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Abstract:
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
−6to3Wm
−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.
