Horseshoe Vortices/Eddies On Striations

[Andrew Khan]

I have a few questions about Horseshoe Vortices/Eddies On Striations.

First, how can these eddies form on the striations of a rotating supercell? Do the eddies themselves actually rotate, as well? Eric Nguyen noted that these things are actually quite rare. How do these things form? Why are they rare? What renders them?




Also what makes horseshoe vortices? How do they form?


Thanks for any help!

 

[nickgrillo]

Yeah, I do know it's a pretty rare event... I chased that storm for about 2-3 hours prior to the "eddies", and by that time of those snaps the storm had incredible banding/striations (and was ready to get munched by the main line behind it). At dinner, I believe Eric said he didn't see this type of occurrence since 1999( ? )... Not quite sure what makes this phenomenon, but I'd imagine Amos or Eric himself would know.

 

[Andrew Khan]

Yeah, I also heard, this is quite a rare event. What do you mean about it getting munched by the line behind it?

 

[Andrew Khan]

Oh okay, so it became a derecho event..wow. Have you seen any horseshoe vortices?

 

[Andrew Khan]

Oh. Thanks Nick. Can anyone provide any further input?

 

[Amos Magliocco]

This was taken at the same time as Eric's image and from the same place, but looking more south southwest instead of west.

Glen Romine gave me a great explanation of the physics behind those eddies once but I won't try to recount what he said. Perhaps if he doesn't notice this thread, someone could PM him?

That was a very memorable storm for all the presentations. A photogenic LP that transitioned into a classic (while producing a needle tornado), to what became a fast-moving HP with forward flank mesos and eddies in the striations. Can't ask for much more.

 

[Andrew Khan]

How fast moving was it? I might PM him.

 

[Glen Romine]

Glen Romine gave me a great explanation of the physics behind those eddies once but I won't try to recount what he said. Perhaps if he doesn't notice this thread, someone could PM him?

Well, I don't know how great it was - but I'll try to offer at least one possible explanation. The visual appearance of the vortices is very similar to the pattern you get from pure Kelvin-Helmholtz instability - which are affectionately called cat's eyes vortices sometimes for obvious reasons. Here's a more classic view:

(from http://www-sccm.stanford.edu/Students/witting/kh.html)

Kelvin Helmholtz vortices are most often seen in the atmosphere as vertically oriented wave clouds - occurring with sharp themal inversions coupled with extreme vertical wind shear, and conveniently clouds only at the base of the shear layer to act as a tracer making the shearing instability occasionally visible. Above note in Amos's example the vortices are developing in the horizontal plane - so to apply the same explanation here the strong speed shear must be in the horizontal which is easily plausible - but the horizontal density gradient is less easily proven. One could infer from noting the very laminar cloud characteristics 'forced' ascent - which would potentially coincide with a leading edge of very deep cold pool, which if surging against a strong low-level jet could provide the needed environment of sharp density gradient (in the quasi-horizontal along the leading edge) coupled with a local extreme wind shear - as well as less humid air being entrained into the storm just above the boundary layer making the vortices visible.

Like most things in the atmopshere - it's tough to know for sure what caused the vortices seen here - but at least a good visual match could be what I've suggested above. The rarity of this visual treat suggests a combination of uncommon factors. Other ideas are welcome.

Glen