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.