Wind shear

[Glen Romine]

I was able to find some stuff at the NCAR archive. Let's look at where the precip was occurring at the time of the sounding:

http://locust.mmm.ucar.edu/case-selection/...00208170000.gif

and the surface map at that time:

http://locust.mmm.ucar.edu/case-selection/...00208170105.gif

You probably noticed the sounding at OAX still had a healthy cap of 50 J/Kg, and what we see is that storms were developing north of a boundary at the surface, where temps were only in the 70's, so convection was not occuring in the region with both large CAPE and large shear.

I think you get a better appreciation for the strength of the cap in this sounding plot:

http://locust.mmm.ucar.edu/case-selection/...020817_0000.gif

Thanks for the suggestion though - and please keep thinking of more.

Glen

 

[Mike Hollingshead]

You probably noticed the sounding at OAX still had a healthy cap of 50 J/Kg, and what we see is that storms were developing north of a boundary at the surface, where temps were only in the 70's, so convection was not occuring in the region with both large CAPE and large shear.

[Broken External Image]:http://www.extremeinstability.com/stormpics/02-08-16(11).jpg

LOL, I can assure you that was not north of the boundary in cool air. Look closer at your sfc temps and where those storms are. Hell that one return is almost on O'Neill at 7pm and O'Neill is 91/63. That would be this storm maybe 1hr later north of Norfolk.

 

[Jeff Snyder]

LOL I've got a good case for a combo of high low-level shear + strong instability...
[Broken External Image]:http://www.tornadocentral.com/now/July210z02MPXsounding.gif

0z July 21, 2002, MPX sounding... The tornado reports are empty because the cap held any deep moist convection at bay this day. I haven't seen too many soundings which show ~5800 J/kg MLCAPE (>6100 SBCAPE) and 262 m2/s2 0-1KM SRH (404 0-3km)... This may have been a good day to see this discussion at work considering that the deep layer shear (0-6km) was not terribly impressive at 40kts.

Originally posted by Glen Romine
Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

I know this comment was directed at Gabe's post, but I just had to interject here... I think there is a pretty strong relationship between storm mode and tornado occurrence, and I don't htink many here would argue that there isn't a relationship between the two. I think storm mode is incredibly important to tornado potential... Yes, environments that are most likely to directly affect tornadoes (mainly low-level shear) are not directly associated with storm mode (e.g. squall lines, supercells, etc) and vice versa. But I do think that the parent mesocyclone plays a very important role in the development of tornadoes given strong low-level shear. Again, I know you're probably saying that we can't really use 0-6km shear for tornado forecasting and 0-1km/0-3km helicity for supercell forecasting, but the 0-6km shear does play a role in storm rotation most likely, which in turn can aid in tornadogenesis if you take the theory of tilting and stretching of streamwise vorticity. This is neglecting the fact that updraft rotation in general can significantly enhance updraft 'strength'/velocity relative to a nonrotating updraft...

 

[Gabe Garfield]

Soundings are just too far apart, and launched too rarely to capture these rare meetings with any real frequency.

Are soundings necessary to determine instability/wind shear parameters? There are a number of other methods for determining these quantities (satellite, wind profilers, etc.).

Can you give even one example?

Does model initialization from 00z 24 May 2004 count? As I recall, CAPE was well over 4000 j/kg.

From the 20 UTC Day 1 Convective Outlook:

THE AIR MASS IS EXTREMELY UNSTABLE AND THE DEEP LAYER SHEAR
IS FAVORABLE FOR TORNADIC SUPERCELLS.

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.

Cap strength does not regulate storm mode? Maybe not by itself, but a stronger cap means fewer storms and less storm mergers (e.g. change in storm mode).

From Edwards and Thompson's 3 May 1999 paper:

When forecasting a threat of tornadoes, the mode of convective initiation and the number and spacing of supercells that form are critical to the number of tornadoes expected. In the same mesoscale region, several supercells may develop in association with different forms of boundaries. These boundaries vary in detectability when using conventional data sources, and storms may form where there are no apparent boundaries. The initial storms in the 3 May 1999 outbreak evolved into tornadic supercells that each lasted several hours, with no early transition to a squall line or other convective mode. Storm spacing and motions were such that the supercells remained in an environment of favorable vertical shear and instability for several hours without numerous storm collisions, thus allowing the supercells to produce a large number of tornadoes.

