The role of deep flaming in violent pyroconvection

Abstract

Violent fire-driven convection can manifest as towering cumulus or cumulonimbus clouds, the latter of which are known as firestorms (pyroCb). These extreme fires can have devastating impacts on environment and society, and appear to be a worsening problem. The major concerns surrounding these large pyroconvective events are that their associated fire spread is highly unpredictable and that they’re generally not suppressible. Indeed, current methods of fire spread prediction, including the use of empirical and semi-empirical modelling approaches, fall short when attempting to predict fire behaviour associated with these events. In particular, they commonly under-predict the rate of spread in such situations, when broader-scale fire-atmosphere interactions are dominant. To date, research into large pyroconvective events has either focused on the processes involved in normal atmospheric convection, or on surface fire weather and associated fuel conditions. While some investigations have combined the two approaches, a definitive study into both the effects of the surface conditions and the fire-atmosphere interactions is still needed to better understand their respective role in the development of extreme fires. This paper incorporates recent insights into dynamic fire propagation into a coupled fire-atmosphere fire modelling framework to test the combined effects of fire behaviour and atmospheric structure on the occurrence of violent pyroconvective events. In particular, we consider the role of factors such as the spatial expanse and intensity of the fire, and the stability of the atmosphere (both in terms of temperature lapse and moisture profile). The effects of these variables on extreme pyroconvective development are investigated in a systematic way, varying these parameters using the coupled fire-atmosphere WRF-Fire (Weather Research and Forecasting Model (WRF)). In the initial work presented in this paper we focus on the case where the fire is represented by a static heat source of variable dimension and intensity. Analyses were conducted to investigate how (a) the size of the fire (i.e. area of deep flaming) and (b) the intensity of the sensible heat source affects the plume development. Results from preliminary analyses of dry, static fires indicated that the areal expanse of the fire strongly influences the development of the column of deep convection associated with violent pyroconvective events. This supports the hypothesis that plumes above zones of deep flaming are less influenced by entrainment than plumes emanating from typical linear fire fronts. While air is entrained at the plume boundary, the inner core of the plume is less able to entrain air. As a consequence, the areal nature of the heat source driving convection has an influence on the dynamics of the plume - in particular, the heights it can attain. The magnitude of the heat flux produced by the fire is also important for areas of deep flaming up to a minimum of 2 km diameter, but as the size of the heat source increases beyond a 2 km diameter, this appears to be less influential on the way the plume develops. Our findings provide motivation for further investigation into the effect of the fire’s attributes on the immediate atmosphere. They also have the potential to significantly improve forecasting of blow up fire events. Indeed, by combining our findings with recent insights into dynamic fire propagation and improved operational information relating to the atmospheric profile (e.g. through enhanced capability to conduct atmospheric soundings) it is feasible to provide fire agency personnel with far more targeted methods and tools that can be used to better distinguish fires that are likely to develop into violent pyroconvective events.