Mechanisms for high cloud reductions with climate warming in HadGEM3-GC3.1-LL

High cloud feedbacks are a large contributor to uncertainty in estimates of equilibrium climate sensitivity. Across the CMIP6 ensemble, estimates in global longwave cloud radiative effect (LWcre) feedback (a feedback strongly tied to changes in high cloud) range from approximately -0.45 to +0.5 W m-2 K-1. HadGEM3-GC31-LL sits close to the bottom of this range and therefore we explore mechanisms for high cloud reduction with warming in HadGEM3-GC31-LL. We find that high cloud reduction in HadGEM3-GC31-LL is closely tied to the parameterized convection scheme as well as a contribution linked to a response consistent with the stability iris mechanism. To estimate the relative importance of the parameterized convection and other processes, conv-off experiments are used to capture the high cloud response in the absence of convection parameterization. In these conv-off experiments a much reduced cloud reduction is seen.