Wind shear

[Jon Miller]

Can there be too much wind shear and , if so, what are the limits for tornadic storms ? Are there any good web sites that discuss wind shear and how it is deterrmined, etc?

 

[Jeff Snyder]

My answer would be.. yes. Given very weak instability (say, 100 j/kg CAPE), an updraft may not be able to sustain itself in a strong-shear environment. Like many other processes, we're talking about a balance between shear and instability. You can see this in action during many of the early-season storm days (Feb-March), which tend to be characterized by very strong winds aloft (presumably enhancing deep-layer shear, though this isnt always the case) with relatively weak instability (<1000 j/kg CAPE).

 

[Amos Magliocco]

There is some indication that increasing values of storm relative helicity, which is in the neighborhood of your question I think (unless you're specifically wondering about deep layer shear), become less supportive of tornadoes. This scatter plot was taken from a document used by the SGF office:

[Broken External Image]:http://www.crh.noaa.gov/techpapers/service/tsp-10/fig6.jpg

Figure 6. Scatter diagram showing combinations of CAPE in J/kg and 0-2 km AGL positive wind shear for 242 tornado cases during 1980-1990 (Johns et al. 1993). Figure reproduced from Johns and Doswell (1992).

This plot makes an interesting case that a sort of "sweet spot" exists between ~1000 - 3000 j/kg and between 100 and 400 m2/s2 SRH. Obviously plenty of exceptions on any of the extremes of the plot, but the cluster is intriguing, particularly how it suggests that, even within the cluster itself, higher CAPE values support strong tornadoes more frequently in lower SRH value environments and vice-versa.

Lots of disclaimers here. I don't know their methodoology; I don't know how you get near-storm SRH measurements with any precision since those values are so extremely localized and vary wildly, and even CAPE estimates for particular tornado studies are tough. (See Rasmussen's Baseline Supercell Climo papers for a discussion of this). However, all that understood, this piece of evidence supports what chasers have contended for a long time: that it's not so much about values reaching particular thresholds as it is about balance for supercells and tornadoes to thrive. Thus the pitfall of placing too much emphasis on indices or derived products.

Here's the paper in which this diagram appears:

http://www.crh.noaa.gov/techpapers/service...0/tbl-cnts.html

 

[Jeff Snyder]

Amos,
I think we need to be careful about drawing conclusions relating tornado frequency to a combination of helicity and CAPE. Sure, there may not be many strong/violent tornadoes with extreme helicity (say, >500), but how many times does that situation occur each year? I mean, how often do we see 500 0-2km helicity and 4000 j/kg CAPE? I think a better plot may be a 'relative frequency' plot... In other words, the probability of a tornado based on a given situation... or the number of tornado in a given combination relative to the number of times that particular combination occurs. Of course, I don't think there's really a way to do this, since it'd be difficult to come up with the "total number of times this combination occurred". In conclusion, I think that we don't see many tornadoes in the 5000 CAPE and 500 helicity range mainly because that situation only rarely occurs (and if it does, it's likely in a strong-cap situation).

 

[Craig Maire II]

Here is a link to a page concerning a late season tornado outbreak that occurred across portions of central Iowa on 09/05/04. There was very limited amounts of instability but very favorable wind shear on this day.

http://www.crh.noaa.gov/dmx/WX_events/Sep-05-2004.pdf

 

[Niki Darnaby]

Also, given a rotating updraft in an environment with high shear and marginal instability, the negative pressure perturbation associated with the initial mid-level mesocyclone often enhances the updraft strength such that large amounts of CAPE are no longer needed. That's why the updrafts often appear stronger on days with extreme shear and low CAPE versus days with marginal shear and high CAPE.

 

[Amos Magliocco]

Originally posted by Jeff Snyder
Amos,
I think we need to be careful about drawing conclusions relating tornado frequency to a combination of helicity and CAPE.

I wonder if your argument has some logic flaws. I think we're perfectly safe relating tornado frequency (and even strength) to combinations of CAPE and helicity. All tornadoes occur under some value of these two parameters, so there's no danger in measuring that occurence and trying to derive some ideas from it. That's why Doswell and Johns did the plot. As well, I agree that tornadoes are rare under rare conditions. Tornadoes are rare even under the most supposedly supportive conditions, so it follows they would be even less frequent in unusual circumstances.

