Abstract
More than 1 Tg smoke aerosol was emitted into the atmosphere by the exceptional 2019–2020 Southeast Australian wildfires. Triggered by the extreme fire heat, several deep pyroconvective events carried the smoke directly into the stratosphere. Once there, smoke aerosol remained airborne considerably longer than in lower atmospheric layers. The thick plumes traveled eastward thereby being distributed across the high and mid-latitude Southern Hemisphere enhancing the atmospheric opacity. Due to the increased atmospheric lifetime of the smoke plume its radiative effect increased compared to smoke that remains lower altitudes. Global models describing aerosol-climate impacts show significant uncertainties regarding the emission height of aerosols from intense wildfires. Here, we demonstrate by combination of aerosol-climate modeling and lidar observations the importance of the representation of those high-altitude fire smoke layers for estimating the atmospheric energy budget. In this observation-based approach, the Australian wildfire emissions by pyroconvection are explicitly prescribed to the lower stratosphere in different scenarios. The 2019–2020 Australian fires caused a significant top-of-atmosphere hemispheric instantaneous direct radiative forcing signal that reached a magnitude comparable to the radiative forcing induced by anthropogenic absorbing aerosol. Up to +0.50 W m−2 instantaneous direct radiative forcing was modeled at top of the atmosphere, averaged for the Southern Hemisphere for January to March 2020 under all-sky conditions. While at the surface, an instantaneous solar radiative forcing of up to −0.81 W m−2 was found for clear-sky conditions, depending on the model configuration. Since extreme wildfires are expected to occur more frequently in the rapidly changing climate, our findings suggest that deep wildfire plumes must be adequately considered in climate projections in order to obtain reasonable estimates of atmospheric energy budget changes.
Highlights
During the record Australian 2019-2020 wildfire season, the aerosol load increased substantially over large parts of mid and high latitudes of the Southern Hemisphere due to the massive amounts of smoke aerosol injected into the stratosphere
Ground-based observations for example at the station Punta Arenas show that the January to March 2020 average aerosol optical thickness (AOT) was 0.10, which is more than a factor 2 increase compared to the year 2019 for at least half a year
Showing the contribution of smoke AOT for the case when no smoke injection by pyroconvection is prescribed in the model, and TP+1 – NoEmiss, showing the effect on AOT of smoke injection into the model layer above the tropopause for the pyroCb days 29 – 31 December 2019 and 4 January, 2020 in Southeastern Australia
Summary
During the record Australian 2019-2020 wildfire season, the aerosol load increased substantially over large parts of mid and high latitudes of the Southern Hemisphere due to the massive amounts of smoke aerosol injected into the stratosphere. Triggered by the intense fire heat, the pyroconvection can grow to pyrocumulonimbus (pyroCb) clouds which are the primary pathway of smoke injection into the upper troposphere and lower stratosphere (Fromm et al, 2010; Fromm et al, 2019), with radiation-induced self-lifting causing plumes to continue to rise (Boers et al, 2010) Due to such events, biomass burning smoke contributes considerably to the global aerosol composition, affecting the Earth’s energy balance through aerosol-radiation and tropospheric aerosol-cloud interactions (Bowman et al, 2009; Streets et al 2009; Boucher et al, 2013). Since the typical horizontal resolution of global climate models is too coarse to explicitly resolve convection, observed pyroCb events are explicitly prescribed and the injection height of the wildfire plume is varied in terms of sensitivity experiments Their results are compared to the original settings for biomass burning emissions as well as evaluated with ground-based and spaceborne remote sensing observations to show how realistically these can be represented if the injection heights for pyroCbs are considered . The model results are used to investigate the impact of pyroconvective smoke injection on plume transport and radiative effects for January to March 2020
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