Quantifying the Cloud Particle-Size Feedback in an Earth System Model

Physical process-based two-moment cloud microphysical parameterizations, in which effective cloud particle size evolves prognostically with climate change, have recently been incorporated into global climate models. The impacts of cloud particle-size change on the cloud feedback, however, have never been explicitly quantified. Here we develop a partial radiative perturbation-based method to estimate the cloud feedback associated with particle-size changes in the Community Earth System Model. We find an increase of cloud particle size in the upper troposphere in response to an instantaneous doubling of atmospheric CO₂. The associated net, shortwave, and longwave cloud feedbacks are estimated to be 0.18, 0.33, and −0.15 Wm⁻² K⁻¹, respectively. The cloud particle-size feedback is dominated by its shortwave component with a maximum greater than 1.0 Wm⁻² K⁻¹ in the tropics and the Southern Ocean. We suggest that the cloud particle-size feedback is an underappreciated contributor to the spread of cloud feedback and climate sensitivity among current models.