Author Topic: Supercell definitions.  (Read 8033 times)

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Offline Kristy Norman

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Supercell definitions.
« on: 29 May 2009, 10:15:52 AM »
Is it possible for a small supercell to be classed as a 'mini supercell', or is a supercell simply defined as....
LP or HP?
What exactly is a classic supercell and where do they fit in?
Are there any more terms for a supercell???

Offline Kristy Norman

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Re: Supercell definitions.
« Reply #1 on: 11 June 2009, 02:55:03 AM »
Thanks John for your detailed response, it definetly makes things much clearer. I had been having a bit of trouble fully understanding supercell classifications, so again thank you.

Offline Shaun Galman

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Re: Supercell definitions.
« Reply #2 on: 25 June 2009, 06:32:25 AM »
Hi guys,
Great description John! A nice (easy) read. ;)

Can I just throw something in the mix here. How do lower level strong winds ie; straight line or outflow/inflow come into play and does that affect a supercell classification along with the usual suspects that you mentioned above?
 
We had a monster, and pretty long-lived storm a couple years back that had unreal straight line winds spread over a broad area along the large 'plow type' shelf cloud. The wind speed wasn't recorded but was strong enough to snap 15inch diameter eucalypt trees of near the ground and took a blower (a large truck of around 12tonnes used to remove opal dirt from the opal mines here) completely off a hole and snapped the heavy hydraulic hoses that were connected to a digging machine underground and throwing in about 30metres. It was pretty well destroyed. This is a small example of the destruction but the damage was very wide spread (well over a kilometre wide). The storm had no signs of rotation or a wall cloud but did the damage of an F3 or F4 tornado.

I think I've said in other threads that this is the closest thing I've seen out here to a suprecell but could not confirm that SC status due to the lacking updraft rotation relevance.

Any ideas or thoughts?
Kindest regards,
Shauno  
Chasing Region: Lightning Ridge. N.S.W.
Website: www.ridgelightning.com

Offline Jimmy Deguara

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Re: Supercell definitions.
« Reply #3 on: 25 June 2009, 08:08:57 AM »
Shauno,

Can you provide more specifics as to the date, approximate time and location of the event in question? If it was a long lived event it could have been a supercell particularly given your descrption of the damaged trees. However, that alone cannot distinguish the characteristics of a supercell. High based storms can produce severe microbursts.

Quote
The storm had no signs of rotation or a wall cloud but did the damage of an F3 or F4 tornado.

Do you know exactly what F3 or F4 damage can do?

If there was a shelf cloud, I would suspect that if there was a wall cloud or rotation, it would have been hidden within the precipitation. It is not uncommon for storms to become outflow dominant and develop outflow characteristics.

Perhaps check our supercell section for samples of supercells. I know it is not fool proof in terms of its accuracy especially in the earlier years, however it could provide some guidance as to the variations in structure.

Furthermore, as years go by, the memories of structure also fade.

Regards,

Jimmy Deguara

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Offline Shaun Galman

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Re: Supercell definitions.
« Reply #4 on: 25 June 2009, 11:05:13 AM »
Hi Jimmy,
I was interested just to see whether or not winds were involved in SC classification?
I find it really interesting.

The link to the thread that I posted this report in is here:  http://www.australiasevereweather.com/forum/australian-severe-storms-significant-weather-events-and-storm-chasing/northwest-nsw-sw-qld-storms-26-feb-1-mar-2007/15/

There was no definable wall cloud as such and nothing in the photos as the storm passed by, though it was very HP in nature and could've been rain wrapped? Though you will probably agree that it doesn't look likely in the photos. It was longer in it's lifecycle than an hour as it had done a bit of damage out near Brewarrina and remained over us for an hour or so before moving on towards Collarenebri where the lightning just went crazy from memory.

The guess as to comparing it to an F3 (or possible F4) were only going against what I've seen in the tornado photos/video as far as damage. It was very much comparable. There was a 22foot Caravan that was smashed into trees at the South Eastern base of the Jag Hill ridge and wrapped like a flattened pancaked around them! Our van was blown away and totally destroyed also. On the other side of the dry lake, about 3 kilometres away several large trucks and various pieces of other very heavy mining equipment were destroyed also. The trees were stripped bare across the tops of the ridges, resulting in several large diameter trees being snapped off at ground level. Our machinery can be far heavier than a decent prime mover and is harder to shift from wind forces alone due to most steel superstructures being spread further apart. So it does take tremendous force to move it!

I understand all too well that an F4 can remove a house and leave nothing but a bare concrete block, embed straw into telegraph poles etc. (somewhere near 200mph plus) there was nothing in the way of structures to compare it to that definable extent but it must have been somewhere pretty darn close. I've heard that straight line winds can be nasty! I recall reading a post here where 7 concrete power poles were snapped off in winds somewhere near Narrabri or Armidale? Those winds must have really been up there speed-wise too! 

