Storm modes in unidirectional flow/shear environment

Verhaegen Yoni

Enthusiast
Jun 22, 2019
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Hi all,

I'm new on the forum but have been passionate with regards to weather and storms for years. Lately, I have been trying to broaden my knowledge on storm development and organization.

Now, I know that DLS (0-6 km shear vector) is used to discriminate between single cells (roughly < 20 kt), multicells (roughly 20-40 kt) and supercells (roughly > 40 kt), but I am wondering what the actual role of unidirectional flow vs. unidirectional shear and directional changes is in these storm modes...

Can a supercell be formed in a unidirectional flow (e.g. SW-lies throughout the whole lowest 6 km, so only speed shear) regime when DLS exceeds 40 kt?

When 20 kt < DLS < 40 kt but unidirectional flow, is it correct that you would most likely see organization into line segments? Does it make a difference to storm modes when most of the shear is comprised in the lower/mid levels or when it is more or less uniformly distributed? Is unidirectional flow (hodograph parallel to its radials) a subtype of unidirectional shear (straight hodograph, not parallel to radials) and hence would you expect similar behavior under a unidirectional shear regime?

I hope I will get some clearance on this one. Thanks in advance!
 

Jeff Duda

Resident meteorological expert
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Oct 7, 2008
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If you're not familiar with hodographs, now is the time to become so. Any two hodographs that have the same relative shape (e.g., a line) are going to show the same storm behavior regardless of where on the hodograph the wind profile is.

And yes, there is some impact on storm behavior of the vertical distribution of a given amount of wind shear.
 
Nov 18, 2006
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Chicago, IL
When 20 kt < DLS < 40 kt but unidirectional flow, is it correct that you would most likely see organization into line segments?
A: Yes, more often than not. Discrete modes early on can sometimes get the job done, especially on mesoscale days with boundaries in play.

Does it make a difference to storm modes when most of the shear is comprised in the lower/mid levels or when it is more or less uniformly distributed?
A: Sometimes. You can see more QLCS type events, or embedded supercells within a line. Especially when you've got more speed shear.

Is unidirectional flow (hodograph parallel to its radials) a subtype of unidirectional shear (straight hodograph, not parallel to radials) and hence would you expect similar behavior under a unidirectional shear regime?
A: This one I'm not 100% sure on so I don't want to give too much an answer. I want to say it is not, especially if you're thinking of terms in streamwise vorticity being ingested into your supercell. I believe in those cases you cant parallel vectors but that relates more to your storm direction. I'll let someone else correct me if I'm wrong.
 
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Aug 18, 2018
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Novi, MI
A supercell is essentially defined as a thunderstorm with a persistently rotating updraft, so (substantial) directional shear needs to be present, whether it’s convergence along a boundary (the dryline) or the sheared warm sector of a surface cyclone, in order to get that classic supercell. You won’t always need mid-level directional shear over 40 kts either, although that it is the minimum of the optimum amount of shear needed. I believe the Jarrell, TX tornado formed in an environment characterized by roughly 30 kts of 0-6 km directional shear amidst extreme instability. Basically if you want a good looking supercell in a weakly-sheared environment, you’re going to want the instability to be cranked up, as extreme values can makeup for the lack of shear somewhat.

Unidirectional shear above 40 kts often favors initially discrete storms merging into a strongly organized MCS or even a derecho, instability allowing. Immediately ahead of a cold front, a QLCS/squall line would be favored as Adam said.

Unidirectional shear in the 20-40 kts range will yield the loosely organized complexes of storms like you said (multicells). If you can get a good cold pool to keep pushing it along, significant wind damage can still occur, especially within a very moist and unstable airmass. Very high amounts of Downward CAPE (> 1200 J/Kg) also suggest waterloading within the storm cores can lead to damaging downdrafts.

Unidirectional shear below 20 kts will give you the pulse storms that form quickly and collapse with some gusty winds. If they can form within an environment charactized with extreme MLCAPE (generally > 3000-4000 J/Kg), very localized damaging downdrafts could occur during their collapse. I’ve found that environments like this can often be capped, so there could be very little to no convective initiation.

Hopefully I gave you a little bit of useful knowledge. If anyone has anything to add to this please do.
 
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