Idealized Analysis
Firebrand generation is primarily dependent on the fire ROS. The fire ROS and corresponding number of generated firebrands 40 min after ignition is shown in Figure \ref{498784}. The figures show a sequence of generation cycles for the uncoupled (panel A) and couple (panel B) cases, occurring between 2-minute outputs given the respective model timesteps and generation cycle periods. On the left side, the figure shows the fire ROS at the latest timestep, and on the right side, the number of generated firebrands accumulated between outputs. The fire ROS advances with wind direction whereas the accumulation of generated firebrands is denser over areas where (1) ROS is above the default threshold of 0.1 m/s, and (2) ROS is slower, because more firebrands were generated from the same grid point during the accumulation interval. In the uncoupled simulation, ROS is symmetrical and winds are uniform. In the coupled simulation, the wind speed and direction are influenced by the fire heat flux, which in turn drives an asymmetrical ROS, leading to a heterogeneous firebrand generation.