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What factors powerpyroconvection?
Nic GellieFire ScientistBushfire CRC
Co-authorsBrian Potter
(United States Forest Service)Tony Bannister
(Bureau of Meteorology)
2006 Tawonga Gap: Neil Wilson, DSE 2011 Tostaree: Unknown
Beechworth Fire – Mount Alice 1 hour after passage of fire frontCredit: John Sweeney
Outline
• Aim of Talk• Background• Theory• Examples• Conclusions
Aim of Talk• Principal Aims
– Present contextual background of well documented case studies
– Outline of approach to estimate pyroconvection top height – based on assumptions and data available
– Illustrate the range of scenarios based on mode of spread, fire size and intensity, and convective potential of the atmosphere
• Summary:– Possible factors in combination that contribute to
pyroconvection– Direction for future research
Contextual BackgroundCase Studies
Australia (8):– Western Australia
• 1969 WA Experimental Fires– Northern Territory
• 1971 Darwin River Catchment Experiment
– Australian Capital Territory• 2003 Canberra Fire
– New South Wales• 2006 Grose Valley and Wollemi
Creek Fires– Victoria
• 2006 Tawonga Gap• 2007 Tatong Fire• 2009 Kilmore East• 2009 Beechworth• 2011 Tostaree Fire
Canada (1):2001 Chisholm fire
USA (1):1980 Mack Lake fire
Contextual BackgroundPyroconvective events and weather
Classification:– Edge of ridge of high pressure (4)– Passage of continental heat trough (4)– Passage of continental heat trough &
small low (3)– Passage of cold front (2)
Contextual BackgroundWeak trough on edge of high pressure
Darwin Catchment10 September 1971WA Experimental Fire13 December 1969
WA Experimental Fire6 December 1969WA Experimental Fire5 December 1969
Prevailing Weather PatternsPassage of heat trough
Tatong16 January 2007
Beechworth7 February 2009
Grose Valley & Wollemi Creek
22 November 2006Tostaree
1 February 2011
One Tree Hill Fire19 February 1968
Contextual BackgroundContinental heat trough and small low
Kilmore East7 February 2009
Canberra Fire18 January 2003
Tawonga Gap10 December 2006
Prevailing Weather PatternsPassage of cold fronts
Mack Lake Fire5 May 1980
Chisholm Fire28 May 2001
Case Studies Theory:Conceptual Diagram
Combustion Region
Heat & moisture mixing region
Mixed Layer
Condensation Region
Pyro-Cuor
Pyro-Cb Cloud
LoftedHeight
Cloud Base
Cloud Top
Moisture flux (fire & entrainment)
Heat buoyancy flux
Adapted from Rio et al. 2010
Windspeed
Idealised types of combustion regions
FlamingDepth (DF)
ResidualCombustion
Pre-trough orfront shape
Speed(V)
Post-trough or frontshape
Speed (V)
Widthof
Fire Front
Idealised
BHFI= HWR
FlamingCombustion
Region
Burnt-out
Generalised modes of fire spread& energy release
1980 Mack Lake2003 Canberra2006 Wollemi Creek2006 Tawonga Gap 2007 Tatong
1. Elliptical spread Limited spotting
2. Elliptical spread spotting
2009 Kilmore East,2011 Tostaree
3. Post-changeFlank Spread
2001 Chisholm
4. Post-changeFlank Spread
2009 Kilmore East (Post SW Change)2011 Tostaree
5. Mass Ignition
1969 WA and 1971Darwin Experimental Fires
Mixing RegionConversion of fuel by fire into heat and moisture
~160–200 m
HM
~3–10 m
Moisture flux– Amount of fuel
combusted– Release of free water
in dead and live fuels
Heat flux ~ average combustion rate in (1) flaming & (2) residual combustion regions
FF D
MRχQφF = where RτDF =
DF
FSM
FF
PFSM CRF =
Rτ = residence time
FD = depth of flaming fire front(s)
Mixing RegionEstimation of Heat Fluxes
Soil carbonburning
0.1–1 kW m-2Headfire heat flux~800–3000 kW m-2
Length or area of each combustion regionAmount of various fuel components burnt upCombustion efficiency
Flank fire heat flux~300–800 kW m-2
Flaming–smouldering~1–10 kW m-2
∑∑∑ )×(+)×(+)×(= 2211 FFFFHH DPWφHDPWφHDPWφHE
Within any given time interval:
∑∑ += SMGDDSMGB AWφHAWφHE
Flaming
Flaming–smouldering
Heat flux budget requires:
Mixing RegionMoisture fluxes
Moisture Flux ~ Average combustion rate based on:• Instantaneous burning areas – combustion regions• Density & moisture of live and dead fuel components• Q(H20) = 55% x (Q) fuel consumed (kg)
~160–200 m
~3–10 m
Height of Mixing Region (HM)
Mixed LayerHeat flux conversion to heated buoyancy
1. Buoyancy, potential temperature, and velocity equations
Viegas (1998)Trentmann et al. (2006)Luderer et al. (2006) Rio et al. (2010)
Heat driven buoyancy-little effect of fire induced
moisture
2. Convection – fire treated like a thunderstorm
Taylor et al. (1971, 1973)Potter (2005)Cunningham and Reeder (2009)Heat and moisture-driven buoyancy
Heat flux conversion to heated buoyancyBelow and above Mixed Layer
Four key questions1. What are the equivalent heat and moisture
perturbations at the top of the mixing region( δT and δM)
