Vortex Arches

[Danny Neal]

Vortex Arches in Supercells: WSR-88D Observations
Matthew J. Bunkers
NOAA/NWS Rapid City, SD
[email protected]
ABSTRACT
Observations from the WSR-88D at New Underwood, South Dakota (KUDX), are used to examine vortex “archesâ€￾ that have been found previously in many dual-Doppler analyses of supercells. These arches—formed by baroclinic processes—are defined as lines of equal vorticity connecting the cyclonic and anticyclonic vortex couplets that often straddle the hook echo of a supercell. A background and conceptual model of vortex arches is given first; then KUDX observations of three cyclonic supercells (one that was tornadic) over western South Dakota and northeastern Wyoming are used to illustrate various aspects of these arches. Evidence suggests that vortex arches (i) are at least occasionally evident in the WSR-88D data, (ii) may exhibit a dominant anticyclonic vortex, even though the supercell is cyclonic, and (iii) can potentially assist the warning forecaster when making tornado warning decisions. In the last example, vortex arching was observed about eight minutes prior to tornadogenesis. Given the recent upgrade of the WSR-88D network to “super resolutionâ€￾ in 2008, vortex arches should be relatively easier to identify now compared to years prior to 2008.

http://sio.midco.net/weather/workshop2010/abstracts/09_Bunkers.pdf

Anyone have any insight or background on this? I have been having a daily email correspondence with Dave Hintz the WCM at NWS ABR talking 5/22. I mentioned about how I thought perhaps one of "field" tornadoes may have been anti-cyclonic. He was interested and said

I would really be interested in seeing your close-up video of the drill bit tornado. Matt Bunkers (SOO in Rapid City) has done some work with vortex arches and seeing an anticyclonic couplet on radar in association with the RFD coming in. So…if the satellite was indeed rotating clockwise…it may have been the anti-cyclonic couplet actually reaching the ground.

I don't have a lot of experience with this phenomenon (to be honest it was the first I ever heard the term) and was wondering if any of you Met. Majors ever heard of this and may want to contribute thoughts?

 

[Jeff Snyder]

Danny,

I'll yield to someone else to explain the latest-and-greatest in terms of vortex arches since it's late and I want to get to bed. That said, do you have access to AMS journals? Markowski, Rasmussen, etc., have worked on the vortex lines and arches theory a bit in an attempt to determine how the how a tornadic circulation actually develops at the ground. Some reading (the first requires access to AMS journals):

Markowski, Rasmussen, Straka, Davies-Jones, Trapp, 2008: Vortex Lines within Low-Level Mesocyclones Obtained from Pseudo-Dual-Doppler Radar Observations. MWR

Straka, Rasmussen, Davies-Jones, Markowski, 2007: An Observational and Idealized Numerical Examination of Low-Level Counter-Rotating Vortices in the Rear Flank of Supercells. EJSSM

Markowski, 2007: Tornadogenesis: Our current understanding, operational considerations, and questions to guide future research. European Conference of Severe Storms

If you are wondering more about the anticyclonic member of a cyclonic-anticyclonic vortex pair often seen straddling the RFD of a supercell, I don't think these are uncommon. There are a few case studies in the literature that document anticyclonic tornadoes that occurred as part of a vortex pair on either side of the RFD, and, perhaps, I'll have another one to add if I ever get around to finishing and submitting a paper on the 4/24/06 El Reno supercell (which produced a cyclonic and anticyclonic tornado concurrently on both sides of the RFD).

If you are referring more to a possible anticyclonic satellite tornado (presumably in the intense shear zone along the periphery of a larger circulation -- when I think of "satellite", I think of a smaller tornado that rotates around a larger one), then I can't share much since I'm not aware of much work in this area. If you imagine an intensely-rotating tornado cyclone, there may be a circular/annular "sheet" of negative/anticyclonic vorticity along the periphery of that circulation. Now, whether that can "wrap up" enough to produce an anticyclonic "satellite" tornado, I'm not sure. I think I've read about such occurrences, however. These would be separate from the cyclonic-anticyclonic vortex pair along the left and right (respectively) edge of the RFD in a cyclonic supercell mentioned in the previous paragraph.

 

[Robert Edmonds]

In stead of throwing you in the vortex knowledge deep end, it might be helpful to first explain what a vortex line is…

I’ll try and give a non-mathy explanation.

