Have question regarding veering/backing and the cyclonic rotation of "most" tornadoes

[Andrew Clope]

I'm working my way through Tim Vasquez' book Severe Storm Forecasting. There was a section regarding veering and backing that got me thinking. So veering is obviously a clockwise change with height (or distance, or time), and backing is a counterclockwise change with height (or distance, or time). Looking at most hodographs that have very good directional shear the LL winds will generally come from the SE, then turn S, then SW, then W... wouldn't this produce a clockwise or veering turn with height? If so, then why do most tornadoes rotate counterclockwise or "backed?"

I understand that the designation for veering and backing change regards rising motion or descending motion, but if veering is a clockwise motion with height ( and tornadoes are a pressure drop, or suction, then I'm assuming that it is a rising motion) then why do they rotate *mostly* cyclonically?"

Thanks

 

[Travis Davis]

The end of page two starts to explain with graphic.
http://www.meteo.psu.edu/~pmm116/pubs/2009/MR09ATMOSRES.pdf


Basically the updraft tilts the horizontal vorticity.

 

[Paul Knightley]

That paper should clear things up, but a couple of notes:

veering and backing are terms used for the wind changing at a certain level, but can also be used to explain how winds change with height, as you've said. The terms were originally just for surface winds, where they either veered or backed around the compass.

Supercells derive their rotation from the tilting of horizontal vorticity into the vertical. Note you don't need winds to turn with height (technically, although in practice it usually helps a lot!) to get storm-scale rotation. So, picture a rolling pin - this is how you can imagine horizontal vorticity borne out of increasing winds with height. Now tilt this into the vertical - you now have a rotation...storms have to areas of rotation when this process starts, the cyclonic (usually southern) and anticyclonic (usually northern) parts. Now, because a vertically veering wind profile is associated with warm advection at low-levels (e.g. in the warm sector of a large-scale cyclone), convergence is highest at the southern part of the developing thunderstorm, and so the southern (i.e. cyclonic) updraught is favoured, and thus tends to become dominant.

Developing vortices under the cyclonic portion can be stretched by the cyclonically rotating updraught to become tornadoes - hence, the reason most (mesocyclone-derived) tornadoes rotate cyclonically is because the parent supercell's (most powerful) updraught does.

 

[Andrew Clope]

I guess I'm still confused. Maybe I'm making this way harder than it needs to be. But let me try to explain:

So, most hodographs that I see that forecasters term as favorable begin with a wind out of the SE down low around 850mb, then turn out of the south, then SW, then Westerly. This would indicate a clockwise rotation with height correct? Or veering? So my question is, if the whole supercell is rotating clockwise, then how does the clear slot (rfd), and the funnel and potentially tornado drop down rotating the opposite direction?

I'm trying to cram as much in as possible lol, so maybe I just went and confused myself! If it's a dumb question I apologize ahead of time.

Thanks!

** EDIT ** Actually, I think I was making it harder than necessary. I think I understand now. Just over thinking it.

 

[Paul Knightley]

Most supercells in the N Hem rotate cyclonically, that is, counter-clockwise...you're confusing looking at how the wind changes with height with rotation.

 

[Royce Sheibal]

The hodograph moves clockwise, however this creates a cyclonic motion (counter-clockwise). Picture a paper tower roll. Stand it on end. Now take a straw and set it on the table with the end of the straw tangent to the roll in the direction of low level inflow, counter-clockwise. Now go up a couple inches and apply the straws in the direction of inlow with elevation and tape them to them, boom homemade hodograph diorama. Notice that you can also apply the straws on the anti-cyclonic side in the same fashion, which is why we get anti-cyclonic tornades. However they usually only accompany cyclonic rotation, because the mesocyclone can tap into large scale cyclonic vorticity (from a surface low, upper low).

 

[Stan Rose]

Note also that the clockwise 'turning' with height you see in the hodo is totally consistent with the structure (radar/visual) of a lot of classic supercells--they have a comma or hook shape that curves around to the right (since the upper winds are veering)