coupled fire-atmosphere research modeling and observations · diagnostics. comparison with previous...

Main contributors:

Janice Coen, NCAR BoulderMorwenna Griffiths,Monash Australia Bill Hall, NCAR BoulderMary Ann Jenkins,York U. Toronto Don Latham, USFS Missoula Montana Don Middleton,NCAR BoulderDavid Packham, Monash AustraliaLarry Radke,NCAR Rhode IslandMichael Reeder,Monash Australia

Coupled Fire-Atmosphere ResearchModeling and Observations

by Terry L. Clark

NCAR University of British Columbia Comox

OUTLINE OF TALK

IR Imagery-overview of IR camera and analysis techniques-some prescribed and wild fire field experiments

Dynamic Model -Overview of nesting,drag etc large range of resolution

Trees Without Fire-Role of Drag

Description of Fire Code-Exp testing effects of canopy drag and heat exchange

INTRODUCTORYOBSERVATIONS

showingFire Atmosphere Interation

Onion sage brush fire in Owensvalley, Ca 1985 courtesy of C. George

Street Patterns observed in Fires

Photo courtesy Brenner -observed in Florida

Montana Finger of Fire

Or effect of Drag+gusts

Modelling the Dynamics

Numerical Model• 3D Non-hydrostatic 2nd order finite-differences• Terrain following - geo-spherical coordinates • Vertical and horizontal grid refinement

– 2-way interaction-

• Vertically stretched grids with grid refinement- Clark (1977,JCP), Clark-Farley(1984,JAS),

Clark-Hall(1991,JCP; 1996,JAM)• Boundary-Initial conditions from NWP• Bulk parameterizations of rain/ice processes• Canopy dynamics and diurnal heat budget

∇2 ∇4 and ∇6 filters for 2 ∇h modes

Coordinates

z = F (ζ )(1 − h ( x , y ) / H ) + h ( x, y )whereF (0) = 0 F (H ) = Handh ( x, y ) is the height of the orography.Model is cast onto constant Δζ grids .

Δ x represents constant longitude incrementsΔy represents constant latitude increments

Horizontal Coordinates

Vertically Stretched Coordinates

Vertical Coordinates used in Fire Modelling

• Inconsistencies can lead to numerical artifacts anderroneous vorticity (e.g. Clark and Hall, 1996)

• Example of grid using multiple passes with 1:2:1 filter

Δz versus z 2

2

zJ

∂∂

• Allows smooth transition between multiple vertical nests

z z

Upscale-Downscale OperatorsMatch mass continuity is matched at all scales

0)()()(______

=++ ωρρρζ

ζδδδ vuy

y

x

x

∑=yzZY

X

etcuxnn

UR_1_

ρ

++++−−= )_

(0)_

(0)_

(_

UREUREUREux XXX

ρ

24/]12)/([2/)1(2)21(0

2/)1(

−ΔΔ=++=+

−−=+−=−

LlEEE

ααεε

αεαεε

where

Variational interpolation Clark-Farley(1984,JAS)

Vector averaging for density weighted velocity terms

Note-Only outermost grid chooses vertical structure-positioning and nesting only flexibility for inner domains

Example of 3-Domain Vertical Grids- (Clark-Hall,1996)

3-Domain vertical grids continueda) b)

Canopy Drag

j

ijii x

uVzadCnudtd

∂∂

+−=τρρ ||)(L

First order formulation, e.g. Wilson et al. (1998)

Where a(z) [m-1] is the one sided leaf area index (LAI)

In numerical experiments presented

a ( z ) dz0

h∫ = 4 . 5 , 7 and ∞

and Cnd = 0.0,.15, .30, .60 and .90

Canopy Drag for Obstacle Flow

SO2 study in Taylor, BC Canada-flow around buildings

-vegetation, bushes and some trees

View of Taylor BC

Drag Outline-Red is high buildings-Magenta low buildings-Green is vegetation

Vertical Vorticity at z= 2 m

Video- Vertical Vorticity at z=2m

QuickTime™ and aVideo decompressor

are needed to see this picture.

Multi-Processing Approach

Message Passing Interface (MPI) software is usedfor multi-processing.

