Recent studies argue that general circulation models (GCMs) are overestimating the second indirect effect because they overestimate the increase in cloud liquid water path (LWP) due to an increase in CCNs. One of the culprits is thought to be the autoconversion parameterization (i.e the process in which cloud droplets grow to a size sufficient to form precipitation). The autoconversion rate is typically parameterized as a function of the cloud droplet number concentration (Nc) and the cloud water content (qc). An increase of Nc leads to a reduced autoconversion rate, delaying precipitation and increasing LWP. Especially, changing the sensitivity to Nc has a strong impact on the indirect effect. Therefore, a careful evaluation of the autoconversion schemes is important for understanding the role of cloud-aerosol indirect effect processes in the overall climate sensitivity of climate models.
In this study, we examine the sensitivity of the second indirect effect to the autoconversion scheme in a development version the Community Earth System Model. We assess the second indirect effect in atmosphere-only simulations with observed SSTs and in fully coupled historical simulations. We evaluate the effect of changing sensitivity to Nc in the autoconversion scheme. Our results demonstrate the primary importance of autoconversion processes in the representation of the second indirect effect magnitude, and the potential impact on the skill of transient CESM simulations.