The Importance of Wildfire Emission Heights for Global Climate Modeling

Abstract:

Wildfires represent a major source of atmospheric aerosols impacting radiative transfer, atmospheric chemistry and cloud micro-physical properties. Although former studies showed that the height of the aerosol-radiation interaction crucially influences the overall climate impact, the importance of fire emission heights in particular has not yet been quantified. In this study we use the general circulation model ECHAM6 extended by the aerosol module HAM2 to investigate the impact of wildfire emission heights on aerosol transport, burden and radiation. We replace prescribed emission heights by an implementation of a semi-empirical plume model. Furthermore, extreme scenarios of pure near-surface and pure free-tropospheric injections provide upper constraints of the emission height climate impact. For evaluation, the modeled Aerosol Optical Thickness (AOT) of all simulations is compared to MODIS data.
Our preliminary results show the unrealistic scenario of free-tropospheric injections to entail a global net surface radiative forcing of -2.5 ± 0.4 Wm^-2. In contrast, the differences in radiative forcing between those simulations with the simple plume height model and those using prescribed emission height scenarios are small (0.05 Wm^-2 to 0.15 Wm^-2). The comparison of simulated AOT to MODIS observations indicates that the choice of the emission inventory is generally more important than the choice of the plume height parametrization. The implementation of the plume model and the additional application of a diurnal cycle in fire emissions slightly reduces the model biases close to the major emission source regions. Nevertheless, on the global scale these improvements in model performance are largely negligible. Thus, the use of advanced plume height parametrizations in global climate models turns out to be required only for specific aerosol or regional climate applications.