The dynamics and atmospheric impact of fire-induced circulations in idealised large-eddy simulations inspired by the Santa Coloma de Queralt fire

We studied the factors governing the existence of fire-induced circulations ahead of the flaming zone and the impact of these circulations on the thermodynamic structure of the atmospheric boundary layer. To study the circulation, we used MicroHH to create a high-resolution (25 m) turbulence-resolving 3D large-eddy simulations (25.6 km×38.4 km) of a stationary fire under realistic atmospheric conditions. The setup was inspired by field observations of fire characteristics and a radiosonde from the Santa Coloma de Queralt fire (Catalonia, Spain, 24 July 2021). The stationary fire enabled us to isolate the persistent impacts of the fire on the atmosphere. Our results indicate that the existence of a fire-induced circulation is governed by the wind speed component aligned with the circulation. In our simulations, the circulation consisted of updrafts above the fire, downdrafts 2 km ahead, and reversed surface winds between the updrafts and downdrafts. With higher wind speeds in the direction of the circulation, the reversal of the surface winds decreases. Consequently, the circulation dissipates, since the reversed winds connect the updrafts and downdrafts into a circulation. Hence, explaining why fire-induced circulations are not always present. The thermodynamic impact of the circulation is driven by the updrafts and downdrafts, causing 2 km of deepening, followed by 2 km of thinning of the atmospheric boundary layer ahead of the fire. Future research with non-stationary fires is required to quantify the impact of the modified thermodynamics and wind patterns on fire behaviour.