Wildfire Modelling of Todayand Into the Future

MITACS/GEOIDE Conference 2009on

Forest Fire Modelling

ByMary Ann Jenkins

York University, Toronto, Canada

Will talk about:

The numerical, primitive equation approach

Dominating influence of ABL (Atmospheric Boundary Layer) winds on the behaviour of wildland fires, especially fire spread.

Demonstrate effect of ambient wind shear, CBL, spotting, hill flow

Unpredictable, ensemble approach, probability forecast

Finally, the problem of scale . . . and its impact on forecast potential

Numerical LES Solution to Navier Stokes Equations

Specifically, the two factors that spread WUI and wildfires and affect their overall behavior are:

• the interaction or coupling between the fire and the fire-induced flow;

• the interaction or coupling between the fire and flow driven by processes in the Atmospheric Boundary Layer (ABL).

Both factors are captured by an LES approach to WUI/wildfire modeling.

The coupling of the UU-LES with an operational wildland fire model (e.g., the Canadian FBP System, Cheney/Gould grassfire spread) is used to simulate grassfire behavior in the ABL.

The coupled UU-LES is designed to simulate fires over landscape scales where a typical computational grid size of 10s of meters is too coarse to resolve physical processes in the combustion zone.

Interaction between the fire plume and the atmosphere is emphasized [as opposed to WFDS’s combustion/fire emphasis].

The Effect of Shear in the Ambient Wind Field

Identical surface windspeed in each case

Tanh Profile

Control(constant with

height)

**Operational fire spread rate formulas**

Tanh (Strong Shear)

(triangle)

Control ConstantWind +

TextText

Control Run No Background Shear

Y Vorticity Budget Time(sec)= 210 Section at y(km)= 1.59

(a) Y-vorticity (s-1) Min, Max=(-0.33, 0.33)

0.0

0.5

1.0

1.5

z (k

m)

-0.3-0.2

-0.1

-0.0

0.1

0.20.3

(b) W (m s-1) Min, Max=(-5.06, 1.98 x 101)

0.0

0.5

1.0

1.5

z (k

m)

-10

-5

0

5

10

(c) Advection (s-2) Min, Max=(-0.79 x 10-1, 0.48 x 10-1)

0.0

0.5

1.0

1.5

z (k

m)

-0.03-0.02

-0.01

0.00

0.01

0.020.03

(d) Pressure Perturbations (kPa) Min, Max=(-0.34 x 10-6, 0.97 x 10-7)

0.0

0.5

1.0

1.5

z (k

m)

5/ 5 m s-1

-4•10-7

-2•10-7

0

2•10-7

4•10-7

(e) Fire Flux (W m-2) Max=(9.32 x 105) Rear/Head (km)= (2.1, 2.6)

1.0 1.5 2.0 2.5 3.0 3.5 4.0x (km)

✓

Time(s)= 720 Section at z(m)= 22

(a) Z-vorticity (s-1) Min,Max=(-0.45, 0.60)

1.0

1.2

1.5

1.7

2.0

2.2

y (k

m)

-0.4

-0.2

0.0

0.2

0.4

(b) W (m s-1) Min,Max=(-2.65, 4.92)

3.5 3.7 4.0 4.2 4.5x (km)

1.0

1.2

1.5

1.7

2.0

2.2

y (k

m)

-4

-2

0

2

4

(c) Divergence (s-2) Min,Max=(-0.14, 0.11)

-0.15

-0.10

-0.05

-0.00

0.05

0.10

0.15

(d) P (kPa) Min,Max=(-0.43 x 10-6, -0.23 x 10-7)

3.5 3.7 4.0 4.2 4.5x (km)

5/ 5 m s-1

-4•10-7

-2•10-7

0

2•10-7

4•10-7

Pay attention tothe Pressure Fieldout ahead of the

fireline/front.

