Impact of Sensible Heating and Water Vapor Emission on Pyro-Convective Plume Characteristics

PyroCb events are important sources of stratospheric aerosol. During the Australian forest fires in 2019/2020, such convective plumes transported quantities of smoke into the tropopause region comparable to those of a large volcanic eruption. In this study, we investigate the heat emission threshold at which wildfire plumes transition into pyroCbs. We examine the sensitivity of the pyroCb to further changes in the total amount of heat released as well as to the latent to sensible heat flux ratio. Using the extreme Australian New Years Eve PyroCb event as a case study, we perform semi-idealized simulations with a regional high-resolution atmospheric model. The so simulated plumes show a pronounced bimodal behavior with an abrupt onset of pyroCb formation when the sensible heat flux exceeds 50 kW m⁻². When a cloud is formed within the plume, the plume top height is mainly controlled by the sum of the sensible and latent heat flux, while the ratio between the two plays a subordinate role. Increasing either heat flux raises the plume water content and temperature anomaly within the cloud. The strong differences below the cloud between plumes with equal total heat flux but different sensible heat-to-latent heat ratios are buffered by changes in the cloud base height. These results show the importance of accurate estimates of heat and moisture released by fires for predicting pyroCb development. Encouragingly, a reliable estimate of the total heat flux might be sufficient to characterize the behavior of pyroCbs, reducing the need for detailed partitioning of sensible and latent heat.