Horizontal Vortices with Wedge Tornadoes

[Jeff Miller]

Since watching a video posted in Reports in the Discussion thread for yesterday, and thinking back to other popular video showing this phenomenon, I am raised with a curious question:

How is it that a horizontal vortex can spin out of a large tornado. Ive been debating in my head this question and have debated some internal theories, however, I can't seem to get them to make sense. So, I am raising this question hoping some of our severe weather experts here can assist.

 

[Dan Dawson]

Since watching a video posted in Reports in the Discussion thread for yesterday, and thinking back to other popular video showing this phenomenon, I am raised with a curious question:

How is it that a horizontal vortex can spin out of a large tornado. Ive been debating in my head this question and have debated some internal theories, however, I can't seem to get them to make sense. So, I am raising this question hoping some of our severe weather experts here can assist.
[/b]

I don't know if I can provide a complete answer, but here are some of my thoughts on the subject. In the case of large and violent tornadoes, the inflow into the tornado (and the storm as a whole) is characterized by high values of helicity. Basically, helicity is a measure of the orientation of the vorticity vectors relative to the velocity vectors of the flow. High helicity is characterized by a small angle between the two, so that the vorticity lines are nearly lined up with the flow streamlines. This is why it is often referred to as a measure of "streamwise vorticity". In other words, you will have a tendency for rotation around an axis nearly parallel to the direction of the flow itself. It stands to reason that with the very strong acceleration of the flow as it approaches a strong tornado, the existing streamwise vorticity in the flow could be stretched to the point where you get a visible (due to the pressure dropping to the point of condensation), horizontally-oriented vortex aligned with the inflow into the tornado. That would be my best guess as to what is going on.