Meteorological impacts on long-range spotting of firebrands

Firebrands, small pieces of burning vegetation, can be detached and transported far away from the main fire front during intense fires. The process of firebrand generation, transport and ignition of a fuel bed is known as spotting. Spotting can start new fires and plays an important role in wildfire spread, presenting critical challenges for containment strategies and risk management. This study utilizes a series of high-resolution simulations to evaluate the influence of wind speed, topographic features, fire intensity and atmospheric stability on firebrand transport and fuel ignition. By coupling a fire-atmosphere modeling with combustion and firebrand transport models, we analyze key processes affecting firebrand trajectories and ignition potential.

To obtain realistic conditions of an intense fire, we use the cloud resolving weather model MesoNH coupled with the fire propagation model ForeFire. Such coupled fire-atmosphere simulations are designed to have a computational domain of the same scale of large wildfires, here 80m resolution for 14 km wide, 28 km length and 16 km high. This coupled fire atmosphere model is run for 36 different conditions:

• Three reference wind speeds (5, 10 and 15m.s^-1)
• Three head fire heat flux (40, 80 and 120 kW.m^-2)
• Three topographies (a flat terrain, a hill and a canyon)
• Two atmospheric conditions: stable and unstable

Firebrands are modelled as point masses with three degrees of freedom (three translations), with a set of aerodynamic coefficients and a combustion model. By combining high-resolution LES simulations with detailed firebrand trajectory and combustion processes, we expect to obtain realistic firebrand trajectories.

The resulting different ground patterns distributions of potentially still burning firebrands show that high wind speeds significantly increase firebrand lofting and horizontal transport distances of up to several kilometers. The maximum spotting distance is increased when topographic elements, such as hills or canyons, are added to the simulation. Furthermore, atmospheric stability exerts a critical influence on firebrand behavior: unstable conditions encourage turbulent mixing, vortices, and upward lofting with increased maximum heights reached by the firebrands.

Our results also emphasize the interaction between fire intensity, terrain-driven wind patterns, and atmospheric conditions. This should allow to identify thresholds where long-range spotting becomes most likely. As a result, this research provides valuable insights into the mechanisms driving firebrand dynamics, advancing predictive wildfire modeling and improving hazard mitigation strategies.

 

These results contribute to the broader understanding of wildfire behavior and have practical implications for fire management, evacuation planning, and the development of tailored mitigation measures to address the growing threats posed by wildfires in a changing climate.