Physical constraints on the stratospheric injection of trace gases from wildfires : Simulation of the 2019/2020 Australian New Year event

The Australian New Year (ANY) event refers to a time period during the 2019/2020 "Black
Summer" wildfire season, when a particularly intense pyrocumulonimbus activity was recorded.
This resulted in a direct injection of aerosols and chemical trace gases into the upper troposphere and lower stratosphere. Emissions at these heights enhance the lifetime of the aerosols and therefore enable long-range transport. Furthermore, they had profound impacts on Earth’s radiation budget, stratospheric chemistry and local circulations.
Wildfires interact with the atmosphere in numerous ways. The sensible heat released by the fire
creates buoyancy, which can enable pyrocumulus or pyrocumulonimbus formation. This strongly
impacts the emission height and the subsequent transport of the emitted chemical tracers and
aerosols. When it comes to modeling wildfire emissions, the injection height is a key parameter.
In this study, the ICOsahedral Nonhydrostatic model with Aerosols and Reactive Trace gases
(ICON-ART), which incorporates a so-called plume rise model, is used to simulate the fire emission height and transport for the extreme fire event ANY.
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The simulation lasts for seven days starting on the December 29, 2019. In a first step, I try to
reproduce the extreme plume injection heights that have been observed and are connected to
extreme pyrocumulonimbus activity with a low-resolution global simulation in ICON-ART. To
achieve larger injection heights of the plume, the sensible heat release by the fire is accounted for in the simulation and enhanced by a constant factor of 6.8 to match the plume heights observed with satellite instruments. CALIOP cross-sections indicate that plume signals around or above 15 km can only be reproduced when accounting for fire-induced sensible heat release. Furthermore, the plume and cloud top heights for this factor get closest to the maxima of the MODIS cloud top height.
The impact of aerosol-radiation interaction is investigated, especially with regard to the self-lofting effect which made the plume ascend for a longer time period after its injection. The self-lofting counteracts the sedimentation of the plume and enables it to rise to greater heights. The magnitude of the self-lofting is heavily influenced by the initial injection heights, as they determine whether the plume gets above cloud level or whether cirrus clouds can be located above them and inhibit warming by scattering solar radiation. Furthermore, the local temperature and vertical wind anomalies caused by the aerosol-radiation interaction are identified in the model data. A diurnal velocity variation of the self-lofting is found and an overall self-lofting rate of almost 500 m per day is derived.
For this thesis, the wildfire emission parameterization in ICON-ART, which currently only considers aerosols, is extended to include chemical trace gases. Therefore, it is now possible not only to simulate the impact of wildfire smoke on atmospheric chemistry but also to analyze the aging effect of aerosols. This permits tracking the transport of the plume by emitting CO and using a lifetime chemistry approach, which can, for example, be compared to IASI CO column load data or to timelines measured by MLS on several pressure levels. Another possibility is to emit more trace gases; these can react and impact the aerosol aging processes. The aerosol aging changes the optical properties; therefore, new optical properties are derived for the mixed soot particles. An analysis of the aerosol aging finds a coating fraction of approximately 15% and a median diameter of 150 nm. These values are used to calculate the new optical properties, which lead to a slight reduction in self-lofting due to the aging.