Author Topic: Bushfire initiated tornadic winds  (Read 4191 times)

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Offline nmoir

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Bushfire initiated tornadic winds
« on: 29 October 2008, 11:29:16 AM »
After visiting a small bushfire in lane cove yesterday i was very interested in the micro scale wind shear and its impact on the smoke and it reminded me of a bad bushfire in dec 2002 in Glenorie in sydneys NW in which the wind shear both in the atmosphere and the local windshear created by the intensity of the fire itself. The column of smoke rotated rapidly. During the Canberra bushfires i was covering them for the SMH  , on the day after a came across a strange scene in the suburb of Kambah where a couple of streets sustained damage from the wind alone and residents described the event like this

http://www.smh.com.au/cgi-bin/common/popupPrintArticle.pl?path=/articles/2003/01/24/1042911549553.html


plus these videos from the US show a rapidly rotating smoke cloud which has separated from the fire itself

http://www.youtube.com/watch?v=OnevoZZvbig

http://www.youtube.com/watch?v=Wwn-PHMnz6Q

I wonder if it is possible for a seriously  destructive tornado to develop rapidly from the intensive updrafts , radical wind shear and instablity present on an extrene bushfire day , i believe the Haines index measures the atmosphere for instability and the atmosheres ability to lift the column of smoke from the fire.

The description in Canberra certainly descibes this scenario , is there any more substantial evidence of this and other events?
Nick Moir
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The Sydney Morning Herald
and www.oculi.com.au

Offline Richary

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Re: Bushfire initiated tornadic winds
« Reply #1 on: 29 October 2008, 04:03:55 PM »
Certainly impressive videos and (luckily) have never seen anything on that scale. I remember seeing one item of TV news footage years ago (it may have been the 1994 Sydney fires) of a fire tornado in the Royal National Park. Just this tornado of fire ripping along a ridge. Obviously the updrafts caused by the fire, it's massive consumption of oxygen creating an inflow effect with the hot CO2 going up created a whirlwind effect.

I would say yes, that a really good fire is obviously going to create it's own wind (something firefighters I have known in the past have confirmed). The oxygen burns, heats and goes up because it is hotter. This creates a low pressure area at the bottom and the surrounding air comes in to keep fuelling the fire. I don't see it as hard to believe this could easily form a whirlwind type effect depending on prevailing breezes and topography. I have witnessed enough dust devils to know that with little input these can start and form their own micro environment with lifting.

Actually the cause of that would probably be harder to answer than fire tornadoes. Where is the energy coming from to get them going?

Offline Michael Thomas

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Re: Bushfire initiated tornadic winds
« Reply #2 on: 30 October 2008, 12:06:19 AM »
This is a subject that really interests me. Pyrocumulonimbus are really quite spectacular but are. unfortunately. so devastating. Concerning the 2003 Canberra bushfire, there is substantial evidence of an tornado. There is in fact a scientific article written about this event-

Title: Violent pyro-convective storm devastates Australia's capital and pollutes the stratosphere
Author(s): Fromm M, Tupper A, Rosenfeld D, et al.
Source: GEOPHYSICAL RESEARCH LETTERS   Volume: 33   Issue: 5 Article Number: L05815   Published: MAR 15 2006

I will have to read this in more detail when I have the time but just having a brief read it seems quite interesting. In the article they state that the pyrocumulonimbus produced an F2 tornado based on a damage survey and they also provide information on the tornado's path. Interestingly, there is also a photo of what they believe to be the tornado. There was also reports of black hail generated by the intense updrafts. The maximum height of echos (by radar) were 15 km.

I certainly believe that the key for these kinds of events, other than extremely dry conditions, is steep lapse rates. Here is the Wagga sounding from that day-

http://soundings.bsch.au.com/skew-t.html?source=wyoming&lat=-35.1333&lon=147.3667&gribdate=&month=01&day=18&year=2003&hour=00&window=on

Keep in mind that Canberra is roughly at 600m above sea level compared to 200m for Wagga Wagga and that Canberra reached 37C that day. There is no doubt that dry adiabatic or super adiabatic lapse rates extended well up into the mid levels of the troposphere. With all the energy released from the fire there would have been quite a bit of instability locally so there would have been nothing stopping air rising all the way to the tropopause. Conversely, at night time an inversion typically develops above the ground which would really put the breaks on pyrocumulonimbus formation (think of it like a cap on a storm day).

Michael

Edit: Typo
« Last Edit: 30 October 2008, 01:24:35 AM by Michael T »

Offline nmoir

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Re: Bushfire initiated tornadic winds
« Reply #3 on: 30 October 2008, 01:18:17 AM »
for people unfamiliar with the damage caused in Kambah (suburb of Canberra) here is a shot of the street and tree damage which i took the day after the firestorm , note the tree in foreground snapped at the base and the leaves on the road in background
Nick Moir
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The Sydney Morning Herald
and www.oculi.com.au

Offline nmoir

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Re: Bushfire initiated tornadic winds
« Reply #4 on: 30 October 2008, 01:24:25 AM »
here is the link to the paper Michael T was talking about and i recommend reading it , it is extremely interesting with a possible tornado photo

http://earth.huji.ac.il/data/pics/Fromm_etal_2005GL025161.pdf
« Last Edit: 30 October 2008, 01:33:15 AM by nmoir »
Nick Moir
Photographer
The Sydney Morning Herald
and www.oculi.com.au