Dry pyroclouds promote longer-lasting extreme wildfire events intensification.

Abstract Global occurrence of pyroclouds leads to an accelerated wildfire rate-of-spread (ROS), creating extreme wildfire events (EWE). Pyroclouds form during wildfires under unstable atmospheres. Recent EWEs unexpectedly created deep moist pyrocloud and accelerated ROS through the night, beyond the unstable atmosphere’s daily cycle. Here, we analyzed the dependence of the ROS acceleration on pyroclouds and atmospheric instability. We used 190 EWEs observed worldwide, supporting their analysis with a coupled fire‒atmosphere model. We find that accelerated ROS depends on dry pyroclouds processes driven by atmospheric instability, particularly that of the free troposphere (FT). Dry pyroclouds form when plume deepening reaches the transition layer between the atmospheric boundary layer (ABL) top and the lifting condensation level (LCL). The depth of dry pyrocloud turbulence, modulated by the FT stability, plays a leading role in further enhancing the downward entrainment of warm and dry air. This entrainment process mixes ABL to form a deeper fireABL, optimizing conditions for moist pyrocloud events. A novel finding is the role of the dry pyrocloud entrainment process in maintaining a decoupled fireABL from surface conditions during the day-to-night transition. This dynamical transition explains the recently observed nocturnal extreme fire events that lasted up to 17 hours, a phenomenon present in 40.2% of observed EWEs worldwide. We argue that during wildfires, the dry pyrocloud persistence shifts the turbulent driving conditions from surface to atmosphere, disrupts the daily cycle of the lower atmosphere, and increases the number of longer-lasting pyroconvective EWEs.