Intensified Diurnal Cycle of Convection Explains Half of Tropical High-Cloud Feedback

Tropical high clouds are the cloud type that contributes most to the uncertainty in climate sensitivity, as their feedback on global warming remains poorly constrained. Part of the reason for this is that global circulation models do not resolve the full spectrum of high clouds, ranging from thin cirrus to thick deep convective clouds. We use a set of storm-resolving aquaplanet simulations that resolve this spectrum to investigate how high clouds with different thicknesses contribute to the total high-cloud feedback. We find that the total feedback is positive, arising in roughly equal parts from all high clouds remaining at fixed temperatures and from the intensification of the diurnal cycle of convection. Surface warming strengthens the nighttime peak of convection while reducing daytime convection, causing fewer deep convective clouds during the day. This reduces sunlight reflection and results in a previously unrecognised positive feedback. The reduction of high clouds in response to warming, assumed to produce a negative feedback, affects thin (warming) and thick (cooling) clouds equally in our simulations, resulting in a near-neutral feedback. By resolving cloud thickness, our storm-resolving simulations show that heterogeneous high-cloud responses to warming combine to yield a positive total feedback.