Funnel Drag

[Scott Nelson]

I've never seen this question discussed on the forum so was wondering if anyone had any specific studies on the movement of the funnel under the mesocyclone?

When a storm drops a funnel from the wall cloud there is "usually" storm movement/motion that will pull the tornado along with it. Yet I often see videos or still pictures in which the funnel IN AN OPEN FIELD is essentially being dragged well behind the updraft. I understand that wind may be slowed as it touches large objects but when it's not impeded by anything more than grass and dirt the funnel should keep up with the storm with little or no effort. Especially a weaker storm. (IE: less restriction... easier to drag.) Yet what I see the more violent storms that should show drag by the nature of the funnel FORCING ITSELF INTO THE GROUND they usually keep up with the storm movement much better. It seems like it should be the other way around so I'm having trouble wrapping my brain around the process.

I'm sure part of the drag is caused by the random spin of the funnel or maybe there might be some outside influences like the rear flank downdraft or inflow that I'm not factoring in. Heck... I think every tornado has multiple vortex columns of air near the main spin that don't always touch the ground that could create drag by blocking the funnel but that's my own thoughts.

Silly topic I know but what do you think?

 

[rdale]

I guess I'm not following your conceptual question from the start. Are you saying that the "friction" or something caused by the ground should hold a tornado's motion in check? And I don't even remotely understand what you mean about larger funnels "forcing itself into the ground." Can you try rewording that? What is being force into the ground?

 

[Skip Talbot]

A tornado is not a solid object physically attached to its parent storm, so it's not "poking" the ground like when you poke a table top with your finger. You generate more friction by putting more force down on the table with your finger as you drag your finger across the table. This analogy doesn't apply to toradoes, however, since they're complex wind fields with winds moving in all directions. Friction plays a big role in the surface winds of a tornado, even if it's over grass or dirt, but I don't think the tornado is "dragged" in the sense that you think it is. It's more of a flowing with the other winds. What you're probably seeing with smaller tornadoes, is that they are more strongly influenced by other winds like the RFD and inflow. Outflow can cause a tornado to bow outward from the direction the outflow is moving.

 

[Scott Nelson]

Thanks Skip. I've never thought of the tornado NOT being attached the the parent storm. Something else for me to wrap my brain around. But it sounds like I was right with my thinking the "dragging" may be the influence of storms inflows/outflows.

Rdale: Sorry the wording of my question didn't work for you. Had I used "trails" behind it might have made things easier to envision. Essentially a more powerful storm would have more suction right? With more suction you have more friction... and thus more drag or tailing will take place. Rather it seems to be the oposite. A tornado with less power tends to trail rather than keep up with the parent storm. This is a thread to learn on so that's why I tossed it out there.

 

[Robert Edmonds]

Even if you consider no friction being present the surface does play a role in the vortex dynamics. If you consider a horizontal vortex to the surface it will "see" an image of itself (with opposite sign in vorticity) across the boundary making it move in the horizontal direction. In fact so long as there is any component of the vorticity not directly perpendicular to the surface the vortex will want to have some horizontal component of velocity (relative to the mean flow).

The surface with friction considered does create small amounts of horizontal vorticity in the parcel at the surface. This can be advected, streched, tilited, and/or slowly diffused through the fluid as well.

I have some simulation with a "toy" model I made last year-ish. I'll post them later.

 

[Scott Nelson]

Looking forward to seeing your model Robert.

 

[Robert Edmonds]

These are simple vortex line and ring models. They do not have friction/viscosity included, but there is a limit to how much the lines can be bent. The lines represent where vorticity has been concentrated, and the points are where I keep track of the line. I developed these initially to see if I could find some nearly stable configuration (I was unsuccessful) with some combination of lines and rings. The surface was handled in different ways. For one the lines of vorticity do not assume to intersect the surface, others the vorticity assumes to intersect the surface perpendicular.

http://www.youtube.com/watch?v=BBp1RvyHTK4&list=UU1A7eejMzFWbdVw9HMO6UdQ&index=10
http://www.youtube.com/watch?v=xMTe93mvqPs&list=UU1A7eejMzFWbdVw9HMO6UdQ&index=9
http://www.youtube.com/watch?v=VmHzwtM4rUI&list=UU1A7eejMzFWbdVw9HMO6UdQ&index=8

You do see some interesting effects like vortex pairing (here lines approach and align with each other). Note, this could have an affect in something sorta like pulling vorticity toward the surface, since it should see an image across the surface.

 

[Scott Nelson]

Very cool. Video three is the most interesting to me but thats from a person knew to this kind of modeling. How would you use this data?