Climate Processes in CMIP5: An investigation of the connections between convection, clouds and climate sensitivity in a Global Climate Model

This study explores the connections between process-level modeling of convection and GCM (global climate model) simulated clouds and cloud feedback to global warming through a set of perturbed-physics and perturbed-sea-surface-temperature experiments. A bulk diagnostic approach is constructed and a set of key variables is derived and demonstrated to be useful in understanding the simulated relationship. In particular, a novel bulk quantity, the convective precipitation efficiency or equivalently the convective detrainment efficiency is proposed as a simple measure of the aggregated properties of parameterized convection important to GCM simulated clouds. As the convective precipitation efficiency increases in the perturbed-physics experiments, both liquid and ice water path decrease, with low and middle cloud fraction diminishing at a faster rate than the high cloud fraction. This asymmetric impact results in a large sensitivity of total cloud radiative forcing to changes in convective precipitation efficiency.

For the global warming experiments, all models explored here produce an increase in precipitation efficiency to warming. The intermodel variation in response of cloud condensate, low and middle cloud fraction, and total cloud radiative forcing are well explained by model variations in response of precipitation efficiency or cloud detrainment efficiency. A substantial fraction of the increase in precipitation efficiency to warming is due to its convective component which depends on the parameterization of cumulus mixing and convective microphysics processes. The differing response in convective precipitation efficiency to warming explains the increased cloud feedback and climate sensitivity in recently developed Geophysical Fluid Dynamics Laboratory GCMs.