However, the reason I posted the plot is to consider various environmental conditions under which the incindence of strong tornadoes is correlated with those particular ingredients, one of which is shear, the topic of the question. What this suggests about the overall frequency of tornadoes or their baseline climatology is outside the scope, though I think it does not run afield of some of the VORTEX conclusions. I think the results suggest that a particular environment of CAPE and SRH supports more strong tornadoes than other sets of those particular values. Nothing more and nothing less. But it seems germane to the idea of too much shear.

If only we could get Dr. Doswell to do a guest post!

 

[Jeff Snyder]

Originally posted by Amos Magliocco
I think the results suggest that a particular environment of CAPE and SRH supports more strong tornadoes than other sets of those particular values. Nothing more and nothing less. But it seems germane to the idea of too much shear.

But does it? Saying so means that one can conclude that there's a better chance of a strong/violent tornado occuring with a 300 0-2km helicity and 3000 CAPE combination than with a 500 0-2km helicity and 5000 CAPE combination. I don't think that graph shows that at all, since it only shows that there have been more tornadoes associated with the 300/3000 combo than the 500/5000 combo only in absolute number of tornadoes. This does NOT imply that, for whatever reason, the 500/5000 combo is less conducive to tornadogenesis, just that there are more tornadoes that occur in the 300/3000 combo. But this is likely because the 300/3000 combo occurs much more frequently than the 500/5000 combo. It's like saying that, because there are more severe hail reports with 3000 CAPE than with 8000 CAPE environment, that there can be too much instability for hail. One cannot conclude that accurately, however, since the 3000 CAPE situations occurs MUCH more often than the 8000 CAPE situation, and thus we'd expect to see more hail reports in terms of the absolutely number of reports.... From the chart, there were more reports with the 'moderate' degrees of helicity and CAPE than with the 'extreme' degrees of both, but again that's because the 'moderate' helicity/cape combo occurs much more frequently than the 'extreme' combo.

 

[Mike Hollingshead]

Their map is for 0-2km helicity. If we are worried about too much shear wouldn't we be talking higher up? I can't see anything wrong with extremely high 0-2km shear for any reason. I don't even think I'd be terribly concerned with extremely high winds up at 300mb or higher under any setup. Between those two heights I'd probably be more concerned with too much shear.

 

[Amos Magliocco]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

<!--QuoteBegin-Amos Magliocco

I think the results suggest that a particular environment of CAPE and SRH supports more strong tornadoes than other sets of those particular values. Nothing more and nothing less. But it seems germane to the idea of too much shear.

But does it? Saying so means that one can conclude that there's a better chance of a strong/violent tornado occuring with a 300 0-2km helicity and 3000 CAPE combination than with a 500 0-2km helicity and 5000 CAPE combination. [/b]

No, saying so doesn't mean that at all, at least not for me. I'm not being very clear, I suppose. Yes there are more tornadoes within the more common parametric conditions. No it does not prove that balanced but extreme conditions are somehow less conducive. While the lack of tornadoes in extreme circumstances may suggest nothing more than the rarity of those circumstances, it does not, in my opinion, invalidate the utility of the graph.

What I view as the utility of the graph--the reason I posted it---is the possiblity that within a fairly well-defined balance, the incidence of tornadoes is higher. I don't think we can dismiss this and say, 'well that's only true because there are more days with those particular values.' If that were the case, the plot would be meaningless. It would be a logical fallacy.

I think there's a little of the operational/theoretical dilemna here. I'm not proposing a theory. I'm not smart enough or qualified to do that. I'm only pointing to a small piece of evidence (which appears in an in-house operational handbook) that, within a balance of shear and CAPE of a particular range, a higher density of strong tornadoes appear. I can't imagine that all they're doing is showing us that more days happen with those ranges. As Chris said, "the cross-section of conditions conducive to tornadic supercells." It seems to me that must have been the intent of the graph.

Obviously somebody should download this paper and post Doswells and Johns conclusions or commentary. Now I really want to know.