First imagine you suspend hundreds (thousands, millions) of paddle wheels in the air and you orient them in the air. You orient them so that they spin as fast as possible (i.e. for example you orient the paddle wheel axis perpendicular, not parallel with river). The faster the paddle wheel is spinning the more vorticity there is at THAT location (assume really small paddle wheel). Now imagine you take a piece of string that goes through the axes of a paddle wheel. You take that string along the same direction of this paddle wheel axis until you run into another paddle wheel, reorient the string slightly and go to the next paddle wheel, etc. Eventually you have paddle wheels strung along this string. This is more or less a vortex line. In an ideal barotropic fluid (i.e. pressure is directly proportional to density) these strings with attached paddles wheels (and vorticity) will be carried with the fluid.

Hmmmm….. that still seems confusing.

Note, to have vorticity you don’t need circulation. For example wind traveling faster above (with all the wind still going in the same direction) could still cause the paddle wheel to rotate.

This is one of the major ideas discussed in the papers referred above (that I haven’t read completely). The papers also discuss ideas where you can’t treat it like a barotropic ideal fluid, so these lines may not get carried with the fluid.

Perhaps may go make some cartoon figures later...

 

[Danny Neal]

Thanks for the replies guys,

I get the concept of it, I think, but had never heard about vortex arches before. Robert your explanation was not over my head and I appreciate you giving me a visual. It intrigues me to know what is going on while I am sitting there watching it. Visually I can tell you what is happening in my eyes, but that may not necessarily be the most scientific of explanations . I have noticed cyclonic and anti-cyclonic rotations coexisting times before, but never knew of "vortex arching" existing. My education is brief in this type of thing (2 years at COD) and the rest I have picked up reading articles/essays/papers. That being said I will read these papers in a bit, thanks Jeff. I always knew there was the cyclonic and anti-cyclonic rotations involved however, I never come across a term to link them.

Evidence suggests that vortex arches (i) are at least occasionally evident in the WSR-88D data, (ii) may exhibit a dominant anticyclonic vortex, even though the supercell is cyclonic, and (iii) can potentially assist the warning forecaster when making tornado warning decisions. In the last example, vortex arching was observed about eight minutes prior to tornadogenesis. Given the recent upgrade of the WSR-88D network to “super resolutionâ€￾ in 2008, vortex arches should be relatively easier to identify now compared to years prior to 2008.

This is of particular interest to me. I would love to see radar evidence of this going on. As I said the whole term as a whole is new to me, maybe there are some examples in the papers? Would vortex arches be more common in left moving supercells? Those generally have anticyclonic mesos/rotation? Or are they speaking more of the classic right moving supercell in which has a dominant anticyclonic rotation? How reliable are these clues of vortex arching in detecting tornado genesis? Every strong tornado I have seen this year, had an anticyclonic couplet that we visually were able to see. This may have been studied for a while, but just learning about it now is fascinating!

 

[rdale]

I would love to see radar evidence of this going on.

Head over to the GRLevelX owner's forum, Mike posted a few examples as well as some researchers that make it very obvious.

 

[Danny Neal]

Thanks Rob,

I will go searching for them. Should be interesting to see and to dig my hands into. I wonder if this will actually be a beneficial clue to tornadogenesis.. or be more of a hit or miss.

 

[Adam Lucio]

I believe the Wakita supercell had one of these going for it as well. We were able to catch those spinups as we missed the main show of that massive supercell. Jon Davies also noted this in his write up of the event.

I can best picture it in my mind like gears, if you have a rotating gear that is spinning counter clockwise and attach another gear to it, it will spin clockwise. That is how I picture it in my working.

 

[Patrick Marsh]

I believe the Wakita supercell had one of these going for it as well. We were able to catch those spinups as we missed the main show of that massive supercell. Jon Davies also noted this in his write up of the event.

I can best picture it in my mind like gears, if you have a rotating gear that is spinning counter clockwise and attach another gear to it, it will spin clockwise. That is how I picture it in my working.

A better way to think about it is to use the tube from a roll of paper towels. Place this tube near the ground and let it roll (rotate). Now, in the middle of the tube, place an updraft. The tube will begin to lift in the middle, resulting in one end of the tube spinning in one direction and the other end of the tube spinning the other direction.

Depending on how the direction the tube was originally rotating determines which end is cyclonic and which end is anticyclonic. Try it!

