Question on Vortex Breakdown

[Caleb Routt]

So I know that Vortex Breakdown occurs in a tornado when centrifugal force basically punches a hole in the middle of a tornado and causes it to break down into vortices when it acts with gravity. But how does a tornado like from Fridley MN become helical like it did? And how do horizontal vortices occur?

 

[Todd Rector]

I can't offer a scientific opinion, but I am curious which Fridley tornado you are referring to. July, 1986?

 

[Skip Talbot]

I've seen some animations that modeled these helical vortices, so I'm sure somebody can offer a pretty in depth and technical explanation. My crude conceptual guess is that you're seeing the condensation of a subvortex interacting with the parent tornado's circulation. The parent circulation is both lifting and rotating, so the condensation of the subvortex is going around and up. Imagine a bead of liquid caught in a screw tread. The liquid takes on the helical spiral as the screw turns.

It's probably a complicated mess of fluid dynamics for some of those multiple vortex tornadoes with all kinds of horizontal vortices, tentacles, and vorticity noodles. I'm guessing the horizontal vortices are rolls induced by intense updraft/downdraft interactions or from tilting. Maybe those have been modeled too, but I wouldn't be surprised if some of the processes behind those complex shapes are not well understood.

 

[Rob H]

I can't offer a scientific opinion, but I am curious which Fridley tornado you are referring to. July, 1986?

Not many pictures of "The Longest Night", so it's almost assuredly the 1986 one:

https://www.youtube.com/watch?v=LclcYJO6qDE#t=129

 

[Jeff Duda]

So I know that Vortex Breakdown occurs in a tornado when centrifugal force basically punches a hole in the middle of a tornado and causes it to break down into vortices when it acts with gravity. But how does a tornado like from Fridley MN become helical like it did? And how do horizontal vortices occur?

I don't believe the centripetal/centrifugal force plays a major role in vortex breakdown. A two-cell tornado features a central downdraft driven dynamically by vertical perturbation pressure gradient forces in the mesocyclone region. Vortex breakdown occurs when this downward punching downdraft reaches the surface and the resulting flow shifts the structure from two-cell to multi-cell (i.e., multiple vortex tornado). The helical appearance of the tornado in question is likely caused by the currently one-cell vortex being mangled and manipulated by the downward moving air (you can't see it because it's unsaturated) that hasn't quite reached the ground yet (that's why the base of the tornado doesn't fit with the helical structure of the rest of the tornado above it). Imagine sticking a conical object into a glass of soda or pot of boiling water and watching as the rising bubbles are forced to detour around it. In that example however, rotation is basically absent, so the effects from rotation of the funnel (and probably of the tornado cyclone or mesocyclone) probably cause the helical shape of the vortex. Any wobbles in the central downdraft probably cause noticeable changes in the physical appearance/path of the helical vortex.