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