Modelling fire-generated thunderstorms: Pedrógão Grande case study

Abstract

The development of PyroCumulonimbus clouds during mega fire events has high impact in the evolution of the fire fronts and their development is frequently associated with strong convective processes due the heat and moisture released by the combustion. In 2017, Portugal was affected by several episodes of extreme wildfires with such a cloud system. The “Pedrógão Grande” mega fire caused more than 60 fatalities and burned a total area of almost 29,000 ha. In general, the atmospheric models do not account for fire-atmosphere interactions. Aiming to investigate the pyroconvective activity during the Pedrógão Grande mega fire, a numerical simulation was run with the Meso-NH model coupled to the ForeFire model. The Meso-NH model was configured into three nested domains. The horizontal resolution is 2000 m for the outer domain (600 km × 600 km). The inner computational grids have grid increments of respectively 400 m (120 km × 120 km) and 80 m (24 km × 24 km) for the innermost model. Initial and lateral boundary conditions for the outer domain are provided by ECMWF analysis, with updates every 6 h. The simulation with the coarsest resolution began on 17th June 2017 at 0600 UTC, with a progressive downscaling up to the finest resolution beginning at 1300 UTC. The vertical resolution is the same for all the nested domains, with 50 levels and a first level above the ground at 30 m height. The study used the reference fire propagation deduced from the official investigation (forced fire) and the emission of heat and vapour into the atmosphere was made using the ForeFire model. The results highlight the importance of the use of cloud resolving models configured with very-high spatial and temporal resolutions (80m, 10s) for representing the development of phenomena associated to pyro-convective activity, namely those occurring in the micro-scale from the cloud microphysics processes, like very-localised microbursts. This study was funded by national funds through FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).