Exploring the uncertainties in the aviation soot–cirrus effect

Abstract

Abstract. A global aerosol–climate model, including a two-moment cloud microphysical scheme and a parametrization for aerosol-induced ice formation in cirrus clouds, is applied in order to quantify the impact of aviation soot on natural cirrus clouds. Several sensitivity experiments are performed to assess the uncertainties in this effect related to (i) the assumptions on the ice nucleation abilities of aviation soot, (ii) the representation of vertical updrafts in the model, and (iii) the use of reanalysis data to relax the model dynamics (the so-called nudging technique). Based on the results of the model simulations, a radiative forcing from the aviation soot–cirrus effect in the range of −35 to 13 mW m−2 is quantified, depending on the assumed critical saturation ratio for ice nucleation and active fraction of aviation soot but with a confidence level below 95 % in several cases. Simple idealized experiments with prescribed vertical velocities further show that the uncertainties on this aspect of the model dynamics are critical for the investigated effect and could potentially add a factor of about 2 of further uncertainty to the model estimates of the resulting radiative forcing. The use of the nudging technique to relax model dynamics is proved essential in order to identify a statistically significant signal from the model internal variability, while simulations performed in free-running mode and with prescribed sea-surface temperatures and sea-ice concentrations are shown to be unable to provide robust estimates of the investigated effect. A comparison with analogous model studies on the aviation soot–cirrus effect show a very large model diversity, with a conspicuous lack of consensus across the various estimates, which points to the need for more in-depth analyses on the roots of such discrepancies.

A global climatology of ice nucleating particles at cirrus conditions derived from model simulations with EMAC-MADE3

“…Kärcher and Lohmann (2002) and Kärcher et al (2006). The extent of this attenuation effect and resulting cirrus and climate modifications have been shown recently for the case of the aviation soot-cirrus effect (Righi et al, 2021).…”

“…In each of the heterogeneous freezing modes the ice-nucleation properties of the INPs are represented by two parameters, namely the active fraction (f act ) of ice nucleating particles, which actually lead to the formation of ice crystals, and the critical supersaturation ratio with respect to ice (S c ), at which the freezing process is initiated. The model setup has been extensively tuned and evaluated with respect to various cloud and radiation variables by Righi et al (2020) with further model improvements described in Righi et al (2021).…”

“…The EMAC-MADE3 setup applied here is largely based on the setup described in Righi et al (2021). We use the recent CMIP6 (Coupled Model Intercomparison Project, phase 6) emission inventory for anthropogenic and biomass burning emissions of aerosols and aerosol precursor species (van Marle et al, 2017;Hoesly et al, 2018) for the year 2014, to provide an emission setup close to present day conditions.…”

“…Mineral dust emissions are calculated according to the online emission scheme of Tegen et al (2002), as described and evaluated in Beer et al (2020). We employ EMAC in the T63L31 resolution, in contrast to the T42L41 resolution in Righi et al (2021), as this was shown to improve the simulated aerosol concentrations, especially for mineral dust, in the upper troposphere (Beer et al, 2020).…”

“…Furthermore, the model includes an additional tracer for tagging soot particles from specific sources, e.g. from aviation emissions (Righi et al, 2021), to analyse the global distribution of aviation soot INPs.…”