Model Configuration for 3 LayersNVRT=0 (no tiling)

SingleProcessorframework

N=1

N=3

N=2

Multi-Processing Configuration for 3 Layers

NVRT=1 (with tiling) Layer 1 details

Multi-processorFramework

Four sub-domainsper layerMCPU=4

N=3

N=1

N=4

N=2

Green=lmx1,lmx2lmy1,lmy2

Blue=mi2mo

Multi-Processing Configuration for 3 Layers

NVRT=1 (with tiling) Layer 2/3 details

N=5 N=6

N=7 N=8

Green=

lmx1,lmx2

lmy1,lmy2

Blue=mi2mo

Multi-processor

Framework

Four sub-domains

per layer

MCPU=4

Diagnostics

Comparison with previous memory shared code:-useful only for legacy codes

Random symmetry tests:-applicable for all codes-used since early 1980s in code development-eliminates virtually all indexing errors

Comparison among three modes of computation:-NVRT=0 IMPI=0 1 processor-NVRT=1 IMPI=0 1 processor-NVRT=1 IMPI=1 multi-processors

Grid Refinement Using 5 Domains

Example fromClark et al. 2000, JAS

Grid size ranges from26 km to 200 m(4:1 4:1 4:1 2:1)

CO

DOMAIN 5 OROGRAPHY

Example fromClark et al. 2000, JAS

Vs= 55 m/s σq = 10 m/s t=14:10:07

Video - 3D Vis5d of jet stream and CAT

QuickTime™ and aCinepak decompressor


Trees Without Fire

Some results from a forest retention study-Question addressed is “What retention pattern results inleast blowdown after clear cutting?”

-My Answer: There isn’t one.

Here I want to explain Reason I believeno such patterns exist and the possible relation to wildfires

As corroboration: Foresters working in the field see almostAll retention pattern logging fail, i.e. Aggregates also blow down

Wind Tunnel SimulationsExperiment Cnd Cd Δt (s) Δx, Δy, Δz (cm) NX,NY,NZ LAI

WTA3D3 .30 .01 .0015/.0005 (3,3,3)/(1,1,1) (12,1090,52) /(32,1634,41)

4.5

WTA3D4 .60 .01 4.5

WTA3D5 .90 .01 4.5

Cross-stream (x-direction) cyclic, Lx = .3 m, Ly = 32.64 m and 16.32 m and Lz = 1.5 m, V0= 8 m s-1

Video:UZ cross stream avg WTA3D3approx 4h



Comment

Note from previous figures how deep effect of tree drag extends above the trees.

-the effect of tree drag that extends above the surface “turns on” for

≈4h

Δx,ΔyandΔz ≤ h/3 to h/4 -I used h/15 in above study-typical surface paramaterizations at low horizontalor vertical resolution miss the physics entirely

-implication for pollution or smoke dispersion modelling

Retention Modelling

Video-Vertical Vorticity from RTB1H2

Adding Single Eddy added at t=1.65

Video- U at Z=10.5 m

Video:Vertical Cross-section w

Video:Vertical Cross-section u

Video-Torque from RTD1H2

QuickTime™ and aVideo decompressor


Aggregate 1 Bending Moment

Aggregate 2-5 Bending Moments

Relevance to Fire DynamicsEarly Time Flow Dominated by Potential Flow

-Batchelor Fluid Dynamics 101

Introduction of Time Dependence to Flow-Breaks streamlining though development ofa potential flow component

-This component follows the “drag” boundary-More eddies would only add to effect-Eddy that increases wind would cause much stronger effect

Possible Relevance to Fires-Forward bursts at firebreaks-Effects due to canopy drag variation before and after fire-Flames shooting across ground, i.e. finger of fire inMontana

Some Further Considerations on how Potential flow might Affect Fire Behaviour

-gusts impinging on a fire front near a drag boundarycause a component of the flow to follow the boundaryi.e. strong downdraft -this effect occurs at speed of sound (rapidly)hot plume being brought to the surface counters withvorticity that makes hot air rise-this effect occurs rather slowly

-changing orientation affects the buoyancy feedback-Wide range of possible paradigmsFIRE BREAK DESIGN?

D ξDt

= g ∂ B∂ x

OBSERVATIONS

A. Infrared Camera- Inframetrics PM380- 3 to 5 m- 256 by 256 array - Sterling cooler

μ

Image Flow Analysis Applications• Understand fire behavior• Calculate combustion zone winds

and their statistics• Use derived data to validate

numerical models

Image Flow Analysis

Assumptions– IR camera sees incandescent soot particles– Motion is on a distorted two-dimensional surface – Local features can be followed for short periods– We can fit data to simple types of motions, i.e.

translation, rotation, dilation and shear

. .

Image Registration1. Reduce image resolution (e.g. 7:1 in x and 5:1 in y)

2. Align image using IR intensity center of mass

3. Refine alignment using correlation analysis

Λ = Σ (φn+1(x + Δx, y +Δy) − φn(x,y))2

Minimize Λ to estimate Δx and Δy.

