What factors power pyroconvection?

Transcription

1 What factors power pyroconvection? Nic Gellie Fire Scientist Bushfire CRC Co-authors Brian Potter (United States Forest Service) Tony Bannister (Bureau of Meteorology) 2006 Tawonga Gap: Neil Wilson, DSE 2011 Tostaree: Unknown

2 Beechworth Fire Mount Alice 1 hour after passage of fire front Credit: John Sweeney

3 Outline Aim of Talk Background Theory Examples Conclusions

4 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

5 Contextual Background Case 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

6 Contextual Background Pyroconvective 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)

7 Contextual Background Weak trough on edge of high pressure Darwin Catchment 10 September 1971 WA Experimental Fire 13 December 1969 WA Experimental Fire 6 December 1969 WA Experimental Fire 5 December 1969

8 Prevailing Weather Patterns Passage of heat trough Tatong One Tree Hill FireBeechworth 16 January February February 2009 Grose Valley & Wollemi Creek 22 November 2006 Tostaree 1 February 2011

9 Tawonga Gap 10 December 2006 Contextual Background Continental heat trough and small low Canberra Fire 18 January 2003 Kilmore East 7 February 2009

10 Prevailing Weather Patterns Passage of cold fronts Mack Lake Fire 5 May 1980 Chisholm Fire 28 May 2001

11 Case Studies Theory: Conceptual Diagram Cloud Top Lofted Height Mixed Layer Wind speed Condensation Region Pyro-Cu or Pyro-Cb Cloud Heat buoyancy flux Cloud Base Moisture flux (fire & entrainment) Heat & moisture mixing region Combustion Region Adapted from Rio et al. 2010

12 Idealised types of combustion regions Speed (V) Flaming Combustion Region Flaming Depth (D F ) Residual Combustion Width of Fire Front Idealised Burnt-out Speed (V) BHFI= HWR Pre-trough or front shape Post-trough or front shape

13 Generalised modes of fire spread & energy release 1. Elliptical spread Limited spotting 2. Elliptical spread 3. Post-change 5. Mass Ignition spotting Flank Spread 4. Post-change Flank Spread 1980 Mack Lake 2003 Canberra 2006 Wollemi Creek 2006 Tawonga Gap 2007 Tatong 1969 WA and Chisholm 2009 Kilmore Darwin Experimental East, Fires 2011 Tostaree 2009 Kilmore East (Post SW Change) 2011 Tostaree

14 Mixing Region Conversion of fuel by fire into heat and moisture Heat flux ~ average combustion rate in (1) flaming & (2) residual combustion regions φqχmr F F = where D R D F = τ F τ R = residence time D = depth of flaming fire front(s) F F SM = CR PF Moisture flux Amount of fuel combusted Release of free water in dead and live fuels H M F SM F F ~3 10 m D F ~ m

15 H Mixing Region Estimation of Heat Fluxes Within any given time interval: Flaming Flank fire heat flux ~ kw m -2 E = HφW ( P D H ) + HφW ( P D F1) + HφW ( P D 2) Flaming smouldering E HφW A + = B SMG HφWDD ASMG F1 F 2 F Soil carbon burning kw m -2 Headfire heat flux Flaming smouldering ~1 10 kw m -2 Heat flux budget requires: ~ kw m -2 Length or area of each combustion region Amount of various fuel components burnt up Combustion efficiency

16 Mixing Region Moisture fluxes Moisture Flux ~ Average combustion rate based on: Instantaneous burning areas combustion regions Density & moisture of live and dead fuel components Q(H 2 0) = 55% x (Q) fuel consumed (kg) ~ m Height of Mixing Region (H M ) ~3 10 m

17 Mixed Layer Heat flux conversion to heated buoyancy 1. Buoyancy, potential temperature, and velocity equations 2. Convection fire treated like a thunderstorm Viegas (1998) Trentmann et al. (2006) Luderer et al. (2006) Rio et al. (2010) Heat driven buoyancylittle effect of fire induced moisture Taylor et al. (1971, 1973) Potter (2005) Cunningham and Reeder (2009) Heat and moisturedriven buoyancy

18 Heat flux conversion to heated buoyancy Below and above Mixed Layer Four key questions 1. 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?

19 Case Studies Theory: Conceptual Diagram Cloud Top Lofted Height Mixed Layer Wind speed Condensation Region Pyro-Cu or Pyro-Cb Cloud Heat buoyancy flux Cloud Base Moisture flux (fire & entrainment) Heat & moisture mixing region Combustion Region Adapted from Rio et al. 2010

20 Dry and wet convective potential atmospheres Dry Wet 2003 Canberra Fire 2001 Chisholm fire Based on concept put forward in Freitas et al. 2007)

21 Note 2: In wet case 500 to 4000 m lift caused by latent heat above mixed layer less lift with more heat flux General effect of fire in dry and wet convective potential scenarios Dry Wet Source: Figure 4 - Freitas et al. (2007) H (dry case) = 2.5xE R^0.1 Freitas et al. (2007) H (dry case) = 1.43xE R^0.25 Manins (1985) Note 1: F F =75 kw m -2 cf kw m -2

