Gravity Waves Make Tornadoes

January 07th, 2009 | Categoria: xn--btvw8bh0b.com

NASA Science News for March 19, 2008

New research by NASA-supported scientists shows how atmospheric gravity waves, the kind we often see rippling in clouds overhead, can hit a thunderstorm and turn it into a deadly tornado.

FULL STORY at
http://science.nasa.gov/headlines/y2008/19mar_grits.htm?list71039 (http://science.nasa.gov/headlines/y2008/19mar_grits.htm?list71039)

I'd like to see more than a NASA press release before completely buying in...
Lots of "mays" and "more likely" which I could agree with. That's not the same as saying "waves MAKE tornadoes"

Yeah I agree with that. I think we tend to just give the seal of approval to anything NASA puts out and while they probably aren't completely wrong I would say they are more wrong than right with their assertion that "gravity waves make tornadoes". I think gravity waves play more of a role in initiation (and even the frequency of that is probably low and very debatable) than the actual mechanics of tornado formation. It doesn't really matter I suppose though since we will never be able to verify or deny assertions like that anyway.

I remember seeing a presentation about this at SLS a little over a year ago. The hypothesis is this: gravity waves are associated with larger environmental horizontal vorticity (wind shear). As a storm encounters this gravity wave, the updraft tilts the larger ambient vorticity and stretches it, resulting in a supposedly stronger mesocyclone-> tornado?

This isn't necessarily far fetched... we see supercells go bonkers near boundaries where you have enhanced shear AND better thermodynamics.

What I question is whether you can directly relate an individual gravity wave to tornado occurrence. Perhaps the mesocyclone is strengthened slightly, but proving the storm is actually encountering a gravity wave AND attributing this to the cause for tornadogenesis is extremely difficult.

Good day all,

SPC even has a link to such events in their "Cool Images" section...

http://www.spc.noaa.gov/coolimg/gwavecb.gif

This was a hail storm that was enhanced by a gravity wave train. Tornadoes, dependant on storm updrafts, also can be influenced for obvious reasons.

Lint to SPC on this event is below...

http://www.spc.noaa.gov/coolimg/gwavecb.htm

It also seems that these waves, like more-common air-mass boundaries, can influence storm propagation as well if the supercell becomes "anchored" on them. What if a strong gravity wave and outflow boundary / dryline interact?

Holy smokes. I was just reading up on Gravity Wave detection. That's LIGO setup is nothing short of amazing!

http://www.lightandmatter.com/html_books/0sn/ch10/ch10.html

After watching that gravity wave video from a year or so ago from Tama, Iowa. I could see how it could affect a storm system. You all have seen that video right? Anyways that LIGO is a few years old now. Is there any way besides visually to easily detect these gravity waves?

I'd like to see more than a NASA press release before completely buying in...

If I read it correctly - the only thing different between a tornadic storm and non-tornadic is speed of the spin and height of the storm. Smaller / faster storms = Tornado. Not onboard with that conclusion.

Also...

"There is also wind shear in a gravity wave, and the storm can take that wind shear and tilt it and make even more spin. All of these factors may increase storm rotation, making it more powerful and more likely to produce a tornado."
"We've also seen at least one case of a tornado already on the ground (in Birmingham, Alabama, on April 8, 1998) which may have become more intense as it interacted with a gravity wave."

How did they determine time of gravity wave impact vs increase in tornado intensity?

Lots of "mays" and "more likely" which I could agree with. That's not the same as saying "waves MAKE tornadoes"

This was a hail storm that was enhanced by a gravity wave train. Tornadoes, dependant on storm updrafts, also can be influenced for obvious reasons.

I don't know if you can say that the hail storm was "enhanced" by a gravity wave train when we have no idea of what the storm was like without the influence of the gravity wave. It is safe to say that the gravity wave (train) aided initiation by aiding lift necessary to overcome a strong cap, but I'm not comfortable in saying that the storm was "enhanced" by this gravity wave.

I'm also not so sure that tornadoes are influenced "for obvious reasons"...mainly because it is not obvious to me.

There is clearly better chances for a tornado when you have a strongly rotating meso, but there's no criteria where you can say "rotation > x = tornado."

