Topic: supercell characteristics

[Mike]

Read a fascinating journal on tornado formation within supercell thunderstorms (will post the link later).  What was so fascinating was that it mentioned that tornadoes can and do form during the storm's disspitating stage.  Quite the opposite to what I thought!

Also that the storm exhausts any cold air and precipitation within the tower to the backside of the storm and the downdraught from the precip actually creates an extreme wind shear that wraps around the inlfow area which blocks the storm inadvertently sucking in cold air, but aids the storm in drawing warmer air!

Added to that that this shear uses the RFD and FFD to an advantage to create the shear to initiate the tornado.  Even with so much cold downdraught that would normally kill a regular storm due to cold pooling, supercells actually get around this by venting themselves of any unwanted cold air thus sustaining growth or creating new storms (right or left movers?) - one dies off and the other carries on.  That tornadoes generally form near the wall cloud if it is present, but can actually form anticyclonically away from the main inflow area.

Perhaps members who know a little more about supercells could enlighten myself and others as I know that other members would like to differentiate between the two and comment on a lot more about them.  I've certainly got some more comments!

Mike

[David C]

Interesting stuff. I'm kinda busy so wont be able to post too much myself for the next month or so, but should be the start of a good discussion, given it wont be long until the winter is over

One thing though, I seem to recall that cold downdrafts do inhibit tornadogenesis...in simple terms, mostly since the cold air wants to sink and diverge and not converge and rise. Tornadic storms seem to possess a somewhat buoyant RFD ie is relatively warm and moist and promotes upmotion in close proximity to the low-level mesocyclone. I noticed this chasing  before I had read anything about it. That's just one reason why LCL height is important (lower reduces evaporative cooling).

Also not being in the field at all, I don't keep up with the latest research, but again, I have read that RFD generation in supercells in thought to be a dynamic process (ie think of downdraft air as being sucked down rather than simply being more dense), due to vertical pressure pertubation gradient. It is really all so complex and so fascinating and one reason why I love chasing supercells, where as simple convection (ie including those damn volcanic eruptions) is not nearly as appealing to me personally Right, Jimmy!

As for the tornadoes, well we did mention couplets some time back. In some cases I have seen subtle anti-cyclonic circulations at cloud base and assume this to be simply the result of the RFD intrusion (I imagine it like a paddle cutting through the water producing two vortices with opposite sense of rotation). In other cases, it may be the result of presence of meso-anticyclone aloft. Then there can be landpouts along the flanking line if sufficient lapse rates and horizontal vorticity co-exist.

Look forward to reading others thoughts!

[Mike]

Indeed, David, that's exactly what this paper disclosed.  I shall read this again tonight and post some more comments assured soon. Here's the link to the paper   There are some serious mathematical formulas included in the report which I have no knowledge of but the mathematicians of this world no doubt would revel in.  It hosts diagrams, schematics and charts and is a plethora of knowledge that could definitely be used as a tool.

http://ams.allenpress.com/perlserv/?request=get-abstract&issn=1520-0469&volume=040&issue=02&page=0359

Mike

[Jimmy Deguara]

Mike and David,

In relation to what David suggested about the surface mesocyclone, what makes the Tornado Alley supercells often unique are their existence along the dryline which favour warmer air cutting through rather than particularly cold air cutting through.

We rarely have the setups in Australia that favour such warmer air cutting through together with the stronger inflow. I imagine the developing tornado struggling like film trying to catch onto its reel. Eventually it organises itself. So with tornadogenesis you will experience spin ups and multivortex behaviour before the funnel organises.

One case in Australia when I was able to experience first hand a relatively warm rear flank downdraft RFD given the fact it was a massive HP with giant hailstones is the 20th February 2005 Singelton supercell. Even on approach to Singleton, a wall cloud existed with possible RFD cutting in. Just after the hailstones had bombarded my vehicle, I noted the air outside despite the fine drizzle and hailstones was cool not cold. I felt mixing of warmer air. Of course no observed tornado occurred here but I think moreso due to lack of organisation under the main base - inflow and RFD. The load of precipitation did not help and I guess wind shear aloft associated with this load and the lack of a triple point focus made it difficult. Inflow although stronger than most storms I have experienced may have been drawn in by the exploding updraft at this point. It was also not as warm as it could be.

David may comment on the 2nd February 2005 HP supercell that traversed Sydney and the 7th February Lithgow 2007 event - pure classic for as long as we saw it! Both we believe should have been tornado warned!

What about the beaver tails and their importance.

Regards,

Jimmy Deguara

[Mike]

The maths sent my grey matter into freeze mode.  I have no idea what it meant - I was more interested in what they said and not how they worked it out!  Thanks for relieving my stress with your explanation of what they were equating.

You're welcome also.

Mike

[David C]

What about the beaver tails and their importance.

Regards,

Jimmy Deguara

Hi Jimmy:  Beaver tails are simply a manifestation of the stationary gust front (pseudo warm front), right?

[Mike]

Re the beaver:  Correct, Pseudo warm front.  Refers to a particular type of inflow band attached to the supecell's general updraught.  The size and strength and shape change as the strength of the inflow changes. Apparently there is a distinction between beaver tail and 'true' tails clouds.  A true tail cloud is most often attached to the wall cloud and as a cloud base at about the same level as the wall cloud itself.  A beaver tail is not attached to the wall cloud and has a cloud base at about the same height as the updraught base - by definition higher than the wall cloud.

Mike

[stormchaser354]

To the Australian storm chasers, I am from the USA and I am not a tornado chaser but I want to be when I grow up, but to get a tornado to form you need the RFD to slam into the wall cloud and that forces it to spin and BAM you have a tornado.

[stormchaser354]

you guys in the Southern Hemisphere have very violently rotating supercells as I saw in a video clip, but still cool

[Mike]

Welcome Stormchaser3!  I personally have not experienced a supercell due to living in the tropics and they are rare up in Darwin, but in the last month the eastern states of Australia have been hammered by them.  No doubt you may have seen the NSW storm thread !  Look forward to any comments you may have or what you have observed yourself !

Mike