22 Energy Release and Top Height based on case studies Expected Height (m) R 2 = Observed Height (m) Height = 2800xEnergy Release^0.2 (R 2 =0.83) Mass fire experiments removed mass ignition

23 Detailed examples of case studies Fire Surface Mixing Ratio Size (ha) Burning Rate (tonne ha sec -1 ) Energy Release (GW) One Tree Hill 9.1 ~ WA Experiments 5 th and 6 th Dec , Tawonga Gap , Tatong , Tostaree , Chisholm , ,100 1,300 Kilmore East (SE) , Canberra , ,000

24 One Tree Hill hrs 19 th Feb 1968 Atmosphere: relatively stable below LCL surface air mixing ratio ~9.1 g Kg -1 DFMC= % Wind:NNW ~22-25 kph Fire s smoke rose to 3000 m PyroCu puffs to ~4000 m LCL 2800 m D P = 12 o C T S = 30 o C ROS ~ m sec -1 Fire spread between 15:00 and 16:00 Energy release ~7-10 GW

25 WA Fire Experiments 5 th and 6 th Feb th December 6 th December δm δt 5 δm δt 5 Top and bottom of smoke plume ~3000, 1420 m Top and bottom of smoke plume ~4300, 2200 m LCL ~ 1340 m LCL ~ 1820 m D P = 7 o C T S = 22 o C DFMC~5.2 WS~10 15 FFDI~15 D P = 9.1 o C T S = 27 o C DFMC~4.6 WS~15 20 FFDI~20

26 Tawonga Gap hrs 6th Dec 2006 Atmosphere: conditionally unstable Surface mixing ratio ~ g Kg -1 δm 4 δt 5 DFMC= % Wind: NNW ~30 35 kph FFDI~38 40 Height of Pyro-CU to ~8000 m Bottom of Cloud Base? LCL ~ 2800 m ROS ~ m sec -1 D P = -1-3 o C T S = 25 o C Fire spread between 16:10 and 16:40 Energy release ~ GW

27 Tawonga Gap Photo Sequence 14:39 15:14 15:36

28 Tatong hrs 6th Dec 2006 Atmosphere: conditionally unstable Surface mixing ratio ~ g Kg -1 δm 3 δt 10 DFMC= % Wind: NNW ~30-35 kph FFDI ~29-35 ROS ~ m sec -1 LCL ~ 2900 m Height of Pyro-CU to ~11,000 m Bottom of Cloud Base? Sub-canopy fire montane peppermint forests Fuel density~2.3 kg m -2 D P = o C T S = 33 o C Fire spread between 16:30 and 17:30 Energy release ~ GW

29 Tatong Effect of terrain on pyroconvection Tatong Fire 30 km Lightning (orange colour) Pyro-CB centred over mountain

30 Tatong Photo Sequence 17:35 Pyroconvection Looking due south at its towards peak Looking Mount due south Buller towards Mount Buller 15:35

31 Tostaree hrs 1st Feb 2011 Atmosphere: conditionally unstable Mixing Ratio ~ g Kg -1 then 11 g Kg -1 δm 3 δt 6 DFMC= % Wind: NW ~30-35 kph FFDI ROS ~4.5 5 m sec -1 LCL ~ 2200 m D P = o C Height of Pyro-CU to ~10,000 m Bottom of Cloud Base? T S = 33 o C Coastal Silvertop Ash forest Fuel density~3.0 kg m -2 DFMC= % Wind: SW ~30-35 kph FFDI Fire spread between 17:17 and 17:55 Energy release ~ GW

32 Tostaree Photo Sequence 16:34 17:18 17:55 18:00 17:25 PyroCu above mixed layer

33 Canberra 14:00 15:00 18 th Jan 2003 Atmosphere: conditionally unstable Mixing Ratio ~ g Kg -1 δm 8 δt 20 Height of Pyro-CU ~14,500 m DFMC= % Wind: NW ~30-35 kph FFDI (adjusted) LCL ~ 4000 m Bottom of Cloud Base? Montane Mountain forest Fuel density~3.5 kg m -2 ROS ~4.5-5 m sec - D P = -3 o C T S = 33 o C Fire spread between 14:30 and 14:45 Energy release ~4,000 4,500 GW

34 Chisholm Fire, Slave Lake, Canada hrs 28 th May 2001 Atmosphere: conditionally unstable Mixing Ratio ~ g Kg -1 to g Kg -1 DFMC= % Wind: NW ~40-55 kph δm 4 δt 15 Height of Pyro-CU ~11,000 m ROS ~4.5 5 m sec -1 LCL ~ 3,000 m D P = 5.5 o C T S = 25 o C Bottom of Cloud Base? Spruce Fire- Lichen Boreal Forest Fuel density ~2.3 kg m -2 Fire spread between 18:30 and 19:30 Energy release ~ GW

35 Chisholm Flame Fronts

36 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