Modelling the atmospheric factors determining the evolution of the boundary layer during a wildfire event

Abstract

Over mountainous terrain, the atmospheric structure becomes much more complex than homogeneous terrain in terms of the Atmospheric Boundary Layer (ABL). In the context of interaction between fire and atmosphere, abrupt changes in the ABL wind often lead to erratic and turbulent flow in the fire environment and expose firefighters to dangerous conditions. The study aims to characterize the ABL conditions associated with the largest forest fire that occurred in Portugal in 2019. The fire event took place in Vila de Rei county, which is surrounded by hills and valleys with large differences in altitudes. In order to study the regional scale, a numerical simulation was performed using the Meso-NH atmospheric model, configured with 500 × 500 grid points at 2500 m horizontal resolution, between 19 July at 0000 UTC and 25 July 2019 at 0000 UTC. The simulation covered the Iberian Peninsula and corresponds to the period when the fire burned more than 9,000 hectares in Vila de Rei. Such a simulation helped to characterize the lower troposphere, which contributed to the evolution of the ABL height over the days. Results indicate that the simulated ABL evolution is characterized by the presence of a coastal low-level jet with a maximum wind speed of 10 meters per second at ~ 600 meters’ altitude (1800 UTC of 20 July). ABL height calculated from Richardson number method depicted a growing in the morning that reached a peak height by mid-afternoon. The ABL height ranged from 500 to 900 m throughout the afternoon and evening during the entire study period. Besides the identification of the fire weather conditions, this study also highlights the factors that contributed to the lower values of the ABL height in the wildfire event. 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).