Self-lofting and radiative forcing of stratospheric aerosol from major wildfires and how it compares to volcanic eruptions

Caused by intense wildfires, Pyrocumulonimbus generate vigorous convective updrafts that inject biomass burning plumes into the stratosphere. Due to the absorption of solar radiation by carbonaceous aerosols, these plumes are uplifted by radiative heating, up to 35 km altitude, which prolongs their stratospheric residence time. In this study we model the self-lofting of these stratospheric plumes as well as their radiative impact.

In the first step, we focus on the physical properties of these plumes and the radiative modeling of their self-lofting. Measurement-based determination of the Single Scattering Albedo (SSA) for stratospheric aerosols is however a challenging task. We thus attempt to constrain the SSA using a combination of radiative transfer modeling and observations from both ground-based and CALIPSO space-borne lidars, as well as use of OMPS-LP extinction profiles. The DIScrete Ordinate Radiative Transfer (DISORT) model, as part of Atmospheric Radiative Transfer Database for Earth Climate Observation (ARTDECO) numerical tool is employed to reproduce the observed self-lofting rate of the plume for varying properties of the plume. We find that the aerosol optical depth and the geometrical thickness of the plume are crucial parameters to model the self-lofting. We also take into account the variations of the underlying cloud cover and surface albedo to better model the self-lofting mechanism.

In the second step, having assessed the SSA, we estimate the radiative forcing induced by these plumes at the top and the bottom of the atmosphere. This method is applied to the Pacific Northwest Event (PNE) wildfire outbreak in August 2017 and the Australian New Year Super Ourbreak (ANYSO) in 2019/20. The results are then compared with previous studies. Finally, we compare the radiative forcing efficiencies of stratospheric smoke with that of stratospheric aerosols from moderate volcanic eruption on local and global scales.