PhD-talk: Jason Müller

Impact of sensible heating and water vapor emission on pyro-convective plume characteristics

PyroCb events are an important source of stratospheric aerosol. During the extreme Australian forest fires in 2019/2020, such convective plumes transported quantities of smoke into the tropopause region that are comparable to a major volcanic eruption. The trigger for the PyroCb clouds is generally the extreme heat generated by the fires. However, measurements of the amount of released heat as well as the ratio of released sensible to latent heat are very challenging and subject to large uncertainties. In this study we investigate the heat emission threshold at which the plume transitions into a PyroCb. We
then examine the sensitivity of the PyroCb to further changes in the amount of heat released as well as to the latent heat to sensible heat release ratio by using idealized simulations with a regional high-resolution model. We show, that the plumes exhibit a strong bi-modal behavior with a transition from cloudless plume to a PyroCb at a sensible heat flux between 50 kW m^-2 and 60 kW m⁻² . If a cloud is formed within the plume, the smoke injection height is mainly controlled by the sum of the sensible and latent heat flux, while the ratio between the two does not play a major role. Increasing either heat flux leads to an increase in the plumes water content and temperature anomaly within the cloud to the same extend. These results show the importance of good estimates of the heat and moisture released by fires in order to realistically capture the sudden development of a PyroCb plume. They also show that water vapor emissions from fires should not be neglected when clouds form in the plume. On an optimistic note, the results indicate that a good estimate of the total heat emitted is sufficient to characterize the PyroCb plume and that a detailed distinction between sensible and latent heat emission is of secondary importance.

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