Dynamic Pipe Effect / Cyclostrophic balance

[Caleb Routt]

In the easyist form what is the Dynamic Pipe Effect? how does Cyclostrophic Balance play a role in this to?

 

[Shawn Schuman]

I'm sure someone else can explain it better, but in the simplest possible terms, cyclostrophic balance is a state in which the inward pressure gradient force (which causes air to move toward the area of low pressure at the center of the rotating column) and the outward centrifugal force reach a dynamic equilibrium. In this state air flows freely around the axis of the cylinder, but flow toward or away from the center is suppressed. That's the dynamic pipe effect in a nutshell. When that occurs, the pressure at the center of the column rapidly drops because airflow toward the center is suppressed. This essentially "sucks" more air in at the bottom of the "pipe," causing an increase in the PGF, which leads to greatly increased tangential velocities until cyclostrophic balance is achieved again. This process continues to propagate downward until, in the case of tornado formation, it reaches the surface.

Here are a couple diagrams that illustrate the process.

 

[Caleb Routt]

So is Cyclostrophic balance also the reason why suction vorticies do not seem to last long?

 

[Jeff Duda]

Cyclostrophic balance is a balance between two forces: the horizontal pressure gradient force, and the centripetal force keeping an air parcel moving in a circle (you can also think of the centripetal force as being inertia).

The cyclostrophic wind is the wind speed at which a parcel would have to travel if it were in cyclostrophic balance. This is dependent on the diameter of the circular path and the magnitude of the horizontal PGF. The only thing cyclostrophic balance really says is what tangential wind speeds you should expect in a tornado of a given size with a given pressure structure.

So to answer your question, I do not believe cyclostrophic balance says anything one way or the other about the longevity of suction vortices. I'm not an expert on tornado flow and dynamics, but from the research I have seen, I would be willing to put a lot of money on meter-scale friction and drag forces as being significant causes of suction vortices. When near-surface tornado winds interact with the ground and the features on the ground (like trees, shrubs, blades of grass, rocks, pebbles etc.) the diffuse nature of the surface causes turbulent eddies on a variety of sizes, anywhere from a few micrometers to several meters and larger. The interaction of all of these turbulent eddies of different sizes probably contribute a significant amount of responsibility for suction vortices. I may be wrong, but the fact that you don't tend to see suction vortices extend very high off the surface supports this theory.

Jeff Snyder is one very knowledgeable person in this topic. I'm hoping he sticks his head in here and sheds some light for all of us.