 

[Glen Romine]

This paper is a good read, and is available online here:

http://webserv.chatsystems.com/~doswell/pu...&doswell_92.pdf

I would note that nowhere in the paper do they try and suggest an upper limit to CAPE and shear combinations - reality of nature is simply that you rarely have one large without the other being small. Both sreh and CAPE have units of energy - and as such it is when you have a synergistic pairing of the two that interesting things can happen. More recent work by Rasmussen and Blanchard add cautionary supplements, such as the need for the high shear layer to also be bouyant - so you's use a parameter such as 0-1 km EHI (they also show the 0-1 km helicty is most important) which combines the low-level buoyancy with the low-level shear. Of course, this alone is not enough, as you need favorable deep layer shear and CAPE to support the supercell storm. So, the tornadic environment is a supplement to the supercell environment.

Glen

 

[Gabe Garfield]

I think that, in general, storm mode is the most important factor. With stronger shear setups, storms tend to form convective lines and complexes rather than isolated cells. Most high shear/low instability tornado days feature fairly discrete storms (not always the case, but generally speaking). Storm mode is highly dependent on the orientation of the shear vector (relative to the pertinent storm initiating boundaries) and also on the cap strength.

So, while extreme instability and extreme shear would seem to be great, there is an extremely delicate balance between lots of storms (MCSs and MCCs) and no storms at all. I think that extreme shear is good only in cases where the overall forcing for convection is weak (which would be related to the instability and convergence along any surface boundaries).

A good case of an extreme shear/extreme instability (pseudo) bust would be May 24, 2004. Many thought NE KS and NW MO would see violent tornado after violent tornado given the extreme set of parameters. However, too much forcing along the warm front (correct me if I'm wrong) caused the convection mode to go to linear much sooner than most had expected. Subsequently, the significant tornado threat decreased considerably.

In my observation, extreme instability with moderate shear is much better than the volatile extreme instability/extreme shear combo.

Gabe

 

[Glen Romine]

Originally posted by Gabe Garfield

In my observation, extreme instability with moderate shear is much better than the volatile extreme instability/extreme shear combo.

Gabe

Can you give even one example? The paper reference couldn't find one, and I suspect you'd have a hard time as well. Extreme CAPE and extreme [edit]low-level shear[/edit] just don't coexist very often - so if you have an example when it happened, I'd be interested in hearing about it (seriously). Soundings are just too far apart, and launched too rarely to capture these rare meetings with any real frequency.

Also, we are talking about extreme low-level shear here - which does not directly affect storm mode (deep layer shear controls this - say 0-6 km). Cap strength does not regulate storm mode in any formulation that I've ever seen. Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

BTW, there where quite a few tornadoes on the day you mentioned - so maybe not a good case of a bust.

http://www.spc.noaa.gov/climo/reports/040524_rpts.html

[edit]
Also, if you want to see the values of CAPE and helicity for the event you mentioned - you can see them here:

CAPE:
http://www.spc.noaa.gov/exper/archive/even...p_040525_00.gif

Low-level shear - note 0-3 km, which is more generous than 0-2 km
http://www.spc.noaa.gov/exper/archive/even...r_040525_00.gif

If you look carefully at the plots - you'll see that the highest low-level shear is offset to the north from the highest CAPE. Where the two overlay the best, you see roughly 450 helicity with 3000 CAPE, which is on the upper portion of the data envelope in the study, but we are using 0-3 helicity here vs. 0-2 in the study, which moves this case right into the average. Of course, these values weren't actually measured by a sounding. The best case sounding from this event was at Topeka, with a CAPE just over 3000 and 0-3 km SRH of only 318.
[/edit]

Glen

 

[Amos Magliocco]

I need to post one correction to my last post. "Logical fallacy" isn't the right term. What I mean is that if all the plot achieves is to document the frequency of DAYS under which these conditions occur, then it is a statistically insignificant scatter plot. I have not looked at their data, but the cluster at least 'looks' statisitically significant for those two variables, and I'm assuming the authors would not have published a plot that did not achieve this.

 

[Mike Hollingshead]

Extreme CAPE and extreme instabilty just don't coexist very often - so if you have an example when it happened, I'd be interested in hearing about it (seriously).

Well I tried to find the data on it, but the spc archive page doesn't have the shear/cape/lapse overlays for it. August 16, 2002 in ne NE had both, if I recall it right. I remember the ruc having an 80knot llj over SUX that night and a powerful surface low with the system, and a nose of like 5000+ SBCape. I think siggy tor was like 12 or something and the supercell index was over 30. But, the boundary was lined up all wrong.

http://www.spc.noaa.gov/exper/archive/even.../OAX_00_obs.gif

Shear and cape both were stronger more towards Yankton SD than this at Omaha.