Helicity... How does it really help?

[Zachary Biggs]

Ok, I can't stand it...
Might as well come out and demonstrate my ignorance.

The Helicity discussion started by Bob Hartig got me thinking.

I know that Helicity is needed. I can see it on the hodograph.
But how the heck does having your rotation axis parallel to the air flow aid a supercell to become tornadic?

As a thunderstorm moves along it picks up these horizantal (vortex lines?) and stretches them into the vertical to produce a counter clockwise rotation on one side and a clockwise rotation on the other... and you often get splitting cells with neither the right split or the left split being favored with a straight hodograph and zero helicity? Am I right thus far more or less?

In a situation with decent helicity one of the supercells will be favored (depending on the sign of the helicity?) and is much more likely to be tornadic.

Trying to think about how to phrase my question here...

Why is a supercell more likely to spawn a tornado when the vortex lines (axes of rotation) are more parallel to the flow than if they are simply perpendicular?

Should I just wait until I start my Meteorology degree?

 

[Donovan Gruner]

Well basically put, helicity is "a measurement of a storm's ability to ingest streamwise vorticity into it's updraft." When stretching occurs, it is much easier for the storm to ingest this vorticity and maximize the potential strength of the mesocyclone, thus maximizing the potential of the storm to be tornadic. Higher helicity values often support stronger tornadoes, however this isn't always true. I'll write more tomorrow when I haven't been drinking. hehe.

 

[Paul Knightley]

Supercells develop, generally, within larger scale ambient flows which are conducive to them, thus reasonable amounts of shear and helicity, along with the pre-requisite thunderstorm ingredients are required to get mid-level rotation going, and keep it going.
Tornadoes typically develop from much smaller-scale features - features which are still not yet understood. For example, there are many days whereby the general synoptic scale may appear favourable for tornadoes but only a few develop. What was special about the environment in these areas? Studies such as VORTEX showed that very local regions of much high helicities can exist, at a scale which is not resolvable by standard surface observations, and whose origin is unclear.

 

[Robert Edmonds]

Environments with helicity favor cyclonic or anticyclonic updrafts (depending on sign) by producing pressure perturbations which can enhance an updraft. Maybe I might make my own graphics to describe it, but for now I will refer you to a review article Klemp 1987 Ann. Rev. Fluid Mech. 1987. 19: 369-402. If you can't find a way to access the article PM me, and I will email you a copy. The paper describes how storms get their mid-level rotation, but how that strong rotation gets transfered to the surface is up for debate.

 

[Ian Miller]

Well it looks ok to me, it's what happens next that makes it really complicated all the interaction
with the upper and mid level winds...