Crown Fire

Modeling passive and active crown fire

Crown fire occurs when fire spreads through the canopy layer of trees. This tutorial covers crown fire modeling in Elmfire.jl.

using Elmfire
using Plots

Crown Fire Basics

Crown fire is classified as: - Passive (torching): Individual trees or groups torch, but fire doesn’t spread continuously through canopy - Active: Fire spreads continuously through the canopy, driven by wind

Canopy Properties

Crown fire requires canopy data:

ncols, nrows = 100, 100
cellsize = 30.0

# Create canopy grid with typical forest values
canopy = CanopyGrid{Float64}(
    fill(0.15, ncols, nrows),   # cbd: Canopy bulk density (kg/m³)
    fill(2.0, ncols, nrows),     # cbh: Canopy base height (m)
    fill(0.6, ncols, nrows),     # cc: Canopy cover (fraction)
    fill(20.0, ncols, nrows)     # ch: Canopy height (m)
)

println("Canopy properties:")
println("  Bulk density: 0.15 kg/m³")
println("  Base height: 2.0 m")
println("  Cover: 60%")
println("  Height: 20 m")

Canopy properties:
  Bulk density: 0.15 kg/m³
  Base height: 2.0 m
  Cover: 60%
  Height: 20 m

Enabling Crown Fire

Configure simulation for crown fire:

config = SimulationConfig{Float64}(
    enable_crown_fire = true,
    enable_spotting = false,
    crown_fire_adj = 1.0,
    critical_canopy_cover = 0.4,
    foliar_moisture = 100.0
)

SimulationConfig{Float64}(true, false, 1.0, 0.4, 100.0, nothing, false)

Surface Fire vs Crown Fire

Compare simulations with and without crown fire:

fuel_table = create_standard_fuel_table(Float64)
fuel_ids = fill(10, ncols, nrows)  # FBFM10: Timber understory

slope = zeros(Float64, ncols, nrows)
aspect = zeros(Float64, ncols, nrows)

weather = ConstantWeather{Float64}(
    wind_speed_mph = 20.0,
    wind_direction = 270.0,
    M1 = 0.04, M10 = 0.06, M100 = 0.08,
    MLH = 0.50, MLW = 0.80
)

# Surface fire only
state_surface = FireState{Float64}(ncols, nrows, cellsize)
ignite!(state_surface, 50, 50, 0.0)

config_surface = SimulationConfig{Float64}(enable_crown_fire = false)
weather_interp = create_constant_interpolator(weather, ncols, nrows, cellsize)

simulate_full!(state_surface, fuel_ids, fuel_table, weather_interp, slope, aspect,
    0.0, 30.0; config = config_surface)

# Crown fire enabled
state_crown = FireState{Float64}(ncols, nrows, cellsize)
ignite!(state_crown, 50, 50, 0.0)

config_crown = SimulationConfig{Float64}(
    enable_crown_fire = true,
    critical_canopy_cover = 0.4,
    foliar_moisture = 100.0
)

simulate_full!(state_crown, fuel_ids, fuel_table, weather_interp, slope, aspect,
    0.0, 30.0; canopy = canopy, config = config_crown)

# Compare
p1 = heatmap(state_surface.burned',
    title = "Surface Fire Only\n$(round(get_burned_area_acres(state_surface), digits=1)) acres",
    color = :YlOrRd, aspect_ratio = 1)

p2 = heatmap(state_crown.burned',
    title = "Crown Fire Enabled\n$(round(get_burned_area_acres(state_crown), digits=1)) acres",
    color = :YlOrRd, aspect_ratio = 1)

plot(p1, p2, layout = (1, 2), size = (800, 400))

Critical Fireline Intensity

Crown fire initiation depends on fireline intensity reaching the critical threshold:

# Calculate critical intensity for different canopy base heights
cbh_values = 1.0:0.5:10.0  # meters
foliar_moisture = 100.0  # percent

i_crit = [critical_fireline_intensity(cbh, foliar_moisture) for cbh in cbh_values]

plot(cbh_values, i_crit,
    xlabel = "Canopy Base Height (m)",
    ylabel = "Critical Fireline Intensity (kW/m)",
    title = "Crown Fire Initiation Threshold",
    linewidth = 2,
    legend = false
)

Higher canopy base heights require greater fireline intensity to initiate crown fire.

Canopy Bulk Density Effect

cbd_values = [0.05, 0.10, 0.15, 0.20]
plots = []

for cbd in cbd_values
    canopy_var = CanopyGrid{Float64}(
        fill(cbd, ncols, nrows),
        fill(2.0, ncols, nrows),
        fill(0.6, ncols, nrows),
        fill(20.0, ncols, nrows)
    )

    state = FireState{Float64}(ncols, nrows, cellsize)
    ignite!(state, 50, 50, 0.0)

    simulate_full!(state, fuel_ids, fuel_table, weather_interp, slope, aspect,
        0.0, 25.0; canopy = canopy_var, config = config_crown)

    push!(plots, heatmap(state.burned',
        title = "CBD = $cbd kg/m³",
        color = :YlOrRd, aspect_ratio = 1, colorbar = false
    ))
end

plot(plots..., layout = (2, 2), size = (700, 700),
    plot_title = "Crown Fire vs Canopy Bulk Density")

Foliar Moisture Effect

fm_values = [80, 100, 120, 140]
plots = []

for fm in fm_values
    config_fm = SimulationConfig{Float64}(
        enable_crown_fire = true,
        foliar_moisture = Float64(fm)
    )

    state = FireState{Float64}(ncols, nrows, cellsize)
    ignite!(state, 50, 50, 0.0)

    simulate_full!(state, fuel_ids, fuel_table, weather_interp, slope, aspect,
        0.0, 25.0; canopy = canopy, config = config_fm)

    push!(plots, heatmap(state.burned',
        title = "FM = $fm%",
        color = :YlOrRd, aspect_ratio = 1, colorbar = false
    ))
end

plot(plots..., layout = (2, 2), size = (700, 700),
    plot_title = "Crown Fire vs Foliar Moisture")

Combined Spread Rate

The total spread rate combines surface and crown fire components:

# Get canopy properties
canopy_props = CanopyProperties{Float64}(0.15, 2.0, 0.6, 20.0)

# Range of surface fireline intensities
flin_range = 100:100:5000

surface_ros = 50.0  # ft/min
combined_ros = Float64[]

for flin in flin_range
    cr = crown_spread_rate(canopy_props, Float64(flin), 20.0, 0.06, surface_ros)
    # SpreadResult fields: velocity, vs0, ir, hpua, flin, phiw, phis
    sr = SpreadResult{Float64}(surface_ros, surface_ros, 500.0, 1000.0, Float64(flin), 1.0, 0.0)
    cros = combined_spread_rate(sr, cr)
    push!(combined_ros, cros)
end

plot(flin_range, combined_ros,
    xlabel = "Surface Fireline Intensity (kW/m)",
    ylabel = "Combined Spread Rate (ft/min)",
    title = "Crown Fire Contribution to Spread Rate",
    linewidth = 2,
    legend = false
)
hline!([surface_ros], linestyle = :dash, label = "Surface only")