The predominance of a supercell convective mode and lack of a squall line on 3 May 1999 may have been attributable to the lack of strong low-level convergence near the dryline(s). It is conceivable that the outbreak would not have materialized in such intense or prolific form had the convergence been stronger along a consolidated dryline, and had numerous storms formed simultaneously and merged into a larger-scale convective system in the weakly capped environment over northwestern Texas and western Oklahoma during the afternoon.

By the way, when I said May 24, 2004 was a (pseudo) bust, I was referencing the lack of a large number of significant tornadoes.

As far as 0-6 km shear determining storm mode, I can think of at least one exception from '04. Large 0-6 km shear was present in C. OK on May 29th (C. OK was under the mid-level jet), yet the storm mode was high precipitation for a good part of its lifespan.

Gabe

 

[Andrea Griffa]

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

LOL I've got a good case for a combo of high low-level shear + strong instability...
[Broken External Image]:http://www.tornadocentral.com/now/July210z02MPXsounding.gif

0z July 21, 2002, MPX sounding... The tornado reports are empty because the cap held any deep moist convection at bay this day. I haven't seen too many soundings which show ~5800 J/kg MLCAPE (>6100 SBCAPE) and 262 m2/s2 0-1KM SRH (404 0-3km)... This may have been a good day to see this discussion at work considering that the deep layer shear (0-6km) was not terribly impressive at 40kts.

<!--QuoteBegin-Glen Romine

Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

I know this comment was directed at Gabe's post, but I just had to interject here... I think there is a pretty strong relationship between storm mode and tornado occurrence, and I don't htink many here would argue that there isn't a relationship between the two. I think storm mode is incredibly important to tornado potential... Yes, environments that are most likely to directly affect tornadoes (mainly low-level shear) are not directly associated with storm mode (e.g. squall lines, supercells, etc) and vice versa. But I do think that the parent mesocyclone plays a very important role in the development of tornadoes given strong low-level shear. Again, I know you're probably saying that we can't really use 0-6km shear for tornado forecasting and 0-1km/0-3km helicity for supercell forecasting, but the 0-6km shear does play a role in storm rotation most likely, which in turn can aid in tornadogenesis if you take the theory of tilting and stretching of streamwise vorticity. This is neglecting the fact that updraft rotation in general can significantly enhance updraft 'strength'/velocity relative to a nonrotating updraft...[/b]

Seeing this case I've seen that in the morning sounding there was already strong cap that limited a spread convection during the day: in the 12Z sounding I see 404 J/Kg of CIN that it's a strong value. Probably the strong heating of the day eroded a part of this cap but this wasn't enough to erode it at all.This maybe can have suppressed convective initiation. Probably the only thunderstorms that developed that night were those that came from South Dakota and did some damagin winds: but there was no primary convection in MN.
I don't know why, but often when there are similar synoptic situation with a big amount of instability, cap is too much high and limited convection.
I think that the best range for having a deep tornadic convection is 1500-3500/4000 J/Kg of cape with a good amount of SRH: I think probably the same cape is the most important factor for having a good convection: when it's too high occur too factors that limit convection. This is obviously not a scientific observation but often things go in this way.
What do you say?

 

[Glen Romine]

Originally posted by Mike Hollingshead

LOL, I can assure you that was not north of the boundary in cool air. Look closer at your sfc temps and where those storms are. Hell that one return is almost on O'Neill at 7pm and O'Neill is 91/63. That would be this storm maybe 1hr later north of Norfolk.

Almost doesn't cut it - the cells were not at O'Neill, and the surface conditions (and the upper air) was not the same there as they were at Omaha, and an hour difference in time is notable as well. Let me see what I can do with what information I can easily get.

First, the warm front passed through O'Neill after 2000 UTC, as at that time it is clearly north of the boundary:

http://locust.mmm.ucar.edu/case-selection/...00208162105.gif

The cells developed within the next hour on the NE/SD border:

http://locust.mmm.ucar.edu/case-selection/...00208162300.gif

The southern edge of this development looks to be by Niobrara, which is a good 35 miles to the northeast of O'Neill. Let's now look at the visible sat image at 22:30, which splits the time difference between the radar and the surface.

http://locust.mmm.ucar.edu/case-selection/...00208162252.jpg

Note the inverted 'V' cloud pattern near O'Neill. That is the location of the warm front and dryline interesection. Now see the cells developing up along the state line, north of the warm front.

