SFIRE variables
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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'][:,:-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.
| 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 |
| Variable name | Description | Unit |
| RTHFRTEN | temperature tendency | K/s |
| RQVFRTEN | humidity tendency |
| 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 |
| 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 |
| 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 |
| 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 |
| 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) ≈ HFRSF0 ∫aΔtτR/Tf dτ | J = HFRSF0/2Tf | J/m/s2 | yes | yes |