2. How much entrainment of air by turbulent mixing? (affects dilution rate of δT and δM)
3. What are the possible effective δT and δM at the top of the perturbed mixed layer?
4. What extra buoyant energy does the latent heat of condensation released above the top of the mixed layer?
Case Studies Theory:Conceptual Diagram
Combustion Region
Heat & moisture mixing region
Mixed Layer
Condensation Region
Pyro-Cuor
Pyro-Cb Cloud
LoftedHeight
Cloud Base
Cloud Top
Moisture flux (fire & entrainment)
Heat buoyancy flux
Adapted from Rio et al. 2010
Windspeed
Dry and wet convective potentialatmospheres
Dry
2001 Chisholm fire2003 Canberra Fire
Wet
Based on concept put forward in Freitas et al. 2007)
General effect of fire in dry and wet convective potential scenarios
Dry Wet
Source: Figure 4 - Freitas et al. (2007)
H (dry case) = 2.5xER^0.1 Freitas et al. (2007)H (dry case) = 1.43xER^0.25 Manins (1985)
Note 1: FF=75 kW m-2 cf. 600–1200 kW m-2
Note 2: In wet case 500 to 4000 m lift caused by latent heat above mixed layer – less lift with more heat flux
Energy Release and Top Heightbased on case studies
Height = 2800xEnergy Release^0.2 (R2=0.83)Mass fire experiments removed – mass ignition
R2 = 0.8272
0
2000
4000
6000
8000
10000
12000
14000
16000
0 5000 10000 15000 20000Observed Height (m)
Exp
ecte
d H
eigh
t (m
)
Detailed examples of case studiesFire Surface
Mixing Ratio
Size (ha) Burning Rate(tonne ha
sec-1)
Energy Release(GW)
One Tree Hill 9.1 ~200 0.5–1 7–12
WA Experiments5th and 6th Dec
7.1–9.1 7,000 0.8–1.2 105–120
Tawonga Gap 3.5–3.7 1,045 2–3 185–200
Tatong 6.8–7.1 3,924 1.5–2.5 270–300
Tostaree 10.3–12.0 1,840 1.5–2.5 320–360
Chisholm 8.4–8.7 2,250 6–8 1,100–1,300
Kilmore East (SE) 5.2–5.4 2,100 6.0–12 700–900
Canberra 3.3–3.8 5,000 8–10 4,000
One Tree Hill1500-1600 hrs 19th Feb 1968
Fire spread between 15:00 and 16:00Energy release ~7-10 GWDP = 12oC TS = 30oC
LCL~ 2800 m
Atmosphere: relatively stable below LCLsurface air mixing ratio ~9.1 g Kg-1
Fire’s smoke rose to 3000 m
PyroCu puffs to ~4000 m
DFMC= 4.9–5.2% Wind:NNW ~22-25 kph
ROS ~0.4–0.6 m sec-1
WA Fire Experiments5th and 6th Feb 1969
DP = 7oC
5≈Tδ
TS = 22oC
LCL~ 1340 m
Top and bottom ofsmoke plume~4300, 2200 m
2.5-2 ≈Mδ
DP = 9.1oC TS = 27oC
LCL~ 1820 m
Top and bottom ofsmoke plume~3000, 1420 m
5≈Tδ5.2-2 ≈Mδ
5th December 6th December
DFMC~5.2 WS~10–15 FFDI~15 DFMC~4.6 WS~15–20 FFDI~20
Tawonga Gap1610-1640 hrs 6th Dec 2006
Fire spread between 16:10 and 16:40Energy release ~185–200 GWDP = -1– -3oC
TS = 25oC
LCL~ 2800 m
Atmosphere: conditionally unstable Surface mixing ratio ~3.5–3.7 g Kg-1
Height of Pyro-CU to ~8000 m
DFMC= 4.1–4.2%Wind: NNW ~30–35 kph
FFDI~38–405≈Tδ4 ≈Mδ
Bottom of Cloud Base?