We saw that this weekend when Atlanta issued a Tornado Emergency because they saw strong rotation on radar, but not even strong winds were reported on the ground.

Gabe you're proposing that the author meant a horizontal compression versus a vertical compression?

If so, I can't visualize a gravity wave doing this...

Aaron, I agree with what you have said, however the article specifically mentionsWhen a gravity wave bears down on a rotating thunderstorm, it compresses the storm. This, in turn, causes the storm to spin faster. This is almost completely contradictory to what you just said about about stretching (which is true). I don't know how to rectify the discrepancy. LoL.

I recall reading some assessments re: the role a gravity wave was suspected to have played in the deadly Jarrell, TX F-5 tornado. As I understand, it was believed to have a role (as a boundary) in convective initiation of the storm complex and also perhaps was a factor in the unusual southwesterly storm motion.

Must admit, though, this new hypothesis of a gravity wave enhancing shear and contributing to tornado strength is intriguing. Perhaps the researchers will take another look at Jarrell, as the gravity wave was well documented in that event.

Good fine Eddie! That helps explain much more what his theory is about...

I think the "compression" the author could be talking about is a decrease in the length scale of the wave (instead of a full 3D compression). In this case, the vorticity associated with the gravity wave could actually stretch and intensify (as the proverbial ice skater pulls in her arms). Definitely a poor choice of wording, though.

Aaron, I agree with what you have said, however the article specifically mentionsThis is almost completely contradictory to what you just said about about stretching (which is true). I don't know how to rectify the discrepancy. LoL.

Now that's funny. I have *NEVER* considered Gravity Waves with anything to do with tornadogenesis. Wow, that puts a whole new *spin* on it!

I'll have to really read that and study into it......

I just skimmed the article so I missed that tidbit. Sounds like a bad PR interpretation to me. Gravity waves travel through a fluid... actual "compression" would have to be negligible given the viscosity of air and the pressure perturbations involved (if I can still trust the fluid mechanics knowledge in my head) . With gravity waves, however, you will see oscillations of air rising/falling which will be associated with max/mins of vorticity, much like horizontal convective rolls.

Question: when one sees mammatus formations that seem to line up in rows instead of the usual random clustering, is this visual evidence of the presence of gravity waves?

Thought this would be a good link for all the details on this research:

http://www.atmos.uah.edu/mips/personnel/tim/

Eddie

yea that's the video I was talking about. The term "Gravity Wave" is really used to loosely so I'd have to retract my gravity wave statement given to this video above.. ;) I would consider a gravity wave to be a invisible gravitational force creating a disturbance in the atmosphere across a flat area. Not a mountain/hill creating a wave in air flow like this:

http://www.aerospaceweb.org/question/nature/thermals/mountain-wave.jpg
Now if you replaced the mountain with "a change in gravity" you would have a Gravity Wave.

After looking at the Google maps terrain mode (Tama, Iowa) where the video was shot. I'd think that video was a "mountain wave cloud".

Far as creating tornadoes I don't see how a real gravity wave would make a difference. I mean wouldn't both air masses be equally affected by differences in gravitational pull? If anything I would think it would disrupt the tornado cycle. Of course I could be 100% wrong. Don't shoot me, I'm still learning here. :o

I understand what you mean and it's true in what you say, purely because the weather is constantly changing, which is what makes the weather as intrigueing as it is :)

Willie

Tim Coleman of UAH is also working on this. We have had several damaging gravity wave events here in Alabama over the last several weeks including some tornadic activity. I believe that most of the action was low topped and shallow based in a weak instability environment if I am not mistaken. I'll be interested to see what conclusions are drawn.

I've searched and can't find a web presence for the Grits tool or anything related other than this story...

Here is an offering from the SPC regarding the Jarrell, Texas tornado. Since I'm not as versed on the terms as a published scientist, I'll leave the explanation to him:

Coincidental with the onset of deep convection near Waco was the passage of the MCS-generated gravity wave. Having originated in Arkansas, the central Texas portion of this feature moved southwestward in-step with the surface wave until around 2100 UTC, at which time the cold front overtook the dry line near Austin. The advance of the gravity wave and its motion relative to the developing storms is readily apparent in the animated satellite imagery shown in Figure 6.

