Too much shear?

[Andrew Clope]

I was wondering if there is such a thing as too much shear? I'm referring probably mostly to speed shear with this question. I'm asking because I've heard folks talk about updraft towers getting sheared off and blown apart. It seems that potentially this environment could inhibit deep convection, similarly to the way that a temperature inversion inhibits convection above a certain level.

I would assume there's more to it than just the wind speed, eg, instability and stronger updrafts (and rotating updrafts) likely would resist being sheared off. But I was wondering generally speaking is there such a thing as too much? If so, under what situation would you expect the updrafts not to be able to survive?

 

[rdale]

On a basic level - it's when the shear is too high for the available CAPE. If you have high shear but low instability, the towers get ripped apart or spread downwind before the storm can get its act together.

 

[Andy Berrington]

January 29th, 2013 seems to have been a case like this (the day before Adairsville). In addition to having some problems with forcing being focused along the cold front as opposed to spreading over the warm sector, the shear was extreme with the instability/lapse rates being rather modest (although not completely marginal as shown below). Discrete storms did try to form in AR, but they never were able to develop robust updrafts and eventually everything became linear (which did end up producing a swarm of weaker QLCS tornadoes in TN later in the evening). This was a large 15% sig-hatched tor that generally busted.

The LZK sounding from 00z 1/30 shows how dangerous it could've been if supercells had dominated...

 

[Jeff Duda]

Look up the bulk Richardson number. There are a number of great reads from the 1980s and early 1990s on it.

 

[Andrew Clope]

Great, thanks for the replies!

 

[Adam Lucio]

Absolutely there can be too much shear. I call those setups "insane-o shear" setups. More common in the fall, winter and early spring. Likewise, there can be too much CAPE too. Its all about the balance. When your updrafts are leaning over like this, its a sign there is a lot of shear.

 

[Marshall Stoner]

It probably does occur in some instances like those listed above. I don't think it's a common reason for busts though. IMO the most common issue when chasers complain of isolated towers being "sheared apart" is lack of a good mesoscale forcing mechanism. It isn't uncommon to see weak sheared-over "turkey towers" in the same general area where tornadic supercells are occurring. To me this is pretty strong evidence that the problem isn't with the large-scale parameters like shear or instability.

An updraft that breaks through the cap doesn't automatically develop into a supercell. The issue is small pulse-type updrafts lose their buoyancy as they mix with the surrounding air. This mixing prevents small updrafts from immediately organizing. Most supercells start out from a sheared multi-cell cluster. The transition from multi-cell to supercell occurs more easily if the storm is located on some kind of boundary where there's extra forcing. The forcing can create a larger updraft that's less easily diluted by mixing. The transition also occurs much more quickly if there's good low level directional shear. Once a rotating updraft is established "too much shear" usually isn't a problem.

I've also noticed it's more common to see areas of sheared multi-cell crapvection despite good CAPE and good hodographs east of the Mississippi. I'm not exactly sure why. It might be the lack of up-slope flow that occurs exclusively in the pains. The farther east you go the more you have to rely on unusually potent synoptic systems or freak mesoscale accidents where things just happen to come together.

 

[Marshall Stoner]

typo correction - I meant "up-slope flow that occurs exclusively on the plains".

 

[joel ewing]

ABSOLUTELY you can have too much shear! I've seen it a couple of times over the years. It'll ruin your most hopeful chase day, and is really quite something to behold. My first experience with it was in the early 2000's in the extreme northeastern corner of the Texas Panhandle....near a tiny little place called Higgins, Texas (Lipscomb Co.). Gorgeous, likely-tornadic towers were billowing up skywards, but were becoming VIOLENTLY shredded and ripped apart as soon as they gained a bit of height. We watched as cell after cell easily began rotating long before their mature stage, but it was futile. It was as if hidden, unseen entities were tasked with the orders to destroy these cells from every direction and angle. We had very low LCL's that afternoon, which provided us with front-row seats to witness this event in it's entirety. What an epic tornadic day we probably would have had if the shear could have been just a little bit less. Certainly, we were all extremely disappointed at "what could'a, would'a and should'a been....but in discussing the day's events as we drove back to our infamous "Chase Ghetto" in Amarillo.....we realized that the Storm Gods had treated us to an incredible weather spectacle in it's own right. It's certainly something that I will never, ever forget.

 

[Des Heyliger]

It probably does occur in some instances like those listed above. I don't think it's a common reason for busts though. IMO the most common issue when chasers complain of isolated towers being "sheared apart" is lack of a good mesoscale forcing mechanism. It isn't uncommon to see weak sheared-over "turkey towers" in the same general area where tornadic supercells are occurring. To me this is pretty strong evidence that the problem isn't with the large-scale parameters like shear or instability.

