Ground level winds in tornadoes

[Bill Hark]

I was recently told by a media person that at ground level in a tornado, the wind speed is 0 and remains so for the first inch off of the ground and she wanted to know why? She couldn't remember her source of the info. I know that the wind speed decreases near the ground due to friction, but I have never heard that statistic. I actually don't think it has been determined including with Tim Samaras' probes. Has anybody heard that statistic and reasons for the lower speed other than ground friction.

Bill Hark

 

[Danny Cheresnick]

I was recently told by a media person that at ground level in a tornado, the wind speed is 0 and remains so for the first inch off of the ground and she wanted to know why? She couldn't remember her source of the info. I know that the wind speed decreases near the ground due to friction, but I have never heard that statistic. I actually don't think it has been determined including with Tim Samaras' probes. Has anybody heard that statistic and reasons for the lower speed other than ground friction.

Bill Hark

Bill, I think this is probably someone who has had the theoretical physics quoted to them. Someone who has more recently dont the math on this should probably correct me, since I'm several years removed. The key is whats caled the log-wind relation. This basically states that the effect of friction at the surface is reduced exponentially as you go up in the atmosphere. This link below show a graph of what that would look like:

http://www.windpower.org/en/tour/wres/shear.htm

So right at the surface, there is a very thin layer where friction is high enough that air is essentially stagnant, basically "stuck" to the surface. I dont think there is really a depth to this though, but calling it an inch just implies very shallow I would guess.

This question is one of those things the ROTATE field project was hoping to investigate with the combination of the DOW, TIV, and PODs, ideally giving wind data at multiple level directly over one another, with PODs only being 1 meter high.

 

[Simon Andersen]

Wouldn't a tornado have to have ground level-windspeeds above 0 mph to whirl up soil and rocks from ground level? Or perhaps pressure is the reason for that. Very interesting subject either way.

 

[STurner]

***** Yeah that is a very interesting topic. Was it the Oklahoma City tornado they clocked 318mph winds but it was well above the surface?

 

[Simon Andersen]

***** Yeah that is a very interesting topic. Was it the Oklahoma City tornado they clocked 318mph winds but it was well above the surface?

Hi Shane, that windspeed was between 160 and 320 feet above ground level. And it may have been less than 318, but probably above 300 mph. As per: http://www.srh.noaa.gov/oun/storms/19990503/may3faqs.html

Sorry to disrupt the thread with this, you'll remove the post if you want, mods

 

[Rob Wadsworth]

Danny - question.
Doesn't the inflow jet move accros the surface as it then moves upward into the funnel?
If we are seing a dust cloud on the surface before the condensation funnel fully forms - isn't it evidence of wind scouring the ground along the surface?

 

[Danny Cheresnick]

Danny - question.
Doesn't the inflow jet move accros the surface as it then moves upward into the funnel?
If we are seing a dust cloud on the surface before the condensation funnel fully forms - isn't it evidence of wind scouring the ground along the surface?

Well, thats where the theoretical part comes in. Obviously dust, grass, andall sorts of things on the ground do get into the air. For wind speeds as high as near a tornado, this layer could be only a tiny tiny fraction on an inch. And thats under the assumption of a perfectly flat "ground" which obviously is never the case. I'm probably the wrong person to explain this, since its been a good number of years that I have been away from the mathematics of it, and it was never really my specialty anyway. I just wanted to provide the likely reasoning behind the inital statment from the reporter.

 

[Joel Wright]

Well, technically if the above is true then tornadoes aren't possible. They'd just be funnel clouds causing significant damage to structures extending above the surface, but not AT the surface.

With pavement scourings that have resulted from strong tornadoes it's hard to believe that strong winds don't reach the actual surface.

 

[Sam Stone]

I think the technical term to describe what you're looking for is boundary layer: http://en.wikipedia.org/wiki/Boundary_layer. In a weather environment, this 'boundary layer' probably varies in thickness considerably as a result of whatever is on the earth's surface winds are going over...asphalt, grasses, prairie grasses, tall corn, trees, buildings, etc.

Also, do not forget that Bernoulli's stuff inversely correlates wind speed to pressure...so there's going to be quite a pressure difference between the 'freestanding' wind speed and that in this lower layer, especially at higher wind velocities.

Sam

 

[Skip Talbot]

There are downdrafts in tornadoes. Not just updrafts and winds with pure horizontal components. Downdrafts clearly impact the ground.

 

[Bob Hartig]

Interesting topic. I'm no physicist and I stunk at math, but I do understand that air has mass and weight, and therefore momentum as a component of its speed. And it seems to me that the momentum of air in a tornado, factored in with all the hellacious, constantly shifting currents, would drive air into the ground--as well as away from it and parallel to it and whatnot--just as effectively as if it were a solid object.

 

[Donald Giuliano]

Bill,

I'm going to agree with Danny's suggestion that she was given an out-of-context theoretical explanation based on boundary layer physics, or perhaps misunderstood the explanation and took it out of context herself. The "1 inch" figure probably refers to what is known as the "roughness length", which is basically the height above ground level where air "always" remains still due to the effects of friction.

As one might imagine, the roughness length varies significantly based on the type of vegetation/structures present on the ground. For example, in a grassland it might be around 1 cm, and in a dense forest it might be around 1 m. Keep in mind, though, that the roughness length approximation is only valid in an idealized boundary layer physics framework, which usually assumes a constant vegetation/structure height and distribution, but nevertheless works pretty well for many common atmospheric and land-use situations.

Obviously, a tornado is hardly a "common atmospheric situation". Among other things, a tornado will tear down structures and vegetation, reducing the height of obstructions and sometimes even leaving bare ground. So what is the roughness length of smooth, featureless bare ground? If I'm remembering my boundary layer physics correctly, it's something in the ballpark of 1/10 the height of a grain of sand.