The laminar appearance of the cloud bases in your pictures from this event is further evidence that surface air was not buoyant until substantially lifted. Also, if we take the OMA sounding and make the (bold) assumption that the upper level environment was similar at O'Neill, then overlay the O'Neill surface conditions on the OAX sounding, you'll see that the CIN is even greater at O'Neill (91/63 vs. 88/70 at OAX) than at Omaha. Further, the wind direction at O'Neill is from the SSW and strong, so there goes much of the helicity as well. The best scenario of surface inflow north of the boundary is at Norfolk 2 hours before the cell crossed north of there, with 89/74 right on the edge of the warm front, but this fell to 86/67 by the time the cell approached this station.

So, I don't see any evidence that would convince me this cell was not north of the boundary. Did you have a thermometer with you to measure the air temperature of the inflow? The would be helpful in determining if the cell perhaps crossed south of the warm front later in it's life.

Glen

 

[Glen Romine]

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

LOL I've got a good case for a combo of high low-level shear + strong instability...

[/b]

Thanks Jeff. I'll take a look at this later... you guys are overwhelming me a bit with all this at once.....

<!--QuoteBegin-Jeff Snyder

I know this comment was directed at Gabe's post, but I just had to interject here... I think there is a pretty strong relationship between storm mode and tornado occurrence......

Thanks Jeff, I guess I wasn't very clear on what I'm trying to say here. Let me try this again and see if it makes more sense. The biggest confusion I have with Gabe's arguments is the lack of distinction between deep layer shear and low-level shear. Perhaps it's best if we step through them, so here is Gabe's first set of statements:

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.

Here I must assume he is talking about deep layer shear. Shear strength alone does not dictate whether cells are discrete or organized, such as a squall line - instead the magnitude of forcing relative to cap strength will control this. If the cap is weak and dynamic forcing strong - mode is likely to be linear. Yes, the orientation of the shear vector relative to the boundary forcing ascent is very important. If the shear vector is oriented perpindicular to the boundary and is a straight-line hodograph - this favors splitting cells along a line which will tend to collide quickly - which often leads to a squall line, but can also have cell interactions that later cause a return to discrete cells. Also, yes you do need some cap to keep cells discrete - but there is a delicate balance in that too much cap means parcels won't become buoyant until high up in the storm - meaning the updraft is weak until you are well aloft. If you want tornadoes, you need a strong updraft as close to the ground as possible.

I think that extreme shear is good only in cases where the overall forcing for convection is weak

Again - are talking about deep layer shear or low-level shear? If you have too strong of deep layer shear for the amount of buoyancy present - storms are simply ripped apart - so you have no storms at all. If the statement here is that you can have too strong of low-level shear, that doesn't hold either. Deep layer shear combined with the amount of buoyancy regulates the type storm that will occur. If you have extreme CAPE, you don't need the as much deep layer shear (example - Jarrell Tx. 27 May 1997 F5), likewise, with extreme low-level shear you can get strong tornadoes with surprisingly little buoyancy (example Van Wert Oh. 10 November 2002 F4). Go back and look at this case - there was LOTS of forcing. That is what is shown in the Johns and Doswell plot, and really nothing more than there is a very broad range of favorable combinations of low-level shear and buoyancy.

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

Again, I'd like to see this in an example - but note here I'm looking for extreme low-level shear, not extreme deep layer shear, combined with large CAPE. A good recent example of the two coming together, but unfortunately not directly sampled, is the Manchester SD tornadoes of 24 June 2003 - which appeared to have ~6000 CAPE and SRH ~500, but no soundings stations are in this area.

Ok.... on to the next one.

Glen

 

[George Tincher]

Good topic!

It does seem we rarely get to see a high shear/high CAPE day, as the early season action is usually characterized by high shear/low CAPE events and the late Spring season by high CAPE/relatively modest shear events.

So it would seem logical that most events are going to occur on days when these values are moderate and not extreme, simply because it's very rare to have an abundance of both on any ghiven day throughout the year.

I can remember one crazy day back on January 3, 2000. Shear was at an insane level (seems it was in the range of 800 m2/s2 at least locally!) while CAPE was barely present at all (500-800 j/kg). Regardless, there was a few tornadoes, including an F3 that touched down near the Owensboro, KY area. But the storms quickly became linear, rather thin and unimpressive looking on radar and lost their tornado potential soon after.

I have seen other examples in late May/June where CAPE was abundant, even extreme, yet shear was very weak. Yet these events can also spawn some tornadoes.

Now, if you could take those early season events where CAPE is low and bump that up into the 2000-3000 j/kg range to go along with the insane shear that's often present at that time of year, then you might be looking at a record outbreak. The same is true for the late Spring/Summer activity. If shear was increased in the presence of the extreme CAPE often available at that time of the season, things would likely go nuts.

