Question about Helicity

[Bob Hartig]

Quote from the latest target area discussion by rdewey:

"...directional shear is not as good as the ETA was showing (helicity below 100m2/s2)..."

As I understand it, helicity is the tendency of a column of air to turn with height, right? What I don't get is the way it's expressed in terms of "m2/s2." What does that mean? I'd like to understand this as it seems so vital to these discussions. I can look at a chart and say to myself, "Aha! SRH is 475--strong possibility of rotating storms." But I don't know exactly why; I just know that the higher the helicity, the stronger the rotational influence it exerts on storms.

So, when you say "100m2/s2," why is that expressed as an equation rather than a simple whole number--and how do you resolve that equation? Or do you even need to resolve it? Does it express storm-relative helicity or something else? What makes for stronger versus weaker helicity? Would it be how greatly the wind directions shift with height? Does a higher wind speed increase the helicity? Seems like it would, but how would that differ from speed shear?

Hope you don't mind all these questions. Thanks to whoever wants to tackle them.

 

[rdale]

It's not an equation - the units are meters squared per second squared.

There are two types of shear, rotational (change of directions with height) and speed (change of wind velocity with height.)

- Rob

 

[Jeff Snyder]

Bob,
Before I start with the question/answer/insight/whatever, do you know how to read a hodograph?

 

[Aaron Kennedy]

Essentially, helicity is the measure of how readily flow will become helical in a updraft (imagine an aircraft ascending while going in a circle). So really, helicity is basically a measure of the streamwise vorticity. Streamwise vorticity occurs when your wind vector is alligned with your vortex lines. Sheared environments create vorticity and vortex lines... when your low level inflow is alligned with these lines, you have a bunch of helicity.

Try this:


Hopefully that helps a bit!

Aaron

 

[Bob Hartig]

Jeff, I recently learned how to read a hodograph--or at least, I think I've learned. It's easier than I'd thought, and I've been using that in conjunction with the wind indicators on the skew-T/log-P. I'm a neophyte at interpreting, but I can make sense of it.

I'll welcome whatever info you can offer, but I want to ask three things here:

1. Am I right to think that helicity and shear are not quite the same thing? Or are the terms interchangeable?

2. Why is helicity expressed in meters squared per second squared? To me, the "per second squared" part doesn't make sense unless you're using a value greater than one. Does that ever happen? Otherwise, what's the point? A second squared is still just a second, so why not state it as, for example, 150m2 per second? Just trying to grasp the reasoning behind this.

3. How does the speed shown indicate the strength of vorticity rather than simply straight-line wind speed?

Thanks, Jeff!

 

[Bob Hartig]

Also, thank you, Aaron. I saw your reply after posting, and it's helpful. I appreciate the diagram and the insight on streamwise versus crosswise inflow. This topic of vorticity is getting bigger all the time, and pretty fascinating.

 

[Aaron Kennedy]

As for #1. Definetly not the same.
Shear can create vorticity, helicity is a measure of a type of vorticity (streamwise).

Shear is created by two different methods in the atmosphere... speed and direction. Combining these two, you end up with vorticity in the horizontal (imagine a pinwheel spinning above the ground) when conditions are right (such as wind increasing and turning clockwise with height).

#2
Try and neglect the units.... Through a bunch of vector math like cross products you get a vorticity which is in m/s multiplied by the inflow which is also in m/s the resultant is m^2/s^2. It is hard to explain this without calculus.

Helicity is a mathematical way to give a value to the potential for helical flow.


Not quite sure I follow you with #3... I think it is along the lines of why is a higher number better. Well since it is simply a measure of "helical flow", of course we would want more. You can increase this number in two ways.
#1: Increase your shear which will increase the magnitude of your horizontal vorticity.

#2 Decrease the angle of your inflow compared to the axis of the rotation. 0 degrees is best, 90 is worst.


Aaron

 

[Glen Romine]

I'm noticing that the "storm-relative" aspect is missing - so I thought I'd chime in on that. Again, knowledge of a hodograph would be helpful - so I'll give a quick intro here. A hodograph is just an x-y plot of wind speed with height. If you make a graph of the e-w and n-s winds, making a dot at each height you have a measurement, and then connect the dots, you are looking at a hodograph. Then, if you make another dot for the observed storm motion in that wind field, and connect the storm motion dot with the surface wind dot, and another line from the observed motion to the 1km or 3km height wind dot, and fill in the triangle between these three points and the hodograph line, the area represents the amount of storm-relative helicity (0-1 km for the former, 0-3 km for the latter). If this area concept sounds similar to CAPE - it is - in fact the units are even the same (m^2/s^2 can be converted to J/Kg).

http://www.theweatherprediction.com/habyhints/290/

As for speed shear with no directional shear, the hodograph is a straight line, and the initial cell motion will be on that same line. So, while a straightline hodograph does not have any streamwise vorticity, the straight line shear, if adequate, can still support storm rotation through tilting of crosswise vorticity (resulting in a splitting supercell), and that rotation will cause deviant cell motion, and then, the cell motion will leave the straight-line environment hodograph line. As the cell motion gets further from the line, the storm-relative helicity gets progressively larger.

Vorticity is intimately related to the hodograph. If you pick any point along the line of the hodograph, and made a short line segment pointing along the plane of the line at that point - this is the orientation of the local shear vector, and perpindicular to this is (directed away from the origin) is the vorticity vector. Then, draw a line from the storm motion to the start of the shear vector arrow, the length of this line represents the storm-relative wind speed. The two arrows will cross at some angle, and the more perpindicular the intersection, more of the vorticity is streamwise, the more parallel, the crosswise component is larger.

You probably then wonder how the storm-relative helicity can be known before there is a storm - as the storm motion is required in order to correctly calculate the amount of storm-relative helicity(srh). This is indeed a problem - and there are several different methods used to estimate the supercell storm motion - often of varying success. So, when an event begins, it is valuable to look at observed storm motions relative to the storm motion estimate used for the srh calculation to evaluate if the environment is better or worse than calculated srh would suggest.

Glen