An updraft that breaks through the cap doesn't automatically develop into a supercell. The issue is small pulse-type updrafts lose their buoyancy as they mix with the surrounding air. This mixing prevents small updrafts from immediately organizing. Most supercells start out from a sheared multi-cell cluster. The transition from multi-cell to supercell occurs more easily if the storm is located on some kind of boundary where there's extra forcing. The forcing can create a larger updraft that's less easily diluted by mixing. The transition also occurs much more quickly if there's good low level directional shear. Once a rotating updraft is established "too much shear" usually isn't a problem.

I've also noticed it's more common to see areas of sheared multi-cell crapvection despite good CAPE and good hodographs east of the Mississippi. I'm not exactly sure why. It might be the lack of up-slope flow that occurs exclusively in the pains. The farther east you go the more you have to rely on unusually potent synoptic systems or freak mesoscale accidents where things just happen to come together.

The highly-variable terrain on both the mesoscale (hills/mountains) and microscale (trees) in the East creates increased mixing which inhibits instability. Same principle applies to the Intermountain West.

 

[James Hammett]

BRN values <10 were found to indicate shear too strong to permit storm growth in numerical simuations (Weisman and Klemp, 1982). But I think too often updraft failure due to lack of buoyancy regardless of shear, a strengthening inversion layer, and/or boundary layer decoupling gets blamed on "too much shear". Any of the above will make updrafts appear to "fall over" and "shear off". It doesn't take a lot of CAPE even in a high shear environment to sustain a supercell (see: Dixie Alley).

Further reading:
http://journals.ametsoc.org/doi/abs/10.1175/1520-0493(1982)110<0504:TDONSC>2.0.CO;2
https://stormtrack.org/threads/wind-shear.2703/

 

[James Gustina]

This previous Friday (April 24th) was a great example. Upper levels were somewhere well over 100 knots and when an incipient updraft hit that belt of winds near Pratt it just got annihilated. It took less then 20 minutes for it to go from this:

To this:

It's "anvil" got blown downstream for at least thirty miles in that short time while the entirety of the updraft got shredded. Not enough CAPE coupled with shear that was way too strong on the wrong side of the upper-level dynamics just murdered it.

 

[Jeff Duda]

But I think too often updraft failure due to lack of buoyancy regardless of shear, a strengthening inversion layer, and/or boundary layer decoupling gets blamed on "too much shear". Any of the above will make updrafts appear to "fall over" and "shear off". It doesn't take a lot of CAPE even in a high shear environment to sustain a supercell (see: Dixie Alley).

I think what's really going on when an updraft gets "sheared off" by vertical shear that is too strong is that the updraft column gets tilted so severely that entrainment increases to the point where the parcel begins entraining enough dry air to cause the parcel to lose saturation and buoyancy as it cools much faster than the MALR, really, much closer to the DALR.

 

[Marshall Stoner]

I think what's really going on when an updraft gets "sheared off" by vertical shear that is too strong is that the updraft column gets tilted so severely that entrainment increases to the point where the parcel begins entraining enough dry air to cause the parcel to lose saturation and buoyancy as it cools much faster than the MALR, really, much closer to the DALR.

Yea. I think even without shear there's always mixing around the outer surface of a cumulus congestus cloud. Only the very center is undiluted. Strong shear tilts the tower which increases the surface area relative to the volume of the cloud. That increases the mixing and decreases the buoyancy.

If you can get a really big fat updraft the shear is less problematic because the volume-to-surface-area ratio is larger. The problem is such an updraft won't spontaneously occur. You have to have some kind of mesoscale forcing that's on a larger scale. On the plains there's usually terrain features to help, along with the dryline. There might have to be some kind of triple-point where two boundaries intersect.

I think all-in-all, really strong shear has a similar effect as a cap. It can really decrease the coverage as only the largest updrafts are able to survive. This can be a good thing though as isolated supercells have less competition and can persist for a longer time before merging into some kind of non-supecellular MCS. Of course it can also lead to a bust if nothing is able to form.

 

[Jeff Duda]

If you can get a really big fat updraft the shear is less problematic because the volume-to-surface-area ratio is larger. The problem is such an updraft won't spontaneously occur. You have to have some kind of mesoscale forcing that's on a larger scale. On the plains there's usually terrain features to help, along with the dryline. There might have to be some kind of triple-point where two boundaries intersect.

Great post and I agree with it except for this part. I don't necessarily think you need forcing on the mesoscale, at least, on the meso-alpha or meso-beta scale. Forcing on the meso-gamma, maybe. I was told years ago that a good relationship between maximum updraft height and width is that the maximum potential height is approximately 1.5 times the width of the updraft, which is consistent with your statement. Applying that rule literally, if you can get an updraft around 5 km in diameter, you can generally get it about to the tropopause. I would argue that's more of a storm-scale feature rather than mesoscale.