4. Extract linear trends in Δx (t) and Δy(t).

5. Registered IR images:

- used in image flow analysis to estimate winds within the combustion zone

Image Flow AnalysisGradient Approach (Helmholtz theorem):Two-dimensional motions can be represented as the sum of six components.

Translation

. .

)( 210 −=∇fdtd

)()( 3022 =+ yfxfdtd

Rotation

)(40=+ yfdtdyfxf

dtdxf

Uniform expansion

And two shear components which most researchers ignore.

Robust StatisticsUsing the two components of translation,

0=∇fdtd

⎟⎟⎠

⎞⎜⎜⎝

⎛−−

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

ztfxtf

wu

zzfxzfxzfxxf

we obtain the matrix equation

which we solve at each pixel for u and w. If |u2 + w2| > S2 thenwe flag that pixel as an outlier and avoid considering it in the calculations. Typically, S= 20 to 40 m/s.

Least Squares Minimization

⎟⎟⎠

⎞⎜⎜⎝

⎛

−

−=⎟⎟

⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

∑∑

∑∑∑∑

ztfxtf

wu

zzfxzfxzfxxf

After identifying outliers we sum over a patch of data as

We typically use 7 by 7 pixels.Outlier points are not included in any of the summations.

This simplest approach requires the inversion of a second order matrix to estimate u and w..

International Crown Fire Modelling Exp

Cameras on 50fttower

•Canadian and US Forest Services •Near Fort Providence NWT Canada• Prescribed crown fires•150 by 150 m plots•June – July 1997•Tower based IR measurements•Plot 6–9 July 1997

Workshop at the Site

Plot 5 Fire Whirl

Plot 6 Ignition

Plot 6 at 2:08

Plot 6 at 2:09

Plot 6 at 2:10

Plot 6 at 2:10 plus

Plot 6 at 2:11

Forward Burst Sequence

Video: Plot 6 Visual



Derived Winds for file=7004

Video: Plot 6 Image Analyzed Fire WindsFrame 13000 to 14300



Fig 10a Clark et al. 1999, JAM

Half Time

Wild Fire Experiment

• NCAR• Sept 1998, Montana, Colorado and California • Infrared Camera mounted on NSF/NCAR C-130• Wildfires were the target of opportunity• First case was in Glacier National Park

-Challenge Fire Complex 4 Sept 98-100 m long hairpin vortex observed with IR-fire about 2 km away from camera

IR Imagery from C-130Fire Finger

Finger shot outabout 100 m in1-2 sec

Indications of burning fuel on sfc after finger retreats

Hairpin or Turbulent BurstOR effect of drag variationwith strong gust

Video: IR Observations over Glacier National Park - 4 Sept 1998



Northern Territory Grass Fire Experiment- Australia

• Spear grass burns 40 km South of Darwin• Kerosene grass burns near Batchelor• Used 19 m high cherry-picker as platform• Platform motion requires apparent motion treatment

Viny’s Cherry Picker

Fuel Type - Australian Spear Grass (Sorgum_Intrans)

Litchfield Park Kerosene Grass

Litchfield Kerosene Grass (19 m up)

Video: IR Data Hughes plot 3Camera ≅ 200 m

from fire

giving ≅ 30 cm pixels



Video: Unprocessed Images Plot-3a Hughes Airfield



Video: Fire Winds using least squareswith registration



Velocity Statistics – Least Squares

Black point valuesRed mean+5sigma

1/2 secrunningaverage

Umax/min Wmax/min

FROSTFIRE

• Permafrost burn near Fairbanks Alaska• 2000 ha prescribed burn • Black spruce and hardwoods• Studying fire behavior and fire ecology• June 1999• NCAR team – airborne observations• CU team – ground based

Frostfire site map

ObservationPost

Vista Geo-monitoring 10 July-escape

Video: Registered images from 10 July 1999A Short Sequence Viewing North

Arrow shows Camera direction

8 times real time



Summary Comments on Observations

• IR Image flow analysis useful to assess statistics of combustion zone winds.

- NWT case (crown fire) estimates between 30 to 40 m/s updrafts - NT case (grass fire) estimates 10 to 15 m/s updrafts

• IR analysis can be used as part of a model validation procedure- model data can be used to mimic what IR camera views.

