Path deviations

[Caleb Routt]

Been reading some papers on terrain effects on the near surface tornado structure the (corner flow region). I’ve came across numerous times that some tornadoes will deviate from the general forward motion. Can anyone explain this? Does it have anything to do with a change in swirl ratio?

 

[Paul Knightley]

I can't profess to be able to answer question as I think that the low-level wind flow into tornadoes is still rather poorly understood. Having said that, overall I would suggest that a tornado's motion is generally governs by a couple of factors: the parent updraught's motion; and interaction with low-level outflow/gust fronts. Tornadoes can also rotate about the parent mesocyclone, probably due to the latter.

So all I can suggest is that low-level flow which is being affected by terrain is going to have some implication to how it arrives at the tornado, which could, in turn have an effect on its motion - or, at least, the motion of its base.

 

[Alex Elmore]

Does it have anything to do with a change in swirl ratio?

Swirl ratio determines a vortex's structure; from single-cell, to two-cell, to multi-vortex. I don't believe it has any direct implications on the motion, although an increase in the swirl ratio will cause an increase in the size of the vortex, which may cause the vortex to appear that it's changing direction.

 

[Jeff Duda]

I wonder if an appeal to potential vorticity would shed some light here. If a tornado moves uphill the vortex will compress vertically, which due to conservation of potential vorticity should (if I remember my dynamics correctly) cause the vorticity to decrease (although I may have that exactly the wrong way). The change in vorticity could possibly then lead to a course deviation somehow, but the exact mechanism escapes me as of this moment.