Vorticity

[Caleb Routt]

So I get that streamwise vorticity is the best for tornado development but why is it when you have crosswise vorticity there still can be strong tornadoes? So does it really matter which type of vorticity there is?

 

[Rob H]

Unless you're modeling fluid dynamics it's probably not that important to think about it too much. We don't have a comprehensive way of measuring vorticity at the misoscale, and we always have some quantity of both present. So for all of us non PHD meteorologists, using model-derived storm-relative helicity (a measure of the streamwise vorticity) is good enough. Quantity of instability, outflow boundaries, interactions with other storms, all sorts of things can affect the storm. I personally find it easiest to put helicity values and tornado chance into the following categories, while leaving some wiggle room for "weather doing weather things that surprise us" and inability to accurately account for misoscale processes:

edit: rough values off the top of my head. You might want to use other values, and always remember that storms don't follow man-made rules

1) Very unlikely (<50)
2) Possible (75-150)
3) Tornadoes likely (150-250)
4) Strong tornadoes likely (250-400)
5) Too much shear, crazy edge cases (400+)

 

[Jeff Snyder]

As noted, horizontal vorticity can be broken down into streamwise and crosswise components. Think of the streamwise component as that which is aligned with the storm-relative wind vector; as air parcels following the storm-relative wind are turned upward in the updraft, there is positive correlation between the location of the updraft and the location of the vertical vorticity max (that is, there is a mesocyclone essentially). If the flow is such that there is only crosswise vorticity, the vertical velocity extrema are dislocated from the vertical vorticity extrema. Since we're often interested in mesocyclones, we're most concerned with streamwise vorticity. The turning of the storm-relative wind vector with height directly affects streamwise vorticity. If the vertical shear profile is unidirectional (which does NOT require that the wind profile be unidirectional -- a nice veering wind profile can still produce a straight-line hodograph and undirectional shear), there is no streamwise vorticity unless/until storm motion deviates off the hodograph, which will then result in a turning of the storm-relative wind vector with height and thus indicate some streamwise vorticity.

As Rob H noted, instead of worrying about streamwise vs crosswise vorticity, use the storm-relative helicity -- SRH is a measure of the streamwise vorticity available to be "ingested" into a storm. Depending upon the hodograph, SRH may be extremely sensitive to storm motion, which can be difficult to forecast with high certainty since the movement of a convective storm can be very sensitive to storm-scale, non-linear processes that are all but impossible to forecast.

Edit: This thread -- Vorticity help -- from back in the day may be helpful!

 

[JamesCaruso]

... If the vertical shear profile is unidirectional (which does NOT require that the wind profile be unidirectional -- a nice veering wind profile can still produce a straight-line hodograph and undirectional shear)...

In what situations would a veering wind profile produce a straight-line hodograph and unidirectional shear? Would wind speeds have to actually be slower the higher up in the atmosphere you go?

 

[Jeff Snyder]

In what situations would a veering wind profile produce a straight-line hodograph and unidirectional shear? Would wind speeds have to actually be slower the higher up in the atmosphere you go?

There's a great figure in the Markowski and Richardson Mesoscale book to highlight this (Fig. 2.13), but I can't find it online. Here's one example from Doswell (1991 -- http://mesoscale.ou.edu/~doswell/hodographs/hodographs.html), though:

In the above image, winds are southeasterly at the surface and veer to southwesterly aloft. Typically, wind speeds would need to decrease from the sfc up to some height (the height at which the hodograph is closest to the origin) and then increase above that height.

A similar one (link: http://tornado.sfsu.edu/Geosciences/classes/m500/Helicity/Straight_Curved.html):

 

[Rob H]

To compound on Jeff's excellent explanation, helicity (aka streamwise vorticity) is denoted by the area marked by the storm's actual motion and the wind profile as plotted on a hodograph:

The crosshairs is storm motion, so if you want 0-3km SRH, you connect the lines: start at storm motion, to go 0km, follow the profile to 3km, then close it off by returning to storm motion. The yellow area is the SRH. Understanding this relationship should make it easier to see why you can have streamwise vorticity with straight line hodos - as long as your storm motion is not resting exactly on the profile, there will always be some area bounded by these points.

Straight line hodos don't mean "wind coming from the same direction" which is a slightly counter-intuitive concept that you need to move past when learning to interpret hodos. The exception to this is if you're considering storm relative winds and you're measuring from somewhere on the profile. In which case you have (near) zero helicity

 

[calvinkaskey]

So according to the SPC with the vorticities in the 300s they are underplaying the chances of strong tornadoes for tomorrow?

 

[Rob H]

So according to the SPC with the vorticities in the 300s they are underplaying the chances of strong tornadoes for tomorrow?

My first post should have huge flashing, blinking lights around it that say "in the presence of an environment with all other parameters and ingredients being favorable for tornadogenesis".

Just a personal observation, but I think your forecasting skills would improve dramatically if you focused less on how you thought the SPC was always doing a poor job.