Large Eddy Simulations of aerosol-cirrus interactions

The climate impacts of global aviation include CO₂ effects and their so-called non-CO₂ effects. Among
those non-CO₂ effects, the effects of aerosol-cirrus interactions are the least understood and the latest
assessment of aviation radiative forcing could not give a best estimate and an uncertainty range [1]. Previous
studies [2, 3] have shown that a perturbation to the ice crystal number leads to a change in cirrus lifetime
and ice water path. In this talk, we investigate whether aerosol perturbations to ice nucleation can produce
large perturbations to cirrus ice crystal number.
We study these interactions using the 3D Met-Office NERC Cloud model (MONC). MONC is a Large Eddy
Simulation model coupled to the cloud micro-physics scheme, CASIM. We simulate two types of cirrus namely
the Gravity Wave cirrus (GW) and the Warm Conveyor Belt cirrus (WCB). The GW cirrus is thicker with
a higher ice crystal number concentration (ICNC) compared to the WCB cirrus, which is consistent with
aircraft observations [4]. We find that perturbing the formation stage of the cirrus cloud by injecting soluble
aerosols leads to an increase in ice crystal number and a decrease in the initial crystal size. Furthermore,
the ice clouds created in the presence of the injected soluble aerosols tend to have a higher ice water content
and an increased lifetime. Both GW and WCB cirrus clouds behave similarly to perturbation by soluble
aerosols at the formation stage.
In contrast, perturbing a pre-existing cirrus with soluble and insoluble aerosols does not change the properties
of the ice cloud. Ice crystal growth by vapour deposition uses available water vapour during the lifetime of
the cirrus [5, 6], which we find comes at the expense of activating aerosols that would lead to an increase
in the ICNC. Hence, although cirrus clouds would be sensitive to a perturbation in their ice crystal number
concentration, it is difficult to obtain such perturbations by injecting aerosols at cloud level, for example from
commercial aircraft exhaust. Furthermore, even though these results do not completely preclude large cirrus
perturbations from aviation aerosols in specific cases, they suggest that the corresponding global radiative
forcing is small.

References
[1] Lee et al. Atmospheric Environment. (2021). 
[2] Verma & Burkhardt. Atmospheric Chemistry and Physics. (2022).
[3] Gilbert et al. In preparation. (2022).
[4] Li et al. Atmospheric Chemistry and Physics Discussions. (2022).
[5] Hill et al. Quarterly Journal of the Royal Meteorological Society. (2014).
[6] Lohmann & Feichter. Atmospheric Chemistry and Physics. (2005).