Modelling fire-induced meteorological phenomena using the MesoNH/ForeFire model

Abstract

Cloud-resolving models coupled to fire propagation models is a powerful tool to understand fire-atmosphere environment. Portugal is one of the countries in Southern Europe with most burned area and numerous ignitions. In 2017, Portugal was affected by several mega-fires with burned areas larger than 10 000 hectares. Some of these fires presented the development of convective clouds, namely pyro-cumulus (pyroCu) and pyro-cumulonimbus (pyroCb). These phenomena can significantly influence the evolution of fire fronts by altering surface winds, increasing burned areas, and accelerating spread rates. This study aims to study pyro-convection during two mega-fires occurred in 2017. In this study, the Quiaios (October 15) and Pedrogão Grande (June 17) mega-fires are chosen as case studies. It was made 3 numerical simulations with the MesoNH atmospheric model coupled with the ForeFire fire propagation model. The experiments were configured into three nested domains with horizontal resolution of 2000m, 400m and 80m. Two simulations were conducted in one-way mode, with the fire front evolution directly imposed by ForeFire and based on a pre-defined map obtained from official reports. The third experiment, conducted only for Pedrogão Grande, utilized the two-way mode (i.e., fully coupled), allowing for the simulation of the impact of fire-induced atmospheric motions on the fire’s own propagation. The results show the development of pyroCu in Quiaios mega-fire and a pyroCb cloud in the Pedrogão Grande event. In the PyroCb case, the simulations showed the development of intense updrafts, that contributed to the vertical transport of water vapor up to the upper troposphere, allowing cloud formation, which was represented by several hydrometeors’ species inside the plume. The extreme fire environment was also verified in the simulation due to downbursts originating from the pyroCb cloud. This study has provided important insights into the numerical modelling of pyro-convective clouds using Meso-NH/ForeFire simulations. This study was funded by FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).