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 mW m−2 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 two 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.

Numerical simulation of the impact of COVID-19 lockdown on tropospheric composition and aerosol radiative forcing in Europe

“…8) the ICNC are reduced (by 30 % at 250 hPa, although with large variability), while no visible effect is found for CDNC. These results are in line with those obtained by Righi et al (2021), who showed that aircraft emissions do increase ice crystal number concentration, although their results were not statistically significant. The ice crystal effective radius seems to be the least affected by the reduced emissions during the COVID-19 lockdown, with a negligible absolute and relative difference.…”

Impact of reduced emissions on direct and indirect aerosol radiative forcing during COVID–19 lockdown in Europe

“…The same effect arises in aircraft flight tracks, where the emitted particles may act as ice nucleating particles, leading to contrail formation (Schumann et al, 2017) and an increase in cirrus cloud formation in the upper troposphere (Boucher, 1999). Still, the magnitude of the aviation soot effect on the radiation budget remains uncertain (Urbanek et al, 2018;Righi et al, 2021). Recently, satellite data have been used to quantify changes in clouds in regions with COVID-reduced airtraffic in 2020 (Quaas et al, 2021;Gettelman et al, 2021).…”

Technical note: Identification of two ice-nucleating regimes for dust-related cirrus clouds based on the relationship between number concentrations of ice-nucleating particles and ice crystals

“…Liquid-origin cirrus completely forms through heterogeneous freezing (immersion and contact freezing) and is then uplifted to the colder altitudes where homogeneous freezing can occur under high updraft conditions in addition to the heterogeneously formed ice crystals. Recently, numerous studies have reported that many types of aerosols existing at the upper troposphere (cirrus altitudes) can serve as effective ice-nucleating particles (Cziczo et al, 2013), such as aviation soot (Tesche et al, 2016;Righi et al, 2021), volcanic aerosols (Sporre et al, 2022), wildfire smoke (Ansmann et al, 2021;Hu et al, 2022), dust aerosols (Kuebbeler et al, 2014), and sulfate and nitrate particles (Che et al, 2021), making the in situ heterogeneous formation at cirrus altitudes possible.…”