Empirical thoughts about tornadogenesis

[Andrea Griffa]

Empirical thoughts about tornadogenesisut down your theories

It's real that storm chasing is a funny practice and overall full of adrenaline and that's the most important reason why we all chase; anyway this doesn't mean that you can't see the sky with a different approach.

Personally even if I'm not a true scientist, I know as many of you, the principal theories of tornadogenesis and often I try to explain to myself why a storm that you rate with a tornadic potential doesn't became tornadic.

Well, the official science has not given a full explanation yet, so it happens that sometime I try to discover some empirical reasons that could lead to the missed tornadogenesis. And I'm convinced that someone of you has already thought of that.

So this is the goal of this thread: let's write down our theories, even though they are bizarre.

As concerns me, observing some tornadic and non tornadic supercells, I found that one of the most important elements that lead or not lead to the tornadogenesis is the one I call "cell interference".

I mean that you can observe a storm for hour, you can see a surfaced base, a tilted updraft, the stiratures start to be more defined, the lowering gets more low, a funnel start to rotate but you never get a tornado. Often in this case I keep 100 eyes on the radar based reflectivity and I try to watch with a critic approach what's going on, and in a high percentage of the cases you could see your supercell that remains isolated till you see that there's some interference with another towering cumulus or a little cell or another supercell that is just forming near.

At this point the interference cells get absorbed by your supercell and this kills the possibility to drop down the tube. Personally I feel on the radar as if the main meso would get less strong and lose the speedy rotational force that had before, whith the eventual hook echo that loses his rotational dimension, and this is very clear on the radar.

So if the old meso was enough strong to support the ingestion of those interference cells, it could assume HP features and drop down a short lived tornado inside the rain but the major part of the times the supercell weakens or it tries to get more reorganized. Anyway if the synoptic system leads to the formation of a lot of cells that compete against each other to get a better source of moist inflow it's always hard to have a tornadic supercell.
At the end I wanna say that to me this is one of the most important reasons why tornadogenesis can fail. I very often have noticed it in a lot of case.
There's another feeling I have about too cold RFD but I don't want to annoy you another time more.
I would be glad to hear some interesting theory from you guys and girls.

 

[Mike Hollingshead]

I did a very small "study" on the steam-nadoes produced by a corn milling plant here in town. Ever since then I've felt tornadogenesis was probably tied into the same sort of need. For what it's worth I'm not one of these chasers that have studied up any of the current ideas, so this is already probably stated as a factor.

These vortices, some large and some intense, pretty much refuse to form when the air is very saturated. If it was a foggy morning, or foggy day, it was hard to ever see one. But if it was a dry day, they'd be generated one after another. If it was windy and very dry they'd still find ways to form. Anyway, they just seemed highly tied to the humidity, far more than any sort of wind shear on the plumes. So when I think of tornado formation, I think of this kind of mixture. I think shear is just a need for the supercell, as well as what brings these differences into place. I do not buy into this horizontally rotating column of air out there being tilted.

It was very interesting to watch these plumes blow out, be blown over by any wind, then have two rotating entities which curled towards each other on the bottom, then away on their top. It's like a supercell's rfd when it cuts in and you can have to strong updrafts right next to each other, one cyclonic, one anti-cyclonic. These plumes would do that, and spread apart as they go out. Then when it's very dry outside, pieces of this half rotating plume would break off into these intense vortices. Anyway, that last step into vortex formation was just excessively tied to the humidity outside. At least in my small small study.

 

[Danny Cheresnick]

I think shear is just a need for the supercell, as well as what brings these differences into place. I do not buy into this horizontally rotating column of air out there being tilted.

Mike, you answered your question, that is exactly why shear is needed for a supercell. Shear in lowest levels of the atmosphere creates a horizonal "spin", I think referred to as an axle on Discovery. This is the rotation that allows a supercell to spin. Somewhere along that axle a bouyant blob of air rises, which becomes our supercell, and it bends that axle upward with it. That is pretty well accepted as the method for getting rotation in an updraft.