It just seems the better setups are a comprimise made by Mother Nature. I suppose that is why the best days often have a good balance of each parameter, but not insane amounts of either. And also not surprisingly these days usually fall in April/May (and for the more NRN folks....June) where such a balance is most likely to take place. Again, it's just unlikely you are going to see a day in February with CAPE at 5000 or a June day with helicities of 800 m2/s2.

So are modest days really the best? I don't think so. I just think they are the best balance we are likely to see and thus, they end up being the best.

And yeah, I know, this is an overly simplistic view of things. But I am an overly simplistic kinda guy. Hehe.

-George

 

[Glen Romine]

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

Are soundings necessary to determine instability/wind shear parameters? There are a number of other methods for determining these quantities (satellite, wind profilers, etc.).
[/b]

These are valuable tools for estimating the environment, but are no substitute for the real thing. Satellite soundings use model output for the first guess, use lot's of assumptions to come up with the profile, and have large error bars. While problems are being solved in this field quickly, they have a long way to go imo.

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

Does model initialization from 00z 24 May 2004 count? As I recall, CAPE was well over 4000 j/kg.
[/b]

I'm a modeler - so I'd love to say yes, but I can't.

<!--QuoteBegin-Gabe Garfield@

Cap strength does not regulate storm mode? Maybe not by itself, but a stronger cap means fewer storms and less storm mergers (e.g. change in storm mode).

You could add hodograph curvature as being equally important. Large curvature favors one updraft over the other after the required split from tilting of environment horizontal vorticity, allowing for streamwise ingestion of the environment shear. If you have a straightline hodograph, then cell splitting is likely, and possibly deconstructive cell interaction and merging of cold pools that force more convective initiation and rapid transition to a squall line. The Edwards and Thompson paper you quoted notes the importance favorable shear and instability - and a weakly capped environment!!! So, a strong cap is not mentioned as a favorable characteristic of this case.

<!--QuoteBegin-Gabe Garfield

As far as 0-6 km shear determining storm mode, I can think of at least one exception from '04. Large 0-6 km shear was present in C. OK on May 29th (C. OK was under the mid-level jet), yet the storm mode was high precipitation for a good part of its lifespan.

Again, you are mixing up shear layers again.

0-6 km shear (really should use a mean BL wind instead of sfc) is generally used for discriminating whether you'll get air mass, multi-cell, or supercell convection.

If your BL-6 km shear is favorable for a supercell, then you can look at the shear above this to determine supercell type to be expected. If the 5-10 km shear is small (~< 5 m/s), expect HP storms, moderate values favors classic supercells and large upper level shear favors LP (~> 30 m/s) storms.

Finally, if we want to talk about one of these supercells having a tornado threat as well, then we need to also have large low-level shear (0-1 km shear vector length of at least 10 knots) and low-level parcel buoyancy (0-3 km CAPE > 50). See Rasmussen 2003 for more on this.

Glen

 

[Gabe Garfield]

I'm definitely getting my layers mixed up...LOL. I understand what you are saying. Writing these posts at 1 am is never good!

So, a strong cap is not mentioned as a favorable characteristic of this case.

I am sorry I implied this (if I did). I'm just saying there is typically a more delicate balance that has to be maintained with extreme deep layer shear and extreme instability.

I agree that extreme instability and extreme low level shear is just as good for tornadoes (if not better) than extreme instability with moderate low level shear. Now, regarding the case of extreme deep layer shear and extreme instability, I still maintain that it is better to have only moderate deep layer shear and extreme instability because the winds that create extreme deep layer shear are often associated with much stronger forcing (and thus greater potential for a linear storm mode).

I think that we agree more than we disagree...I failed to specify which type of shear I was referring to.

Gabe

 

[Amos Magliocco]

This is a cool topic.

I need to revise myself yet again (if this were the Everything Else forum I would make a joke about a particular candidate I supported last year) and say that Doswell and Johns do NOT suggest statistical significance to the cluster in their plot. As Glen wrote, the plot serves their paper only to illustrate the range of CAPE and SRH under which strong tornadoes occured in the cases they studied.

However, using the same data in another plot, they draw a line that suggests the lower limit "of combined CAPE / low level shear values that would support development of strong and violent tornadoes."

[Broken External Image]:http://www.cycloneroad.com/images/chase2004/figure18.JPG

Here they are not simply documenting the scarcity of low CAPE / low shear environments but proposing that those environments are unsupportive. They do not, however, go so far as what I first proposed (admittedly without having looked at the data or even having read their paper!), which was that the cluster suggested a significance to that intermediate range beyond the frequency of its occurence.