Wildfire Modeling

Rational for Wildfire Modeling• Wildfire propagation physics is poorly understood• FS spread models use empirical fits from

– Low intensity small fires– Laboratory fire tunnels– Neither can hope to represent the vast parameter space of intense fires

• Understanding fire behavior involves– Combustion winds interacting with the fire and ambient flow– Fire-atmosphere heat exchange– Fire-fuel heat exchange – Chemical release and transport by the convection

• Some Applications of Coupled Fire-Atmosphere Models – Study burn paradigms – Understand fire related sources/sinks to atmospheric budgets-- Understand and help improve design of fire breaks– Develop suppression techniques

NCAR FIRE CODE

Only aspects used in the current fire model.-spread rate treatment-contour advection scheme (I didn’t know aboutlevel-set method at the time!).

Spread Rate Treatment01 RwspS )( φφ ++=

)( nvww φφ =

φs is the slope coefficientis the wind coefficient

nv is the wind normal to the fire front

R0 depends on fuel type and moisture content.A BURNUP type curve is used to describethe rate of mass loss for each fuel cell.

BEHAVE formulation

• Strong need for improved spread rate parametrizations appropriate for coupled fire-atmosphere models

Fire Line Propagation Scheme

•Contour advection scheme•Avoids assuming shape such as ellipse•We want the physics to determine shape

Schematic of tracer and fire line

Tracer Code

• Area inside tracers designates ignited fuel•The Δx and Δy of fire model range from 1 to 3 m • Fuel cell Δxf and Δyf range from .5 to 1 m

•Grid point method used to track fuel and fire

• Four particles (or tracers) for each fuel cell

•Local contour advection scheme used to move tracers

Fig3 Normal Vectors

Video: Animation of six tests



Weather Fire Integrated System(WFIS) FIRE CODE

•Multi-fluid approach similar to bulk microphysics parameterizations.

•Physical parameterization of fire at the level of sophistication where we can begin to compare with the natural event.

•True parameterization in the sense that model does not converge to physical equations as resolution is increased.(i.e. not a poorly resolved treatment of the physical eqns)

Clark, Griffiths, Reeder and Latham, 2003: J. Geophysical Research, 42, 970-983.Clark, Griffiths,Reeder,Packham,Krusel, 2004:Meteor. & Atmos Phys.

Fire Model Variables

Mf(x,y,z,t) - mixing ratio of combusting materialTb (x,y,z,t) - temperature of combusting materialMb(x,y,z,t) - mixing ratio of smoke

τb - Time scale for combustion e.g. 1-2 s.μ f - Volume mixing ratio of Mflf - Effective volume/area ratiols - Interfacial diffusion length scaleSb - Conversion rate for MfSf - Conversion rate of pyrolysis Sv - Conversion rate for water vapour

Field Variables

Parametersτ

μf - volume mixing ratio of combusting material

Assumptions1. Assume a time scale for combustion τb, e.g. 1-2 s.2. Assume combusting material and air are two

interacting fluids3. lf determined so that Tb (max) = 1200k. 4. Exchange of heat between the combusting fluid and

air 1/(ls lf )5. H20 and CO2 absorption contribute to heating of air

Above assumptions designed for models using Δx Δy and Δz 1 to 3 m.

∝

≈

Governing Equations

)( fMKbSfSzfM

dtd

∇⋅∇+−∂∂

−= ρρ

S b = ρ M f / τ b

Sf = S o exp( −z / λ s )

ρ ddt

Mb = Sb − Sv+ ∇⋅(ρK∇Mb)

Sv = 0.56Sb = 0.56ρMf / τb

Conservation equation for Mb (smoke)

Conservation equation for the mixing ratio of combusting material, Mf

Governing Equations (cont.)

)(

)(42

*

bTKpc

airTbTfs

pcbSbCf

bTbT

dtd

pc

∇⋅∇+

−−⎟⎟⎠

⎞⎜⎜⎝

⎛+

−=

ρ

ρνσερρρ

lll

)()( θρρνθρ ∇⋅∇+ℜ+−= KairTbTfsdt

dll

Thermodynamic equation for temperature of combusting material,Tb.

Conservation equation for θ showing fire - atmosphereexchange terms

Governing Equations (cont. again)

Effective buoyancy that drives updrafts is now defined as

bTfffef μθμθ +−= )1(

where

μf is the volume mixing ratio oUsing heat flux arguemenburnoff raμfis expected to be in the ra

A Numerical ConsiderationThe diagnostic equation for Tb withoutconduction and eddy mixing reveals the timescale

τf =300cpτb

CbMf

meaning that without taking any avoidance measuresΔt /τf ≤1In the present grass fire caseτf is about .1 to .05 s.

Figure 8.Horizontal and vertical cross sections from experiment CFA. (a) is at the surface, (b) is at x = 150 m, (c) is a section through

y = 150 m, and (d) is taken at a height of 1 m.

