Understanding the Extreme Spread in Climate Sensitivity within the Radiative-Convective Equilibrium Model Intercomparison Project

The Radiative-Convective Equilibrium Model Intercomparison Project (RCEMIP) consists of simulations at three fixed sea-surface temperatures (SSTs: 295, 300, and 305 K) and thus allows for a calculation of the climate feedback parameter based on the change of the top-of-atmosphere radiation imbalance. Climate feedback parameters range widely across RCEMIP, roughly from -6 to 3 W m(-2) K-1, particularly across general-circulation models (GCMs) as well as global and large-domain cloud-resolving models (CRMs). Small-domain CRMs and large-eddy simulations have a smaller range of climate feedback parameters due to the absence of convective self-aggregation. More than 70-80% of the intermodel spread in the climate feedback parameter can be explained by the combined temperature dependencies of convective aggregation and shallow cloud fraction. Low climate sensitivities are associated with an increase of shallow cloud fraction (increasing the planetary albedo) and/or an increase in convective aggregation with warming. An increase in aggregation is associated with an increase in outgoing longwave radiation, caused primarily by mid-tropospheric drying, and secondarily by an expansion of subsidence regions. Climate sensitivity is neither dependent on the average amount of aggregation nor on changes in deep/anvil cloud fraction. GCMs have a lower overall climate sensitivity than CRMs because in most GCMs convective aggregation increases with warming, whereas in CRMs, convective aggregation shows no consistent temperature trend.