How Sensible Heat Release and Water Vapor Emissions from Fires Impact the Characteristics of Pyro-convective Plumes

Large wildfires are a major source of atmopsheric aerosol. The lifetime of the smoke aerosol in the atmosphere, and thus their impact on the climate, is strongly controlled by the altitude in which the smoke is injected into the atmopshere. While most fires release their smoke in the lower troposphere, so called pyrocumulunimbus (PyroCb) events have the potential to transport smoke aerosol upwards deep into the troposphere and even into the lower stratosphere, extending the lifetime of the smoke by several orders of magnitude. These PyroCbs are thunderstorms that are triggered by extreme heat release and occasionally form above particularly intense wildfires.

For example, during the extreme PyroCb event now often referred to as the “Australian New Year’s Eve Event” of 2019/2020, deep pyro-convective plumes generated by record-breaking wildfires injected vast quantities of smoke into the tropopause region that are comparable to those of a major volcanic eruption. It is therefore crucial to understand which fires produce deep PyroCbs and why. In this study, we investigate the critical heat emission threshold at which shallow wildfire smoke plumes transition into pyroCbs that penetrate deep into the tropopause region. We further examine the sensitivity of the pyroCbs to further changes in the total amount of heat released by the fire and analyze how changes in the sensible heat emissions and water vapor release impact plume dynamics. 

To do that, using case studies of extreme fires such as the Australian New Years Eve PyroCb event, we perform semi-idealized simulations with a regional high-resolution atmospheric model. Based on the so simulated plumes, we uncover a pronounced bimodal behavior of the fire-induced convection with an abrupt onset of pyroCb formation when the sensible heat flux emissions by the fire exceeds 50kW m^-2. We show, that whenever cloud formation is present within the plume, the plume top height is mainly controlled by the sum of the sensible and latent heat flux by the fire, while the ratio between the two plays a subordinate role. Increasing either heat flux will simultanously raise  both the plume water content and temperature anomaly within the cloud.  These results show the importance of accurate estimates of heat and moisture released by fires for predicting pyroCb development. Encouragingly, these results suggest that 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.