Figure 4.Maximum and minimum u, v, w and maximum

versus time for experiment CFA.

Figure 5.Vertical cross-sections of u (a) and w (c) for experiment CFA at t = 1.4 min. Panel (b) shows the variation of u at z = 1 m,

panel (d) shows the variation of w at z = 1 m.

Figure 6.Plan view of fire line at t = 1.4 min showing the wind vectors

at z = 2m above ground and plotted every 2 m for experiment CFA. The bold line marks the position of the fire front. Only a section of

the domain is shown to highlight the variations along the fire.

Figure 7.(a) Maximum combustion temperature, (K), (b) spread rate (ms-1), (c) heat flux (MWm-2) and (d) total sensible heat flux (GW) from

the fire versus time for experiment CFA.

Figure 9.Horizontal cross section of heat flux at z = 3 m above the surface for experiment CFA

Video showing Isosurface of W (1 m/s)(yellow) with Tb (600K) in blue shown near surface



Effect of Atmospheric Heating

ls small so that

)( airb TTfs

−ll

ρν

has a significant effect Two-Dimensional Framework

Two-Dimensional Test Experiments

All experiments used a specified spread rate of 1 m/sSpecified fire width of 7 m

ls sensitivity exps - used four settings 1, .5, .25, .125 cmRepresenting small to strong heat exchanges.

Canopy drag experimentsExp Cnd ls (cm) Canopy Sp (m/s) Comments

B1 0.15 0.5 36 m break 1.0 35 k heating

B2 0.0 0.5 36 m break 1.0 35 k heating

B3 0.15 0.5 continuous 1.0 35 k heating

B4 0.0 500.0 36 m break 1.0 No heating

B5 0.15 500.0 36 m break 1.0 No heating

Figure 2. at t = 24 s from a series of Australian grass fire experiments. The values of are 1., .5, .25 and .125 cm, respectively. The contour

interval of is 8 K.

l

ls =1 cm ls =.5 cm

ls =.25 cm ls =.125 cm

Observations Needed to Estimate ls

Canopy DragExperiments

in 2D

FireB1-B5 comparisons t= 18 and 37 sCanopy Drag ( ls) = 0.5 & 500 cm Uo=3m/s

t = 18 sls = 0.5Cnd=.15heating

t = 18 sls = 500.Cnd=.15no heating



FireB1-B5 comparisons t= 56 and 75 sCanopy Drag ( ls) = 0.5 & 500 cm Uo=3m/s





FireB1-B2 comparisons t= 18 and 37 sCanopy Drag (Cnd) = 0.0 & .15 Uo=3m/s


t = 18 sls = 0.5Cnd=0.0heating


t = 37 sls = 0.5Cnd=0.0heating

FireB1-B2 comparisons t= 56 and 75 sCanopy Drag (Cnd) = 0.0 & 0.15 Uo =3m/s



t = 56 sls = 0.5Cnd=.0.0heating

t = 75 sls = 0.5Cnd=.0.0heating

AVI FIREB1 Cnd = .15 ls =.5 cm



AVI FIREB2 Cnd = 0.0 ls =.5 cm



Summary Comments on Modelling

• Coupled dynamics and thermodynamics works well in the MPI environment.

• New Parameterization is proving useful to test physical concepts• Field variables easily added

-treat transport of chemical species. -treat additional physical effects, e.g.

• split Mf to consider flashover effects• firebrands for spotting

• Canopy drag has a significant impact on fire behaviour - case treated suggests wind driven to plume type conversion

• Need to validate approach and parameter setting using observations• Model ready to consider fire-to-fuel heat exchange

Conclusions

• High Vertical and Horizontal Grid Resolution-Important to capture fire dynamics-Important to have tree drag physics “turn-on”-Capture local variations in topography

• Importance of Tree and other Drag -Can cause regime change from wind driven to convectivelydriven

-Can capture drag boundary effects affected by time dependantflow, i.e. forward bursts, extruding surface fire fingers etc.-Can be used to approximate flow around buildings

Conclusions (cont.)•IR Imagery

-Bargain at only $70k for 3-5micron camera-Image flow analysis good for winds and thermal structures-Comparison with simulations helps model verification-Some questions about what are we really looking at

•Fire Parameterization-Captures a range of important fire effects-Robust enough to add further physical effects

-fire brands,fire to fuel heat exchange

• Role of Potential Flow In Wildfire-This component follows the drag boundary-Largely ignored?-Important in considering fire break design-Interplay between the two components of flow may help explainflapping like fire behaviour during spread

coupled fire-atmosphere research modeling and observations · diagnostics. comparison with previous...

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