Modelling pyro-convective clouds using the MesoNH and ForeFire models

Abstract

Extreme wildfires are frequently associated with strong convective processes due the heat and moisture released by the combustion. Such a fire-atmosphere coupling environment lead the formation of convective clouds, also referred as PyroCumulus (PyroCu) or PryroCumulonimbus (PyroCb) clouds. In 2017, an extreme fire season affected Portugal. The present study aims to investigate how a wildfire can influence the occurrence of violent pyro-convective activity. In general, the atmospheric models do not account for fire-atmosphere interactions. To investigate the pyroconvective activity, a numerical simulation was run with the atmospheric Meso-NH model coupled to the fire propagation ForeFire model. The present study considers the wildfires occurred in Pedrógão Grande and Góis on June 17, 2017. The fire propagation map was constructed based in the fire propagation map presented by official report. Fire spread associated with extreme pyro-convective activity is currently highly unpredictable and difficult to suppress. In this study, the findings show the benefits of the use of cloud-resolving models in order to assess the potential for dangerous fire conditions associated with pyroconvection. Moreover, the preliminary results indicate that the model, when configured in high temporal and spatial resolution, is able to represent the pyro-convective plume height from a wildfire and the main processes associated to the interaction of the fire with the atmosphere. In the Pedrógão Grande mega fire event, the violent fire-driven convection manifested as a PyroCumulonimbus cloud. The study is on-going and will help us to better understand pyro-convective events, namely the atmospheric processes in a micro-scale context, and its interaction with the fire behavior through changes in local wind, temperature and humidity that drive the fire line evolution and the local rate of spread. An improved understanding of these events is important for fighting extreme fires.