High-Resolution Simulation of the Extreme Fire Event in Central Portugal, Pedrogão Grande (2017)

Wildland fire spread and behaviour are complex phenomena owing to both the number of involved Physicochemical factors and the non-linear relationship between variables. In Portugal, one of the European countries most affected by wildfires, forest and bushfires occur every summer and are often exacerbated when extremely dry weather sets along with high temperatures. On the 17th of June 2017, an extreme heatwave associated with a severe drought and compounded by unusual levels of atmospheric instability led to a multiplicity of wildfires with many active fronts, and the formation of pyro-cumulus with explosive fire behaviour. All these factors contributed to the catastrophic fires that occurred in Pedrogão Grande on that day, with more than 100 fatalities and heavy impacts on livelihoods and assets.

The June 2017 extreme fire event in Pedrogão Grande is simulated with the WRF- Fire and Sfire model using a nested framework with increasing spatial resolution, including high-resolution regional scale (2km), local (0.4km) and Large Eddy Simulation (0.08km) resolutions. In this simulation 68 hybrid vertical levels are used, the model top is fixed as 20hPa, the first level is set at approximately 15m from the ground. Initial and boundary conditions for the outer domain were extracted from the ECMWF operational analyses, at 6-hourly intervals. Three microphysics schemes and three boundary layer parameterisations were employed to evaluate the best combination that suits robust reproduction of this complex event. The fire module is a simple 2D model of a surface fire, where the fire spreads through fuels on the ground. In every time step, the fire model inputs surface wind, which drives the fire, and outputs the heat flux from the fire into the atmosphere, which in turn influences the atmosphere. Among the different unusual features, we were particularly interested in assessing the model’s ability to reproduce a series of downbursts that occurred prior to and during the event and that have contributed decisively to atmospheric instability.

It was found that WRF can simulate those features, as well as the pyro-cumulus formation, yet their development is highly dependent on the interaction between the chosen microphysics and the boundary layer schemes. As in the observed event, the fire spread is accelerated westwards in association with the pyrocumulus. The initial simulated fire spread is faster than the observed in all simulations while the extent of the pyro-cumulus is shorter. The FWI (Fire Weather Index), the CHI (Continuous Haines Index) and the FWIe index (blending of FWI and CHI) were high prior and during the fire, observed in all domains, indicating extreme fire hazard and the presence of large instability conditions that can enhance fires that might become out of control, and with erratic behaviour.

Acknowledgements: This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020. L.C. Santos is supported by the EarthSystems Doctoral School, at University of Lisbon, supported by FCT project UIDP/50019/2020-2023, University of Lisbon. M.M. Lima was supported through the PhD FCT programme grant PRT/BD/154680/2023.