As previously noted, while the lower tropospheric environment over central Texas on the afternoon of 27 May 1997 was very unstable, such instability is certainly not unprecedented. The arrangement and behavior of mesoscale surface features was also not especially noteworthy: slowly-moving dry line/frontal mergers of the type observed occur in that region several times a year. Readily apparent gravity waves are also not uncommon in the area. Nevertheless, the fact that supercells continued to form over central Texas through much of the afternoon despite the presence of weak vertical shear suggests that some unusual circumstances were indeed coming into play. While a more conclusive answer must await careful modelling studies, it is the author's opinion that the fortuitous orientation and timing of the MCS-induced gravity wave, relative to both the slowly-moving surface boundaries and to the diurnal heating cycle, were critical in realizing and maintaining supercellular convection.

The source for the paper is directly from NOAA. (http://www.spc.noaa.gov/publications/corfidi/jarrell.htm) I thought this would be interesting to add to the discussion (if I missed it in earlier posts, I apologize).

You're confusing two things.... the gravity field (whatever that link was you posted) and gravity waves in the atmosphere. In atmospheric gravity waves, buoyancy and gravity act as opposing forces.

While different in some ways to their atmospheric counterparts consider this wave scenario: Consider letting a water droplet impact the surface of water. You cause a perturbation in the water surface that forces it down. This in turn causes a pressure pertubation in the water that forces the water back up. What happens is this overshoots the mean water height, and then gravity acts as a restoring force to push the water back to its original height. Over time, you'll see the waves decay as the fluid eventually comes to balance. In the atmosphere, gravity is also the restoring force. The perturbation is typically caused by bouyancy... cold pools from thunderstorms for example. In many cases, we can use clouds as an indicator for gravity waves. Either as a constant field like the Iowa video, or as rows when you are near saturation and the gravity wave is strong enough to cause condensation.
http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2008/03/080315_modis_truecolor_waves.jpg

I'm glad Eddie posted the link to the guy's webpage with the papers...b/c the press release was laughable. The paper first paper listed on the page was decent...however, a big problem with it is that in the simulation he sends the trough first then the ridge for the interaction. Looking at the graphs, if the ridge is sent in first it looks as if the vorticity in the mesocyclone would significantly decrease and the increase later wouldn't overcome this decrease. I didn't read the entire paper (thumbed most of the examples), but I would like to see the results of his model ran that way and an example where one of these waves weakens a mesocyclone.

Short YouTube video of gravity waves in action:

http://www.youtube.com/watch?v=yXnkzeCU3bE

This may make it a little more visually clearer - in a way. Though for me it just raises more questions than anything.

We had a gravity wave driven gust front that went thru here on Jan 8th...The forward speed of the front was 65-75 mph...It produced EF-0, EF-1 tornadoes and several non-tornadic vortices...
973

Ok, here's an kinda side topical question that I have been wondering, although it's not related to gravity wave interaction of supercells, but on the question of a supercell's rotation itself :)

Has there ever been a study of a supercell's rotation speed, between those that are tornadic and others that are non-tornadic? :)

I have been wondering for the past 8 - 10 years about the likelihood of being the supercell's rotation speed, being an indicatior of which supercells are more likely to be tornadic, than those that are non-tornatic?

I imagine that there has already been a study on this os some form, but I can't help but feel that it is something that could be looked into a bit more, if it hasn't already.

Now back to the topic itself.....

I agree with rdale on this, purely because I haven't been aware of gravity waves and so, I don't know much about them, but at the same time, I think that there wouldn't be too high a likeihood of a gravity wave influencing a storm, after all, if you look at how many stroms there are in the US every year, and thinmk about the number of gravity waves there are each year, then I imagine that the number of potentially gravity wave influenced storms would be quite low, but in saying that, I don't know of there being any evidence to prove that gravity waves do influence storms in one way or another.

Now I'm saying this as a personal view because I haven't read the report about it, but I will do at a later time :)

I remain totally open minded on the possibilities of gravity wave interaction of supercells, as like anyone else, I'm here to learn :)

Willie

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Gravity Waves Make Tornadoes