Modelling the atmospheric boundary layer conditions during a wildfire event

Abstract

The atmospheric structure becomes more complex than homogeneous terrain over mountainous 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, which can be dangerous for firefighters. The study intends to determine the ABL conditions associated with the largest forest fire that occurred in Portugal in 2019. The present case study is the wildfire that occurred in Vila de Rei county, which is surrounded by hills and valleys with large differences in altitudes. In order to study the regional atmospheric environment between 19 July at 0000 UTC and 25 July 2019 at 0000 UTC, a numerical simulation was performed using the Meso-NH atmospheric model, configured at 2500 m horizontal resolution with 500 × 500 grid points. The simulation involves the Iberian Peninsula scale and coincides with the period when the fire burned more than 9,000 hectares in Vila de Rei. Such a simulation helped to characterise the lower troposphere and the ABL development. In addition to the ABL height, the simulation has allowed exploring the turbulent kinetic energy and its dynamic and thermal production. The ABL height calculated from the Richardson number method depicted a growing in the morning and ranged from 500 to 900 m above the terrain throughout the afternoon and evening during the entire study period. Also, the coastal low-level jet was identified above 500 m altitude, with maximum wind speed at around 600 m altitude at 1800 UTC on 20 July. This study demonstrates the importance of the use of numerical models to explore the ABL evolution over complex terrain, which its development plays an important role during wildfire events.