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.
Highlights
The aviation sector contributes about 2.4 % of the global anthropogenic CO2 and is one of the fastest growing anthropogenic sectors, which makes it one of the key targets for mitigating the anthropogenic impact on climate (Lee et al, 2010; Grewe et al, 2017; Lee et al, 2021)
The aviation soot emissions are largest in the northern mid-latitudes (Fig. 4a), with maxima in the Northern Hemisphere at typical flight altitudes (200–250 hPa) and close to the surface, due to the climb and descent phases and to the paths of short-range flights, which are mostly common over the continents
The reason for such sharply structured patterns is that particles in the Aitken mode, which dominate total particle number, are characterized by a shorter lifetime due to particle–particle interactions, which effectively reduce their number concentration away from sources, while their mass is conserved
Summary
The aviation sector contributes about 2.4 % of the global anthropogenic CO2 and is one of the fastest growing anthropogenic sectors, which makes it one of the key targets for mitigating the anthropogenic impact on climate (Lee et al, 2010; Grewe et al, 2017; Lee et al, 2021). In addition to the well-understood impact of CO2 and the related mitigation measures (Fuglestvedt et al, 2008; Dahlmann et al, 2016), aircraft emit a number of nonCO2 components, whose climate impact is still uncertain (Grewe et al, 2017) This concerns, for instance, the role of nitrogen oxides (NOx=NO + NO2), which control ozone formation and affect methane lifetime (Grewe et al, 2019), aerosol particles, and their interactions with clouds Gettelman and Chen, 2013; Righi et al, 2013; Penner et al, 2018), as well as the formation and growth of contrails and contrail cirrus (Burkhardt and Kärcher, 2011; Chen and Gettelman, 2013; Bock and Burkhardt, 2016) Among these various aviation effects, the impact of aviation soot on natural cirrus clouds has gained attention in recent years due to its potentially large climate impact, possibly exceeding the contribution of most of the aforementioned This concerns, for instance, the role of nitrogen oxides (NOx=NO + NO2), which control ozone formation and affect methane lifetime (Grewe et al, 2019), aerosol particles, and their interactions with clouds (e.g. Gettelman and Chen, 2013; Righi et al, 2013; Penner et al, 2018), as well as the formation and growth of contrails and contrail cirrus (Burkhardt and Kärcher, 2011; Chen and Gettelman, 2013; Bock and Burkhardt, 2016).
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