Ulitmately, this becomes more of a Stats novelty since we all know that two variables don't get you very far. Everything we've learned since 1992 demonstrates that supportive tornado environments come in all shapes and sizes and that the lowest levels are far more important, for both stability and shear, than was imagined fifteen years ago.

Great Stormtrack topic!

 

[Mike Hollingshead]

I suppose by 8pm it had slipped back to boundary instead of being north of it? Or is it still north in these two below at 8pm? Certainly no chance it was on the boundary. I know of, oh, at least 6 others on here that were there. Be nice if just one of them would chime in. But, oh well. (yes I know a storm isn't going to slip back to the boundary)

[Broken External Image]:http://locust.mmm.ucar.edu/case-selection/kit/surface/pir/20020817/sfc_pir_200208170205.gif

[Broken External Image]:http://locust.mmm.ucar.edu/case-sel...plains/20020817/north_plains_200208170100.gif

Someone should tell the mets at OAX too that it was north of the boundary so they can take their tornado warnings back.

 

[Glen Romine]

Originally posted by Mike Hollingshead
I suppose by 8pm it had slipped back to boundary instead of being north of it? Or is it still north in these two below at 8pm? Certainly no chance it was on the boundary. I know of, oh, at least 6 others on here that were there. Be nice if just one of them would chime in. But, oh well. (yes I know a storm isn't going to slip back to the boundary)
Someone should tell the mets at OAX too that it was north of the boundary so they can take their tornado warnings back.

Well Mike, you were there, and I wasn't (I was in San Antonio at the time), but turns out I'm not the only one who thought this storm was north of the boundary. Jon Davies came to the same conclusion:

[Broken External Image]:http://members.cox.net/jdavies1/waf796/fig07.gif

Here is a link to the paper this is from:

http://members.cox.net/jdavies1/waf796/waf796.htm

Glen

 

[Mike Hollingshead]

Well there was never any doubt it crossed the boundary, but to say it was never on the boundary seems silly to me.

[Broken External Image]:http://stormguy.com/site/chases/081602/nebraska7.jpg
Image above is Dave Crowley's

I always thought of something like that as being surface based and not elevated. The boundary was damn sharp, I don't see why that would of never been ON it and not north and/or how you could say it was always north? Yes, I realize some storms can be surface based north of some bondaries, but not this one. It was obvious when it crossed the boundary.

[Broken External Image]:http://www.extremeinstability.com/stormpics/02-08-16(14).jpg

That is crossing the boundary, lol....at least to me. When it crossed you knew it.

 

[Glen Romine]

Originally posted by Mike Hollingshead

I always thought of something like that as being surface based and not elevated. .... Yes, I realize some storms can be surface based north of some bondaries, but not this one. It was obvious when it crossed the boundary.

Thanks for sharing the images Mike. It is a well structured storm, with great sculpting of the updraft with the layered striations, along with a wall cloud with attendent tail, but that alone is not indication that the storm inflow is buoyant at low levels. There was plenty of CAPE in the air mass north of boundary, more than enough to sustain the storm for quite some time. And, lot's of supercells have wall clouds - both tornadic and nontornadic. Did you ever observe strong rotation at the base of the wall cloud? I've seen several other chase reports on this event, and none that I saw ever mentioned seeing rotation at the base of the wall cloud. Clearly, the storm is spinning like crazy at mid-levels, but doesn't appear to be at low-levels.

Anyway, the wall cloud is there because the storm updraft is ingesting rain cooled - and humidfied - air from the forward flank of the storm, not that the cell is surface-based. See how laminar the updraft base is in your image? The contrast is subtle, but it looks very smooth to me. That is a charateristic of a cloud arising from forced ascent (in this case, it is likely the vertical perturbation pressure gradient force), not buoyant ascent. So, the cloud base is the LCL, but how high up is the LFC? In this case, it appears too high for there to be a significant low-level updraft. Why? Because the cinh of the surface air was too large. At least, that's my argument.

The fact that the storm later died is not surprising - don't all storms? The air mass it was moving into was increasingly stable - which eventually becomes too much for the dynamic pressure forcing to overcome, and the cell chokes off and dies.

If the air north of the warm front had been just a bit warmer or more moist, this could have been a much different chase. If you were to normalize the temperature/moisture combination by using a variable such as equivalent potential temperature - you'd find on this day the values were smaller north of the front. But, some days, the thetaE values are actually larger on the cool side of the boundary - and you can get amazing tornadic cells like Pampa. Seems you got a pretty assume cell anyway.

Glen