SFIRE variables

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Back to the WRF-SFIRE user guide.
This page is under construction. More contents should be coming soon.

Introduction

Firemesh.png

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
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
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 :

  • HF : Heat contents of the fuel (J/kg)
  • RS : Fire spread rate (m/s)
  • F : Fuel remaining at time t (kg/m2)
  • F0 : Initial fuel load (kg/m2)
  • Fn : Fuel load when reaction ends (kg/m2)
  • Fr : Fuel fraction burnt in the flaming (or reaction) zone (1)
  • Tf : fuel burn time (1-1/e 63% of the fuel burned) (s)
  • τR : 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) ≈ HFRSF0aΔtτR/Tf dτ J = HFRSF0/2Tf J/m/s2 yes yes

See also