Wind shear

Dec 11, 2003
1,194
1
5
Denton, Texas
cycloneroad.com
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-selection/kit/RadarComposites/north_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.
 
Apr 22, 2004
998
1
0
CMI
www.atmos.uiuc.edu
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.
 
Apr 22, 2004
998
1
0
CMI
www.atmos.uiuc.edu
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
 

Mike Hollingshead

Who knows, maybe it wasn't ever ON the SFC boundary and was always north, I can be wrong. I don't even remember the sfc winds during it. It just seemed when I first got to it, after several others were already on it, that it was very much sfc based and the wall cloud was much more warm and moist looking. Oh well.