ROS ~0.4–0.6 m sec-1
Tawonga Gap Photo Sequence
14:3915:1415:36
Tatong1610-1640 hrs 6th Dec 2006
Fire spread between 16:30 and 17:30Energy release ~185–200 GW
DP = 11–12oC TS = 33oC
LCL~ 2900 m
Atmosphere: conditionally unstable Surface mixing ratio ~8.6–8.9 g Kg-1
Height of Pyro-CU to ~11,000 m
DFMC= 4.1–4.3%Wind: NNW ~30-35 kph
FFDI ~29-35
Bottom of Cloud Base?
01≈Tδ3 ≈Mδ
Sub-canopy firemontane
peppermint forests
Fuel density~2.3 kg m-2
ROS ~0.4–0.6 m sec-1
Tatong Effect of terrain on pyroconvection
Pyro-CB centred over mountain
Tatong Fire
30 km
Lightning (orange colour)
Tatong Photo Sequence
15:35
Looking due south towardsMount Buller
Pyroconvection at its peakLooking due south towards
Mount Buller
17:35
Tostaree1517–1755 hrs 1st Feb 2011
Fire spread between 17:17 and 17:55Energy release ~320–360 GWDP = 13–14oC TS = 33oC
LCL~ 2200 m
Atmosphere: conditionally unstable Mixing Ratio ~7.8.–8.1 g Kg-1 then 11 g Kg-1
Height of Pyro-CU to ~10,000 m
DFMC= 3.2–3.3%Wind: NW ~30-35 kph
FFDI 49-546≈Tδ3 ≈Mδ
Bottom of Cloud Base? Coastal
Silvertop Ash forest
Fuel density~3.0 kg m-2
DFMC= 3.5–3.8%Wind: SW ~30-35 kphFFDI 49-54
ROS ~4.5–5 m sec-1
16:34PyroCu above
mixed layer
Tostaree Photo Sequence
17:18 17:2517:55–18:00
Canberra14:00–15:00 18th Jan 2003
Fire spread between 14:30 and 14:45Energy release ~4,000–4,500 GWDP = -3oC TS = 33oC
LCL~ 4000 m
Atmosphere: conditionally unstable Mixing Ratio ~3.0–3.8 g Kg-1
Height of Pyro-CU ~14,500 m
DFMC= 3.2–3.3%Wind: NW ~30-35 kphFFDI 45–70 (adjusted)
02≈Tδ8 ≈Mδ
Bottom of Cloud Base?
MontaneMountain forestFuel density~3.5
kg m-2
ROS ~4.5-5 m sec-
Chisholm Fire, Slave Lake, Canada1830–1930 hrs 28th May 2001
Fire spread between 18:30 and 19:30Energy release ~1600–1800 GWDP = 5.5oC TS = 25oC
LCL~ 3,000 m
Atmosphere: conditionally unstable Mixing Ratio ~6.0–6.2 g Kg-1 to 8.5–8.7 g Kg-1
Height of Pyro-CU ~11,000 m
DFMC= 8.0–11.0%Wind: NW ~40-55 kph
51≈Tδ4 ≈Mδ
Bottom of Cloud Base? Spruce Fire-
Lichen Boreal Forest
Fuel density ~2.3 kg m-2
ROS ~4.5–5 m sec-1
Chisholm Flame Fronts
Summary• More detailed and systematic fire energy
intelligence during going wildfires• Better characterisation of energy release• Still need to work our role of fire and
environment moisture– Dry convective days – extra fire energy needed –
mostly Pyro-cumulus– Wet convective days – fires blow up like
thunderstorms – potential for lightning• Extreme blow-ups happen just after passage of
heat troughs – change in air mass and change in convective potential