 

[Jared Farrer]

I am the only one that finds this easy to conceptualize. I am not saying its easy. But pretty basic to wrap around. Lift a tube of spinning air and Boom u got urika! I am eating pork rinds and reading this somhow for the first time. But wondering where these posted Dualdoppler radar grabs are, I would love to see this tornado porn!!! As we call it! I am not saying it doesnt exist I just havent run accross it yet. Cause I probably havent searched the net enough. LOL. I wish people linked stuff better. I never get the new stuff. I have seen all the old stuff, but not anything recent (2008 and beyond). Hey, if anybody has anything recent and in the general area as this post please chime in, I love to soak up information. And knowing me I might have missed it. Any radar material or the like, oh yeah!!!

 

[Steve Miller TX]

Fascinating stuff. I just wanted to add some historical data and observations to the mix. This is from May 15, 2003 and has some great DOW images captured of a large tornado with a smaller satellite. It is a little study writeup I did as there were conflicting reports of the smaller tornado being anticyclonic or not. Some of the links on this page are expired, but the images are still available.

http://texastailchaser.com/special/051503/

 

[L.B. LaForce]

http://sio.midco.net/weather/workshop2010/abstracts/09_Bunkers.pdf

Anyone have any insight or background on this? I have been having a daily email correspondence with Dave Hintz the WCM at NWS ABR talking 5/22. I mentioned about how I thought perhaps one of "field" tornadoes may have been anti-cyclonic. He was interested and said



I don't have a lot of experience with this phenomenon (to be honest it was the first I ever heard the term) and was wondering if any of you Met. Majors ever heard of this and may want to contribute thoughts?

Ha, interesting, I thought I was the only one who noticed that the rotation on radar was anticyclonic, yet visually (proof by many videos, and self witness), it was cyclonic. Unfortunately, our "stuck" position was the white circle with dot, as we were surrounded by 2 circulations. That's a whole other subject, and we'll leave that out, but when I went back to the radar data, I was intrigued myself, and went and watched several videos including my own of the tornado.

This tornado for anyone wondering was the beautiful photogenic "drill bit" tornado. Here is a picture of it taken by Dick McGowan, and a video of it can be seen in my youtube version of it, or a few other chasers have the same tornado on cam.

Here is my footage of it: http://www.youtube.com/watch?v=hDywhbdZ13k
the tornado we are referencing can be seen from 3:12-3:56.

On a final note, I plan on using this event for my analysis for my mesoscale met. class, so it should be a lot of fun!

Also for anyone interested, I have zipped up 6 scans from the KABR radar that day showing the two circulations. Feel free to download, and play around with these, as well as the different scan levels. The files may appear as text files, as for some reason that's what my computer saved them in, but you should be able to drag them into GR2. Also note this is LEVEL 2 Data.

http://www.lblaforce.com/stuff/5.22.2010_SD_KABR.zip

 

[Nick Nolte]

This might be slightly off topic, but I'm curious...what causes the radar to display an anticyclonic rotation contradictory to what is really happening as LB posted above?

 

[rdale]

what causes the radar to display an anticyclonic rotation contradictory to what is really happening as LB posted above?

I'm not sure I understand your question. The radar isn't "contradicting" the observations... It's likely showing a vortex arch, which has cyclonic and an anti-cyclonic component.

 

[Nick Nolte]

I'm not sure I understand your question. The radar isn't "contradicting" the observations... It's likely showing a vortex arch, which has cyclonic and an anti-cyclonic component.

I was referring to the following quote, perhaps I'm not quite understanding what L.B. is referring to.

Ha, interesting, I thought I was the only one who noticed that the rotation on radar was anticyclonic, yet visually (proof by many videos, and self witness), it was cyclonic.

 

[Patrick Marsh]

Hi Nick,

I'm not familiar with this particular case, but let me offer this counter question. Why do chasers capture anticyclonic tornadoes underneath a cyclonic mesocyclone? The answer most likely lies in the fact that there is probably two separate processes at work. You have the processes that are leading to the development of a mesocyclone - which the radar observes - and you have the lifting of a vortex arch at the surface that results in the tornado(es) - which the radar does not observe.

Again, without knowing the specific details of the event in question, one hypothesis is that the radar was sampling an anti-mesocyclone aloft and the lifting of low-level vortex arches beneath the anti-mesocyclone result in the development of a cyclonic tornado.