Anyhow, just wondering how this plays out in the SC characteristics is all, no disrespect meant by all means, but I have to tell it like it is, or was in this case :)
Kindest regards,
Shauno
 
Chasing Region: Lightning Ridge. N.S.W.
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Offline Jimmy Deguara

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Re: Supercell definitions.
« Reply #5 on: 25 June 2009, 02:02:28 PM »
Shaun,

Not trying to downplay the severeity of the storm as caravans moving and being destroyed is no joke. It takes significant wind strength to cause such damage. However, I would have thought F2 damage could cause similar damage. Sudden winds can also create sufficient momentum to make larger objects airborne. The type of damage you mention although I did not see it all, could be done by winds causing F2 damage.

As to the supercell winds, yes to a certain extent the inflow contributes to lower level rotation and also may be sufficient to assist in supercell development. However, it is beyond doubt that wind shear aloft is more associated with supercell development. As winds increase with height in specific conditions, and you add instability, this assists the virtual forward vorticity to be tilted into the vertical once clouds erupt and interrupt the horizontal flow.

Try this link for some explanation including diagrams:


http://en.wikipedia.org/wiki/Supercell

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/modl/spr/struc.rxml

Regards,

Jimmy Deguara
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Offline Michael Thomas

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Re: Supercell definitions.
« Reply #6 on: 25 June 2009, 03:58:26 PM »
This is a nice paper discussing the different supercell classes-

http://www.ejssm.org/ojs/index.php/ejssm/article/viewArticle/44/45

There is certainly plenty of info out there which can be found by a few google searches.

Shauno, I believe it is very difficult to say whether that storm was a supercell or not. I believe more information would be required. Based on the damage though it certainly was a dangerous storm regardless of its classification! Strong winds are unfortunately not a good indicator of storm mode since strong winds can occur with supercells well as other convective modes. Derechos for example can cause massive damage from strong straight-line winds.

http://en.wikipedia.org/wiki/Derecho

Hail on the other hand is a great indicator, anything larger than 5cm likely is the result of a supercell.

The moree sounding that morning looked pretty good, decent low level moisture, good turning of the winds with height. Upper level winds looked good too though a little weak around 500 mbar. I could believe that there was sufficient CAPE and wind shear for supercells in the afternoon period. Radar shows what would appear to be a left-moving supercell near Walgett. Further north one particular cell came screaming SSE over the border (moving to the right of the steering winds). The 256 km Moree radar doesn't give me that much confidence though.

Finally, John, a would be hesitant to make general statements about conditions leading to LP, classic and HP supercells. The article above has some good discussion on this (effect of moisture distribution in the atmosphere, wind shear etc). Also, wouldn't low-topped supercells be the result of a low tropopause rather than the presence of a capping inversion? I don't want to sound critical or start an argument, we both have the same passion and interest in the weather! Just making a few points.

Michael

Offline Shaun Galman

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Re: Supercell definitions.
« Reply #7 on: 26 June 2009, 05:21:50 AM »
Hi guys,
Thanks Jimmy I understand the F2 comparison very clearly, and it is possibly a better comparison given the widespread nature of the damage overall. Some areas were far worse than others. I was just glad no one was in those caravans at the time! There were 2 or 3 camps on the Western facing side of that ridge that escaped with only minor damage but the guys living in them said is was a total white-out. These were the ones who raced into town and started off the whole "a tornado has struck Lightning Ridge" stories. I soon dismissed this with a visit to the area and a quick call to the newspaper. I still live for that day however lol.

Hi Michael T.
Thanks for the obs! There was a large storm to our N-NE with beautiful updrafts and structure that was heading SE while this other large storm was arriving from the West. They converged just NE of town luckily. It has always been a favorite storm of mine as I had never seen such a monster shelf cloud before.

Can I throw another possible SC classification into the mix for this thread.. How about Cyclic Supercells? Not sure of it's relevance to us in Aus. well.. possibly given the right conditions I guess anything can happen? Thinking this is more for the US however? They fascinate me also and are stunning to see on the footage and photos.

Kindest regards,
Shauno 
Chasing Region: Lightning Ridge. N.S.W.
Website: www.ridgelightning.com

Offline Michael Thomas

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Re: Supercell definitions.
« Reply #8 on: 26 June 2009, 08:01:21 AM »
Hi John,

I guess you have a point the the tropopause is pretty much a massive capping inversion. Just not used to it being referred to as such. I typically think of a capping inversion as inversion around 900-800 mbar putting a lid on deep convection.

Also, it is my understanding that embedded mesocyclones can exist with intense squall line/derechoes so yes I should be careful in making such statements. Just trying to make the point that extreme severe wind events can occur with non-supercellular storms.

You sure know your stuff. Please feel free to pick a bone when I am saying something which is not quite right. I am by no means an expert :)