

Just a guess, but perhaps this paper on the energy helicity index by Jon Davies might be what you were referring to:

http://www.stormtrack.org/library/forecast/ehi.htm

 

[Matt Hunt]

Just a guess, but perhaps this paper on the energy helicity index by Jon Davies might be what you were referring to:

http://www.stormtrack.org/library/forecast/ehi.htm

Don't think so. I believe it was actually a powerpoint presentation. It might not have been Jon Davies, but I thought it was.

 

[Matt Hunt]

This one has a plot in it that might be what I was thinking about: https://www.google.com/url?sa=t&rct...=j5KdIHa8JZb5Qit8D_Zo4Q&bvm=bv.61965928,d.dmQ

(if that works) If that link doesn't work, just Google "supercell tornado environments" and it's the first result. I also have a presentation saved on my computer that is pretty good, and might be the one I was thinking of. It is by Jeff Evans from the SPC, but I can't find it online.

 

[James Gustina]

For those who are interested, the postulation and observation of a secondary rear flank downdraft/gust front behind the main updraft was put into a paper on the Goshen County, WY tornado from back in 2009. It has a lot of interesting theories on the mechanisms of tornadogenesis due to that secondary RFD surge, but it does get a little above general understanding of tornadogenesis so heads up.
http://www.cswr.org/MWR-FINAL-G1-Kosibaetal2012.pdf

 

[Jeff Duda]

Paul Markowski is one among a dozen or so leading researchers in this field. He is a genius. Here is a review paper he had published back in 2002 that thoroughly, but not completely, addresses your questions. It's a long read and sections may be difficult to understand, but it's a top notch review. He has a more recent review on this, but it predates VORTEX2. It's also not free: http://www.sciencedirect.com/science/article/pii/S016980950800255X

I'd have to refresh myself of the research coming out of VORTEX2, but I seem to recall from an SLS conference a few years back that they were starting to question whether the RFD had any limiting role in tornadogenesis, as there have been tornadoes observed from supercells with weak or non-existent RFDs. When it really comes down to it, tornadoes are just a very concentrated and intense blob of vorticity. Look up the vorticity equation. Each of the terms in it have physical meanings that can be used to understand tornadogenesis. Essentially, the atmosphere needs to convert the plethora of pre-existing horizontal vorticity produced in and around the storm into vertical vorticity via tilting and stretching, but also via other terms. My former officemate studied one particular method of tornadogenesis for his PhD by investigating the role that surface friction/drag has. Amazingly, when surface drag was turned off in idealized supercell model simulations, the storm failed to produce a tornado that it otherwise produced when drag was turned on. (My apologies to Alex if I misrepresented his research...this is merely a paraphrasing of what he did)

 

[James Gustina]

The mechanism for stretching with baroclinic vorticity generated in the RFD and then stretched/tilted really is really kind of cool. The Goshen tornado apparently had a very cool secondary RFD surge which is surprising considering that tornado maintained itself for so long, even though colder downdrafts tend to destroy tornadic potential.

That's interesting that nonzero MUk at the surface wouldn't inhibit stretching and expansion of the vortex. It'd be interesting to see if frictional/drag forces have any observable effect during tornadogenesis.

 

[JamesCaruso]

Given VORTEX 2 and related research projects since then, I would have to think there is much more current research available than the two 2002 papers referenced by Jeff Hawkins and Jeff Duda... Is this just not yet publicly available for some reason?

Further to James Gustina's point about warm vs cool RFDs, I remember hearing something a few years ago about the favorable role of a moist/humid RFD in tornadogenesis... But this recollection is anecdotal at best.

In the criteria listed in Jeff Hawkins first post, I believe a low LFC (level of free convection) (low temp/dew point spread) should also be included. While ample moisture is one part of a low LFC, you can have that but also have temperatures that are too high, resulting in higher updraft bases that are prone to being undercut by their own downdrafts.



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[rdale]

Given VORTEX 2 and related research projects since then, I would have to think there is much more current research available than the two 2002 papers referenced by Jeff Hawkins and Jeff Duda... Is this just not yet publicly available for some reason?

Certainly available - it's the main highlighted link at the top of the V2 page

http://www.eol.ucar.edu/projects/vortex2/publications/publication_refs.html

 

[rdale]

You'll also found many AMS 2014 recorded presentations on the current state of the science from Markowski, Ziegler, etc. at https://ams.confex.com/ams/94Annual/webprogram/SLSSYMP.html

 

[Karen St John]

Wonderful links, everyone! Thank you all, from a relative neophyte!