Wildfire-atmosphere interactions during the Santa Coloma de Queralt fire: the development of a fire-induced circulation

Abstract. High fireline intensities during extreme wildfire events can trigger pyro-convection, causing unpredictable fire spread behaviour, including faster-than-predicted fire spread and continued burning throughout the night. Earlier studies hypothesised that the main impact of pyro-convection on the fire behaviour is through the acceleration of the rear inflow. To assess this hypothesis, we used MicroHH to create a high-resolution (25 m) turbulence-resolving 3D large-eddy simulation (25.6 by 38.4 km²) of the Santa Coloma de Queralt fire. We validated the in-plume virtual potential temperature using sounding measurements, to our knowledge, a novel approach for validating large-eddy simulations of pyro-convection. In-depth analysis of the wind patterns revealed an increase in rear inflow due to pyro-convection, as well as a frontal inflow of comparable magnitude, as part of a fire-induced circulation ahead of the fire. The frontal inflow could counteract the accelerated rear inflow and is associated with fire-generated vortices and long-range spotting. Additionally, we found that the fire-induced circulation simultaneously deepens and lowers the boundary layer in the 4 km ahead, thereby disrupting the transition from the convective daytime to a stably stratified nighttime boundary layer. This disruption provides a plausible explanation for the sustained nighttime burning during the Santa Coloma de Queralt fire. Therefore, we argue that the primary impact of pyro-convection on wildfire behaviour depends on the balance between wind patterns at the rear and in front of the fire (revised hypothesis), rather than solely on the acceleration of the rear inflow (original hypothesis).