A Parameterization of Interstitial Aerosol Extinction and Its Application to Marine Cloud Brightening

Abstract

Abstract Marine cloud brightening (MCB) is a geoengineering approach to counteract climate change by the deliberate seeding of sea salt aerosol particles that, once they activated to cloud droplets, directly increase cloud reflectance and hence global albedo. However, a large fraction of the seeded aerosol may remain interstitial, i.e., unactivated particles among cloud droplets. Because the consideration of interstitial aerosol optical properties usually requires computationally expensive simulations of the entire particle spectrum and direct Mie calculations, we develop a simple parameterization to be used with computationally efficient bulk and even bin cloud microphysical schemes that do not treat the unactivated aerosol explicitly. Using parcel and large-eddy simulations with highly detailed Lagrangian cloud microphysics and direct Mie calculations as a reference, we show that the parameterization captures the variability in the interstitial aerosol extinction successfully. By applying the parameterization to typical MCB cases, we find that the consideration of interstitial aerosol extinction is important for the assessment of MCB in shallow clouds with weak updrafts, in which only a small fraction of aerosol particles is activated to cloud droplets. Significance Statement The optical properties of clouds are not only determined by the number and size of cloud droplets. Unactivated aerosol particles, so-called interstitial aerosol, can contribute substantially to the optical thickness of shallow clouds with weak updrafts in aerosol-laden conditions. The consideration of interstitial aerosol optical thickness has been computationally challenging, but the new parameterization presented here allows for an efficient representation in various types of cloud models. The parameterization is shown to be an important addition for the assessment of marine cloud brightening (MCB), a potential geoengineering technique to counteract global warming by increasing the cloud albedo through the deliberate seeding of aerosol.