In this section we illustrate the parameterization components through a set of idealized simulations using WRF-ARW v4.5.1 for two idealized scenarios:

1) Uncoupled

The first scenario is an “uncoupled” scenario in which the feedback from the fire to the atmosphere - i.e., the release of fire heat fluxes to the atmosphere - is turned off, and the surface boundary condition is set to be free-slip. In this configuration, the atmospheric conditions, including the wind speed and direction, remain constant during the simulation. This scenario is configured to eliminate environmental factors that can affect the firebrands generation and transport in order to validate the implemented parametrization. For this scenario, the atmosphere’s horizontal grid interval is set to 40 m, with refined fire grid interval of 5 m. The model top is set at 2 km with 51 equally spaced grids.The timestep of the simulation is set to 0.5 s with open lateral boundary conditions. The simulations are initialized with a surface temperature of 305K and an air temperature vertical profile constant in the lower 1 km (300 K) below a statically stable layer in the upper 1 km, with temperature increasing linearly to 310 K at the top. The fuel in the fire grid is homogeneous and set to Anderson’s 13 fuel category 10, which corresponds to timber litter with understory, and the fire is ignited using a 1 km long and 100 m wide ignition line 10 s after the simulation start time.

2) Coupled 

The second idealized cases scenario is configured in large-eddy simulation (LES) with two-way feedback, in that the fire fluxes are transferred to the atmosphere allowing for turbulent eddies and a fire-induced atmospheric circulation. In this scenario, the atmosphere’s grid interval is set to 10 m and the refined fire grid interval to 5 m. The model top is set at 2 km with 51 vertically stretched grids. The timestep of the simulation is reduced to 0.125 s to account for the finer grid mesh and fire updrafts. The lateral boundary condition is periodic and the Deardorff’s turbulent kinetic energy subgrid-scale model is utilized with coefficient of 0.1. The simulations in this scenario are initialized using the same temperature profile employed in the uncoupled scenario. We impose an initial zonal wind speed of 10 m/s (unless otherwise specified), interacting with an idealized surface with heat flux of 100 W/m2 and drag coefficient of 0.005. To facilitate the turbulence development in the model, a temperature perturbation bubble with magnitude of 0.5 K and depth of 40 m was utilized. The simulations were allowed to “spin-up” for 0.5 h prior to fire ignition in order to develop the turbulent boundary layer. After the spin-up, the fire is ignited using a 1 km long and 40 m wide ignition line on a homogenous fuel bed set to Anderson’s fuel type 10.
In both of these scenarios, fire spots remained off to reduce degrees of freedom and allow a proper assessment of the parameterization components. Unless otherwise specified, the parameterization settings used in these simulations were set to the values indicated in Table 1, under Value in Ideal Scenarios. 
Scenarios.