Strong Tanh Shear

inBackground Flow

Y Vorticity Budget Time(s)= 720 Section at y(km)= 1.59

(a) Y-vorticity (s-1) Min,Max=(-0.33, 0.25)

0.0

0.5

1.0

1.5

2.0

z (k

m)

-0.3-0.2-0.1-0.00.10.20.3

(b) W (m s-1) Min,Max=(-4.14, 1.46 x 101)

0.0

0.5

1.0

1.5

2.0

z (k

m)

-10

-5

0

5

10

(c) Advection (s-2) Min,Max=(-0.33 x 10-1, 0.37 x 10-1)

0.0

0.5

1.0

1.5

2.0

z (k

m)

-0.03-0.02-0.010.000.010.020.03

(d) Pressure Perturbations (kPa) Min,Max=(-0.12 x 10-6, 0.87 x 10-7)

0.0

0.5

1.0

1.5

2.0

z (k

m)

5/ 5 m s-1

-4•10-7

-2•10-7

0

2•10-7

4•10-7

(e) Flux (W m-2) Max=(7.11 x 105) Rear/Head(km)=(1.978,2.84)

-0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5x (km)

?

Behavior of fire plume and fire spread can be influenced greatly by vertical shear in

the ambient wind.

Since fire spread is determined by upper–air plume dynamics and atmosphere/fire

interactions.

Flow conditions in the Atmospheric Boundary Layer are crucial to numerical

rendition of wildland and WUI fires.

Conclusions

A Convectively-Driven Boundary Layer

Behavior of fire plume and fire spread can be influenced greatly by turbulence in the

convectively active flow in the Atmospheric Boundary Layer.

Here, an ensemble/probablistic forecasting approach is recommended.

Uncoupled vs Coupled

Ensemble fire spread after 5 min from 24 fires

uncoupled coupled

0 500 1000 1500x length (m)

0

200

400

600

800

y length

(m

)

0.5 0.

51.0

0.0

0.2

0.4

0.6

0.8

1.0

Ensemble fire spread after 5 min from 24 fires

convective rolls unorganized convection

0 500 1000 1500x length (m)

0

200

400

600

800

y length

(m

)

0.5

0.5

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Intense vorticity observed in wildfires plays importantrole in evolution of such

extreme wildfire behavior as

development of fire whirls in and ahead of fire front,

erratic and/or accelerated fire spread,

and massive or area ignition byfirebrand spotting.

Spotting

0 5 10 150Wind speed [m s-1]

0

10

20

30

40

50

0

Heig

ht [m

]

0 50 100 1500X [m]

0 5 10 1500

10

20

30

40

50

0

Heig

ht [m

]

0 50 100 1500

“They never

underpredict”?

Spottingby non combusting

particles of different sizes

Spottingby non combustingversus combusting

particles

Probablistic

Spottingby non combusting

particles each given random in

magnitude and direction initial flightvelocities

Flow Over the Askervein Hill

(Example of validation of LES wind simulations)

Flow first slows as it “feels” the hill, then speeds up, peaks in speed at hill

top, and then quickly decelerates

Flow is first laminar, becomes turbulent at hill top, and then like a wave, breaks on the downwind side

of the hill.

The Question of Scale and the Future of WFDS

• Combustion & mixing (mm to cm)

• Flames, fire-spread (1 to 10 m)

• Convective plume, fireline (100 m to 100 km)

• Atmospheric turbulence (1 mm to 1 km)

• mesoscale flow (mountain/valley circulations, convective downdrafts, low-level jets)

• fronts (few kms to several hundred kms)

• The range of scales involved is therefore enormous (1 mm to 100 km, 108).

• Given the current state of knowledge & computing power, explicitly resolving this entire range of scales in a 3D CFD model of wildfire is not practical now or in the foreseeable future.

• The question here is, with respect to representing ABL conditions and fire/atmosphere interactions that are crucial to WUI and wildland fire behaviour,

• what model developments are reasonable in the meantime?

• The answer is that, as a field model, the WFDS must in the very least have access to a real-time forecast of ABL flow and process.

• The ability to provide this is likely coming within the next decade with

• WRF (Weather and Regional Forecasting) Model, a numerical weather prediction model that (within a year) will be designed to simulate atmospheric phenomena of spatial scales spanning the several hundred kilometres to several metres ranges, where the smallest scale ABL flow is predicted by LES. [LES research mode only for now; operational grid down to 4 km.]

• Philosophy behind this approach

• Adopt models that improve automatically as numerical grids become more refined.

• Only by adopting fundamentally sound physical mechanisms can we automatically shift from empirical to deterministic descriptions of wildland and WUI fires as computers get faster, and temporal and spatial resolution improves.

Thank you

The End