Evaluation of WRF-Sfire Performance with Field Observations from the FireFluxexperiment

Mary Ann Jenkins1,4,

Adam K. Kochanski1, Jan Mandel2, Jonathan D. Beezley2, Craig B. Clements3, Steven Krueger1

11Department of Atmospheric Science, University of Utah, Salt Lake City, UT2Department of Mathematical and Statistical Sciences, University of Colorado Denver, Denver, CO,USA3Department of Meteorology and Climate Science, San José State University, San José, CA4Department of Earth and Space Science, York University, Toronto, ON, Canada

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accepted for publication by Atmosphericchemistry and physics (ACP)

WRF-SFIRE’s performance?

Pretty good.

Examples of impact ofwildfire-atmosphere coupling on

wildfire behavior

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Two ExamplesImpact of the background wind field

An illustration of the importance ofvertical wind shear in the upstream

flowProof of Concept

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ByMary Ann Jenkins, York University,

Toronto, Canada(Adjunct UofU)

Adam Kochanski, Steven Krueger,

University of Utah

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Clark et al 1996 IJWF. Schematic of SurfaceConditions at Head of Fire

“kinematic” explanation.

But it is completeand correct?

where δ = ∇⋅V is

convergence,

moving apart of

fluid particles.

V is velocity.6

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kinematicexplanation too simple?

Two counter-rotatingvorticies

produce fire front (solidblack line)

Also simulated& observed

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Idealized (no surface friction) WRF-Sfire simulations of propagatinggrass fire lines in environments

without or with vertical wind shear

all with identical upstream surfacewind speed (5 m/s)

TextWRF-Fire

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tsurface windspeed 5 m/s

0 200 400 600 800 1000 1200 1400 1600 18000

500

1000

1500

2000

2500

3000

3500

X po

sitio

n of

the

fire

front

(m)

0 200 400 600 800 1000 1200 1400 1600 18000

0.5

1

1.5

x 1010

0 200 400 600 800 1000 1200 1400 1600 18000

1

2

x 1010

0 200 400 600 800 1000 1200 1400 1600 18000

2

x 1010

0 200 400 600 800 1000 1200 1400 1600 18000

1

2

x 1010

Time (s)

Tota

l Fire

Hea

t (W

)

CONTROLSHEARLOGTANHdata5

tPropagation of Fire Front

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tTANH

tLOG

tCONTROL

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tSHEAR

Z-vorticity, CONST

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Z-vorticity, CONST

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Z-vorticity,LOG

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Z-vorticity,LOG

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where ω = ∇×V is vorticity,

rotation of fluid particles, ∇ is

gradient operator,

V is 3D velocity, and p is

pressure.

p∝−|ω2|

tVVortices and propagation of thefront

tThese low pressures (perturbations) are NOThydrostatic, they are DYNAMICAL

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tLookingdown on

the surfacefire . . .

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tLookingdown onthe LOG

surface fire. . .

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Z-vorticity,TANH

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Control Run No Background Shear

Z-vorticity,TANH

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tLookingdown on

the TANHsurface fire

. . .

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Steady state fires exist in constant or shearedbackground wind profiles . . . that are not unstableto perturbations in the background flow.

Non steady-state fires occur in a backgroundwind profiles that are unstable to perturbations(supplied by surface heating, and in this case bythe fire).

Because it contains an inflection point, theTANH background wind profile is known to beunstable to perturbations in the wind field.

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tX-Z crosssection of the LOG

fire . . .

tFirefront/backposition

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tX-Z crosssection of

the TANHfire . . .

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Background wind profiles can be inherentlystable or inherently unstable to perturbationsintroduced by surface and plume convection.

Fire front propagation (and plume behavior) isdependent on low pressures associated with thedevelopment of vorticity.

Vorticity and vortices are a natural part of thebasic fluid dynamics of fire convection and willalways be part of the fire perimeter and front.

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Discussion

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ConclusionsThe background wind field does matter! Based on

this study, fire/atmosphere interactions can be responsible for a lot of severe and/or erratic firebehavior that is observed.

Coupled atmosphere/fire simulators such as WFDSmust be given proper flow boundary conditions.

ABANDON the prescribed simple near-surface log-linear wind profile --- it does not work!

The most promising candidate to supply these BCs(either by one-way coupling or two-way coupling) isWRF and/or WRF-Sfire.

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Realistic Experiment

WRF-SFIRE validated using Fireflux I•

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29E−W distance (m)

N−S dista

nce (m)

1000 2000 3000 4000 5000 6000

400

800

1200

1600

2000

2400

2800

0.00

0.25

0.50

0.75

1.00

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