Simulating Wildfire-Atmosphere Interactions during the Santa Coloma de Queralt Fire: An Extreme Wildfire Event Continuing into the Night

The Santa Coloma de Queralt fire, a two-day wildfire in Catalonia, Spain (2021), exhibited exceptional wildfire behaviour. It spread four times faster than expected and continued burning into the night while maintaining significant intensity. Under normal circumstances, a wildfire would significantly reduce intensity during the transition to nighttime due to decreasing temperature and ambient turbulence in combination with increasing humidity. Additionally, the Santa Coloma de Queralt fire became extreme for a six-hour period, meaning that it became stronger than the extinguishing capacity of the fire service (10,000 kW/m). This combination of exceptional behaviour and extreme intensity makes it impossible to implement mitigation and evacuation measures timely (e.g. evacuation). To improve the predictability of future extreme wildfires, we investigated the Santa Coloma de Queralt fire, for which extensive documentation and measurements are available.

We hypothesised that exceptional wildfire behaviour could be explained by the wildfire modifying the local atmospheric conditions through its convective plume, thereby improving the burning conditions. Previous studies show that wildfires can significantly alter local wind patterns around the flaming zone by creating strong convective plumes. However, limited effort has been focused on fires' ability to change the local atmospheric conditions.

Hence, we simulated the first day of the Santa Coloma de Queralt fire with MicroHH, a three-dimensional large eddy simulation tool designed to resolve turbulent atmospheric convection, such as wildfire-induced plumes. To ensure realistic results, the simulation was validated against an in-plume sounding.

In line with previous work, we find that a convergence zone developed parallel to the fire front. Developing a convergence zone is typically associated with the acceleration of the wind upwind of the flaming zone. However, for the SCQ fire, our simulation shows the most acceleration inside the flaming zone instead of upwind. Furthermore, we find a significant reversal of the flow downwind of the fire, which leads to downdrafts from the overhanging plume towards the surface. These altered wind patterns downwind of the wildfire change the atmospheric stability up to 3 km downwind of the fire.

In conclusion, our results confirm our hypothesis that wildfires can create an environment with improved burning conditions surrounding the plume.

Acknowledgements: This study was part of the EWED project, funded by the European Union (Project no. 101140363).