Drag on a Tornado

[Scott Nelson]

This question has been on my mind for some time and although I'm doubtful that there is a specific answer to it... I figured I'd throw it out there. Maybe someone even has a video to illustrate their take--pro or con on the subject.

When a strong Ef1 or a weaker Ef2 tornado moves through a heavily wooded areas, do you think the bottom 30 feet of the funnel can be slowed by the drag/pull of the large volume of limbs and tree trunks?

Being from New England, we really don't have the open space some of the country has. I've been doing a research paper on the Flint Michigan/ Worcester Tornado and the Great Barrington Tornado is Western MA. Although these storms aren't comparible to my EF strength storms in question, (and the data will not techically be comparible) they all moved through heavily wooded areas.

Please note that I understand that with more trees you will by nature have more debris in the air. Thus more damage and my "drag" question doesn't seem relevant. But this question really originated from the photos of powerlines being literally BENT to the ground in a wooded section of Worcester county. The damage path tracked right through a heavily wooded area-- bent the stuctures to the ground and continued on. This damage to a man made structure was done by the wind speed only in that one area and it makes me wonder if the damage would have been even worse had the trees not surrounded these high voltage poles. I plan on featuring information on the closest buildings on both sides of these poles in my paper.

Thanks for reading and I look forward to your throughts on my question.

 

[Dave Marshall]

In a word: Absolutely. DOW measurements have shown many times that the fastest winds in a tornado are actually just above ground level. Air is a fluid, and fluids follow certain unbreakable rules. When encountering an object denser than the fluid, it must flow around the object. This will inherently impart some of the fluid's energy on the obstruction, as well as increasing the path length between two points (basically, the air can't travel in a straight line, so it takes longer to get somewhere). Both of these will result in a decrease in overall energy of the fluid once it flows past the obstruction. Whether or not that results in an appreciable impact on the damage inflicted by a tornado, I couldn't say. One of the Gurus will have to speak up on that (Where's Tim Marshall when you need him?).

I will say that while the effect might not be really visually evident on tornadoes I've seen, it is *very* evident with waterspouts. Non-tornadic waterspouts, such as those seen on the Great Lakes in the fall, lose energy very quickly if they encounter any obstruction at all, even just a small rock wave-break 4' higher than the surface of the water and maybe 100 square feet is enough to interrupt the circulation of the weaker ones and cause them to dissipate, at least temporarily.

 

[IAN GIAMMANCO]

Mean wind speeds typically follow a logarithmic increase with height which is governed basically by surface roughness or frictional effects, this holds true even in extreme conditions such as a hurricane. There is a little evidence out there for tornado events to support this structure as well, but a tornado is a more transient wind phenomenon. Now gust profiles may differ somewhat but are still essentially tied to the logarithmic increase in wind speed.

Given a very wooded area, there is also an issue of displacement height where the flow essentially does not "see" the underlying surface as te tree canopy acts as the surface. Now does this hold true in an event such as a tornado? possibly but no data exists to confirm this. But it would likely impart a signficant shear stress on the object.

And the concept of damage being worse if there were no trees is correct. The flow reaches its equilibrium at a lower altitude for smoother surfaces. This is a big issue in the hurricane realm, homes with a more open exposure often see enhanced damage, but there is a trade off with damage from falling trees as well.

 

[Jason Foster]

Some sources and tornadoes to check out:

Get Bill Harks Eastern Fury DVD, which will show many tornadoes with similar conditions since it deals with tornadoes along the east coast.

Two here, nearly in my back yard:

LaPlata & University of Maryland tornadoes both moved through wooded areas. In fact, the University of Maryland tornado moved through an area that had very large 100 foot Poplar and Maples. It basically cleared the forest after hitting the campus. Interestingly, the tornado lifted just before hitting the Home Depot and the Beltway (I-495). That would be one to check as if frictional affect is rather influential, it certainly save a large number of lives that day (considering the Home Depot and beltway are packed with cars and patrons).

 

[Scott Nelson]

Ian writes...
Given a very wooded area, there is also an issue of displacement height where the flow essentially does not "see" the underlying surface as te tree canopy acts as the surface.

Wow. I've never thought of it that way. Are you essentially saying that the full power of the storm might not contact the ground to it's fullest but rather use the tree tops as a temporary surface? If so__amazing!!!

Jason... thanks. I'll look into those Eastert storm videos for sure.

Dave... Thank you for your input. I'd love to hear what Tim thinks of this subject too.

 

[Caleb Routt]

Not sure if anyone is keeping up with this thread.. However it's not well understood about friction and tornadoes. From looking at papers of tornado sub-structure its evident that the strongest wind speeds are likely where the highest Angular Momentum flow is. Because tighter the radius, the less there is a distance from the central axis of rotation and the radius of the vortex.

Now when the Angular Momentum is high like that then usually it would be conserved because it would not take a great deal of energy to traval aroud the perameter of the vortex. So its really hard to tell which would win Angular Momentum or surface friction. The friction of the wooded area would have to depend on a lot of other variables than just the tornado's wind speeds.

Example: Swirl Ratio is the measurement of the tangental velocities to the upward motion of the vortex. In other words a high swirl tornado would support a large wedge tornado because there is so much "spin" aka "Angular Momentum" that the updrafts get centrafuged away from the core flow and inviting a downdraft through the core breaking down the tornado into multipul-suction vorticies as it interacts with the corner flow.

So what happens when high swirl tornado encounters surface friction? The tornado would most likely swich from turbulant flow at the boundary layer to mostly laminer flow which is a fancy way of saying a tornado consisting of mostly updraft. But this dosnt mean the tornado is going to become really weak enough to do damage though. Once the tornado moves out of the area with high amounts of friction, it then has a large upward moving core and the tangental velocities can then return making the tornado even stronger than before as strong upward velocities cannot be stopped because of inertia and drawing in large amounts of angular momentum. This is called "Corner Flow Collapse"

Finally, say if you had a medium swirl tornado with friction, The vortex would most likely be in a state of cyclostrophic balance which occrs when the "Pressure Gradient Force" and centripetal force are the same. The vortex at the boundary layer would then take on a low swirl tornado. When this happens the strong state of cyclostrophic balance aloft would then allow the downdraft to penitrate down to the surface and breakdown the tornado into vorticies. So the awnser to your question would depend on a number of things but most likely the effects on tornadoes that encounter friction usually result in vortex breakdown.

Hope this helps! I love Fluid Dynamics! And only 18 and in highschool!