 

[Shane Adams]

If I'm not mistaken, I think H was referring to tornadogenesis, not supercell formation, when he said he didn't buy the horizontal tube theory.

 

[Mike Hollingshead]

Mike, you answered your question, that is exactly why shear is needed for a supercell. Shear in lowest levels of the atmosphere creates a horizonal "spin", I think referred to as an axle on Discovery. This is the rotation that allows a supercell to spin. Somewhere along that axle a bouyant blob of air rises, which becomes our supercell, and it bends that axle upward with it. That is pretty well accepted as the method for getting rotation in an updraft.

I don't think of shear as horizontal, but vertical. I can see how vertical shear/spin works and happens. I just don't see how you'd get air to spin horizontally. Even if you have strong unidirectional speed shear, I can maybe see things trying to rotate horizontally, but I don't see it becoming organized rotation. Just like supercells and vertical spin, I can see that almost creating vertical rotation, but it's really more like vertical curl as it moves on. I think you need some other factor to get things past what the shear would sort of dictate and into good rotation.

It sounds like a neat idea to have this horizontal rotating air, that then gets tilted upward by a storm/updraft...I just don't buy that. I don't say there's no tendancy towards something rotating horizontally, but I have a hard time seeing an air mass really spinning horizontally.

 

[Mike Hollingshead]

Why I'm talking about a topic a million smart scientists haven't answered yet, I have no idea.

Basically I think of tornadogenesis as this. The tornado is the feed back loop end result of low level moisture being properly directed up into a storm, mixed with a proper amount of dry air, which then cuts in behind the storm, adding some pressure back there, which influences the whole deal of mixing....which is the tornado...the center pressure area of that mixing...the tornado stays there while that mixing and pressure from it, is in the right balance. Chop one off in varying degrees and the tornado dies in varying degrees.

Talk about a basic set of thoughts, lol. Sorry for my boredom.

 

[Shane Adams]

Why I'm talking about a topic a million smart scientists haven't answered yet, I have no idea.

Basically I think of tornadogenesis as this. The tornado is the feed back loop end result of low level moisture being properly directed up into a storm, mixed with a proper amount of dry air, which then cuts in behind the storm, adding some pressure back there, which influences the whole deal of mixing....which is the tornado...the center pressure area of that mixing...the tornado stays there while that mixing and pressure from it, is in the right balance. Chop one off in varying degrees and the tornado dies in varying degrees.

Talk about a basic set of thoughts, lol. Sorry for my boredom.

I think that's an aweosme theory. It kinda agrees with the idea Chuck Doswell threw at some of us during the rememberance for Eric, that the tornado has a distinct purpose, serving a distinct atmospheric need. Until that night, I'd always considered tornadoes more of a random accident.

H's theory also kinda goes along with my long-standing opinion that supercells that do not produce tornadoes are incomplete processes. You have the contiental flow, the synoptic flow, mesoscale flow, mesocyclone flow, tornado cyclone flow, and finally the tornado. Each level rotating due to influence by surrounding windfields. You get the mesocyclone from mid level shear, you get the tornado cyclone from LL shear, and then you get the tornado (using H's opinion) from a combination of moisture influx VS dry air descent....as he says, a mixing thing.

So what's interesting to me about this is the tornado really doesn't rely on shear so much as a balance of different humidities, which would make one think that perhaps the rotation in tornadoes is already "taken care of" by the meso/tornado cyclones, and is ready and waiting for the final process to either happen or not. Because when you think about it, the tornado is already there before you first see it. They don't "drop" but rather condense, from that central pressure drop. I'm sure this has been rehashed to death on dozens of more private, more knowledgeable boards, but the way Mike explained it above really made me look at it differently, and it's kinda blowing my mind (so please forgive my repitition). Just very excited to understand it this way.

It's not about the shear, or even rotation itself...it's about balance.

 

[Mike Hollingshead]

I think that's an aweosme theory. It kinda agrees with the idea Chuck Doswell threw at some of us during the rememberance for Eric, that the tornado has a distinct purpose, serving a distinct atmospheric need. Until that night, I'd always considered tornadoes more of a random accident.

