SFIRE variables

Back to the WRF-SFIRE user guide.

Introduction

The fire mesh is 2D. Every atmosphere cell is divided into sr_x by sr_y fire cells. The submesh ratios sr_x and sr_y are specified in the file namelist.input. These values are not stored in the NetCDF files, but can be computed from the dimension sizes.

sr_x=west_east_subgrid/west_east_stag = west_east_subgrid/(west_east + 1)
sr_y=south_north_subgrid/south_north_stag = south_north_subgrid/(south_north + 1)

For historical reasons, the fire grid dimensions in the output files are larger than what is actually used by the code internally. The extra space is at the end of the variables of size sr_x in x and sr_y in y. The following is an example of python code for correctly reading FGRNHFX from the file wrfout.

from netCDF4 import Dataset
f=Dataset('wrfout','r')
sr_x=len(f.dimensions['west_east_subgrid'])/(len(f.dimensions['west_east'])+1)
sr_y=len(f.dimensions['south_north_subgrid'])/(len(f.dimensions['south_north'])+1)
fgrnhfx=f.variables['FGRNHFX']
fgrnhfx=fgrnhfx[...,:-sr_y,:-sr_x]

Variables on the fire mesh are located at the centers of the fire mesh cells of a 2D fire mesh.

List of variables

The authoritative information can be always found in the Registry.

Fire variables on fire grid

Variable name	Description	Unit
NFUEL_CAT	fuel data
ZSF	height of surface above sea level	m
DZDXF	surface gradient x	1
DZDYF	surface gradient y	1

Fire variables on atm grid (outputs to atm model)

Variable name	Description	Unit
RTHFRTEN	temperature tendency	K/s
RQVFRTEN	humidity tendency

Diagnostics

Variable name	Description	Unit
AVG_FUEL_FRAC	fuel remaining averaged to atmospheric grid	1
GRNHFX	heat flux from ground fire	W/m2
GRNQFX	moisture flux from ground fire	W/m2
CANHFX	heat flux from crown fire	W/m2
CANQFX	moisture flux from crown fire	W/m2
UAH	wind at fire_wind_heigh	m/s
VAH	wind at fire_wind_heigh	m/s

Sfire variables on fire grid(also using inputs: zs,z_at_w,dz8w,nfuel_cat,zsf)

Variable name	Description	Unit
TIGN_G	ignition time on ground	s
LFN	level function	1
FUEL_FRAC	fuel remaining	1
FMC_G	fuel moisture contents	1
FIRE_AREA	fraction of cell area on fire	1
UF	fire wind	m/s
VF	fire wind	m/s
FGRNHFX	heat flux from ground fire	W/m2
FGRNQFX	moisture flux from ground fire	W/m2
FCANHFX	heat flux from crown fire	W/m2
FCANQFX	moisture flux from crown fire	W/m2

Diagnostics for the actual modeled fire

Variable name	Description	Unit
ROS	rate of spread in the normal direction to the fireline	m/s
FLINEINT	fireline intensity	W/m
FLINEINT2	alternative fireline intensity	J/m/s2

Diagnostics for fire risk rating - independent on any actual fire going on

Variable name	Description	Unit
F_ROS0	base rate of spread in all directions	m/s
F_ROSX	X component of the spread vector driven by wind and slope	m/s
F_ROSY	Y component of the spread vector driven by wind and slope	m/s
F_ROS	max spread rate in any direction	m/s
F_INT	fire reaction intensity for risk rating, without fire	J/m2/s
F_LINEINT	Byram fireline intensity for risk rating, without fire	J/m/s
F_LINEINT2	alternative fireline intensity for risk rating, without fire"	J/m/s2

Constant data arrays

Variable name	Description	Unit
FXLONG	longitude of midpoints of fire cells	degrees
FXLAT	latitude of midpoints of fire cells	degrees
FUEL_TIME	fuel
BBB	fuel
PHISC	fuel
PHIWC	fuel
R_0	fuel
FGIP	fuel
FZ0	fuel roughness height
FWH	fuel fire wind height
ISCHAP	fuel

Fire intensities

We introduce three fire intensities:

1. Reaction : The heat release rate per unit area of the front.

2. Byram : The heat produced per unit length of the fireline in unit time (J/m/s) in the so-called flaming zone behind the fireline.

3. Fireline, new : The amount of heat generated by the advancing fireline from the newly burning fuel only, in the unit of time.

Definitions and implementations of fire intensities include variables :

• H_F : Heat contents of the fuel (J/kg)

• R_S : Fire spread rate (m/s)

• F : Fuel remaining at time t (kg/m²)

• F₀ : Initial fuel load (kg/m²)

• F_n : Fuel load when reaction ends (kg/m²)

• F_r : Fuel fraction burnt in the flaming (or reaction) zone (1)

• T_f : fuel burn time (1-1/e 63% of the fuel burned) (s)

• τ_R : Reaction time (s)

Intensity	Definition	Implementation	Unit	Depends on rates spread RS	Depends on rates burn 1/Tf
Reaction	IR=HF(F0-Fn)/τR	IR=HFF0Fr/(-Tfln(1-Fr)	J/m2/s	no	yes
Byram	I=HFRSF	I=HFRSF0Fr	J/m/s	yes	no
Fireline, new	J(Δt2) =	J = HFRSF0/2Tf	J/m/s2	yes	yes