QLCS Tornadoes/ Angular Momentum

[Caleb Routt]

Hey i've been looking around and can't seem to find an awnser to this.

First How exactly do mesovorticies in squall lines form?

And if those are such small circulations should'nt they be stronger than the usual EF-0/EF-1 because of the conservation of angular momentum?

 

[Kendra Sand]

My understanding is, horizontal speed shear initializes the vortex. That's why a bow echo is a good clue as to where one might form.

 

[Jeff Duda]

Not sure exactly what question you're asking, but...

IMO, the term "mesovortex" is ambiguous. There are tornadoes that spawn from semi-mesocyclonic processes in QLCSs (what some call QLCS tornadoes), and then there are bookend vortices.

Regarding the first type: you can get spin ups, pretty much exclusively along the front edge of an advancing (usually progressive) squall line. These types of tornadoes are typically weaker than mesocyclonic tornadoes spawned in supercells, but there have been a number of cases of EF2 and even EF3 tornadoes spawned in this manner. They typically form quickly and don't live long and are thus difficult to forecast. However, they generally exist on the same scale as mesocyclonic tornadoes typical of supercells. You'll generally see either an inflow notch dig into the precip, or a hook echo develop and extend into the clear air ahead of the convective portion of the squall line. When that happens, that's generally an indication of a mesocyclone that has developed along the front edge of the squall line. Again, they typically aren't long lived or very big, but they can and do produce tornadoes.

The second type of vortices form along the flanks of mesoscale convective systems/squall lines. They are related to what @Kendra Sand described as being formed from horizontal wind shear, usually the difference between rear inflow near the apex of a bowing segment of the squall line and the ambient storm relative flow just off to the side of the convective portion of the squall line. Such vortices are much larger in scale than tornadoes or mesocyclones (10 to 100 times larger), and have weaker vorticity. Bookend vortices have spawned tornadoes before, and there is some relationship between the relative position of the bookend vortex to the rest of the squall line and the tornado likelihood, but such tornadoes are pretty uncommon and usually very weak.

Regarding the angular momentum conversation part...yes, conservation of angular momentum is likely a significant contributor to ALL tornadogenesis events. I don't think there's anything special about it for QLCS tornadoes or bookend tornadoes.

 

[Pierre-Marc Doucet]

I actually have a question to submit for that.

Last week in western Quebec we had a big squall line. At some point, there was this feature embeded in the line:

(Sorry about the low quality of the velocity product, Canadian radar are very noisy.) Radar is to the east-south-east.




We had a disscussion with fellow chasers and meteorologist. Some think it was a broad area with rotation but it could not be a tornadic circulation (more like a a very small MCV) and the damages resulted from a downburst. Some think it was a QLCS tornado signature or maybe even an embeded supercell. It does not seem to be a bookend vortex since the shape was not really like a bow echo but a very long squall line. Note that this feature was there for about 30-40 minutes. They were wind damages reported with it and 75 mph winds.

I would like to know what do you guys think about that ? I am all about learning so if you have an answer I would really love to see it.

 

[Simon Andersen]

Looks like a broken-S signature to me Pierre, though I've never seen a weak-echo hole with those before.

 

[John Farley]

Pierre-Marc - Except for the reflectivity hole, your radar imagery looks like a pretty typical QLCS mesovortex/embedded mesocyclone signature to me. Jeff explained these very well (the first of the two types he mentions) a few posts above. Two things I would add is that 1) these are most common with bow-echo type systems, and most often occur near or somewhere to the left (usually north) of the apex of the bow echo, and 2) they often form when the bow echo/squall line interacts with some kind of pre-existing boundary. Sometimes this is visible as a line of TCU ahead of the squall line and at a little less than a right angle to it.

 

[Pierre-Marc Doucet]

Thanks all for your answer's !

 

[Taylor Stone]

typically weaker

Here are a few more links to ponder:
- Assessing QLCS MV potential
- Case Study by NWS
- Climo QLCS vs Cell (both have similar intensity distributions)