My theory lacks any details, is broad, and is certainly already much much better thought out, lol. I guess it's just annoying to me to see this crap all the time(storms, tornadoes....less so on tornadoes anymore!) and not have some great explanation for exactly how a tornado is formed and what it represents. Ever since the whole realization that the steamnadoes at my old job seemed to require a dry airmass, I've been that much more annoyed. Like anything, it's best to work backwards to find the answer. Knowing what the tornado is exactly is the first thing to start with. To me more than a "fluke" area of wind that gets caught up in a spin, it seems like something highly tied to mixing and changing pressures caused by that mixing and feeding of the different airmasses. Hell that is probably already stated somewhere.

 

[scott r currens]

I don’t know if this qualifies as a theory, it is really more of an observation. The more tornadoes I see the more I realize that all tornadoes are made-up of smaller scale vortices. I don’t think that there are any true single vortex tornadoes. It is just a matter of how tightly wound the vortices are within the larger scale vortex that differentiates a multi-vortex tornado from a “singleâ€￾ vortex tornado. If a tornado is a collation of smaller scale vortices that are stretched and bundled together, where do the vortices come from?

When we have strong LL-shear I think the lowest ~3000ft of the atmosphere is filled with small to medium-scale gently rotating vertical and horizontal vortices. It is the ability of a storm to gather and bundle pre-existing vortices that dictates its tornado production.

Ok, so that wasn’t exactly ground breaking but that is the way I think of it.

 

[Kiel Ortega]

One of the original thinking points in the thread was along the lines of why does a potentially tornadic storm not go tornadic? I think this is an incorrect way for the science to go forward b/c that's how the science has been looking at it. The opposite seems much more interesting...why did the storm that didn't look tornadic go tornadic? I was reminded of this over the past spring/summer around Norman, as several tornado events were very boggling (though many can cheaply be written off as QLCS events). One that I surveyed, up in Grant/Garfield Cos, was not tornadic when on radar it looked to be the most tornadic.

I think the biggest problem that scientists, especially those looking at the tornadogensis/tornado problem, is that they keeping trying to look at low level shear...which, from my minimal mesoscale learnings, leads to mid-level rotation, not the low-level rotation; low-level rotation being a result of baroclinic effects along the gust fronts. The tie b/w the strong low-level shear and the tie to tornadogensis is never really spelled out---just implied. Low-level shear I think helps in generating a strong mid-level meso and holding back the RFD from undercutting the storm. I also think the correct combo of low-level shear, to steer the low-level meso, and the mid-level shear, to steer the mid-level meso, is more likely. The more vertically stacked those two can get I think the better chance of tornadogenesis, and then into strength and life time of the tornado.

The biggest problem is the thermodynamics mix into all of this. This would mostly be from microphysics aspects in the cloud, which we are close to no where of understanding. I think especially for tornadogenesis, there are some things that we'll probably never know because of the ability to measure (especially in the vertical) near the tornado isn't possible. I think Dr. Saski's presentation on the role of entropy and others' work in entropy's role of tornadogenesis are very exciting as entropy combines both the dynamics and thermodynamics.

 

[Mike Hollingshead]

Here is another thing from work that drove me nuts and was "tornado related" too. Driving those big steam stacks are massive fans, which have massive intakes to them. The main one was probably 3 feet in diameter, with the center about 8 feet off the ground. There's a large volume of air sucked through that thing.

So one day I'm working out there with another co-worker when I noticed something in the water on the ground that looked like a bug flooping around. The water on the ground was black and nasty, almost soupy from the crap in it. What it looked like was a cricket sized bug going completely nuts in there. Well the cement floor slanted so it went back to dry ground at a slant. This "bug" in the water would vanish into the concrete when it left the water's edge. It was driving me nuts! I figured it out fairly quickly though. The co-worker did not however, and I finally had to fill her in because she was in danger of losing her mind over it. She really thought that "bug" was going into the concrete. It was a vortice from the fan intake way above.

So of course I had to dwell on the why's of this whole thing. I wasn't fully understanding why the vortice had to reach downward. The pipe ran perfectly horizontal. I was like, why can't the vortex just go straight out and be happy. The other thing was where it was contacting the ground first. It wasn't arced out and coming down. It was almost bent at a 90 and forming right under the end of the intake(several feet below). Then it would behave the same way every single time. It would tear up the water and gradually extend outward, away from directly below the stack. It would extend and lift or "vanish" into the pavement. Then it would reform directly under the stack again. I don't remember it ever moving back into that area. Just seemed odd it had to form there, then strech out each time as it tried to suck up the water...then repeat. So then I began to just try and relate that to tornadogenisis, and maybe anvil level flow/vacuum evacuting large volumes of air. I'd love to understand why that vortex had to reach down to begin with, and why it had to do it right at the edge and never out in an arc first. Needless to say I never got a whole lot of real work done at that place.

 

[Doug Lee]

My thoughts on tornadogenesis:

Tornadoes might form several different ways, and each method has its own key factors. The end result is a violently rotating column of air in contact with the surface attached to the base of a thunderstorm, and the winds in that column have linear speeds at a minimum specified in the Fujita scales.

One factor that I've not seen discussed much is what I call vertical coupling. In one Nova episode, Lou Wicker recreated the Manchester SD tornado in a digital model. The model indicated the whirlwinds developed at the surface and merged together to formthe tornado. What could be happening is that the rotation occurs at the surface and in the storm, and if the rotations get close enough, they combine or couple together to form one singe rotating column.
Another possibility is that the only the upper part of the updraft is rotating, and surface level rotation starts spinning the bottom of the updraft.

The rotation in the upper levels of the updraft could also intensify enough to cause the rotation to grow down towards the ground.

Horizontal rotation does happen in roll clouds, but as far as an updraft tilting a horizontal roll upwards, I am skeptical of that. To me, the updraft would have to be very strong to pull the horizontal vortex upward. I believe a more likely scenarion is that if the horizontal vortex contacts the updraft, it starts rotating the updraft.

I'm not proposing any one hypothesis as correct. AFIK, they might all be right, they might all be wrong.

 

[Paul Knightley]

I've never thought that the horizontal vorticity thing was there to explain tornadoes as such, more the rotation in supercells.

Anyway - in theory, perhaps tornadoes per se should be quite easy to understand - just pull the plug out in the bathtub and watch the vorticity tighten up into a narrow tube! In the atmosphere, the same thing must surely happen, albeit that the vorticity can be generated in different ways.

Vorticies forming on gust fronts and then being ingested/stretched by a passing updraught could explain tornado formation in a number of situations. Maybe a unified theory of tornado formation could be sought, and then applied to various "types" of updraught/thunderstorm?

 

[Adam Lucio]

To me the idea of the updraft pulling a horizontal vortex upwards is only plausible if there is unidirectional shear under the base of the storm. In cases of strong directional shear this doesnt make sense to me because I would think the shear/vortex would already be verticle in nature and wouldn't need to be tilted upwards.

We all know that directional shear gets our sups cranking, but I think that something unique as far as air movement has to be going on under the base...the old "pinwheel effect," and that is what I can see being "titled" upwards

Another theory I have is part of the mesocyclone or tornado cyclone becomes compressed and stretches...think of a ball of playdough, if you roll it between your hands it stretches into a piece of spaghetti but still has the same amount of dough.

I know a contradictory point is the fact that if we have a tornado, there is still an area of larger rotation above it. I was just tossing out non-conventional ideas in my head.

Interesting theories and points everyone, it gives me allot to ponder.

 

[David Wolfson]

I mentioned another possible factor in http://stormtrack.org/forum/showthread.php?p=139061&highlight=upper#post139061
What I call the "rock in the stream" dynamics may be a significant factor in the RFD mechanism which pins the tornado vortex to the ground. FWIW.