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

Abstract

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.Plain Language SummaryEffects of clouds on Earth’s radiation budget vary with their spatial and temporal distribution and their physical properties, including water content and its partitioning between liquid and ice, and cloud particle size. Changes in cloud distribution and physical properties can amplify or damp anthropogenic global warming and is the largest source of uncertainty in predictions of future climate. The simulation of cloud physical properties in climate models is limited due to a lack of understanding from theory and observations about what controls these properties. Recent progress has been made in some models to predict cloud particle sizes based on physical processes. In this study, we find an increase of cloud particle size in response to anthropogenic warming and estimate the resulting cloud radiative effects. The larger particles increase scattering of solar radiation in the downward direction leading to an amplification of surface warming. We suggest cloud particle‐size changes play a role in the large spread of warming in model predictions of future climate.Key PointsCloud particle size increases with warming in an Earth system modelThe associated cloud particle‐size feedback is estimated to be 0.18, 0.33, and −0.15 Wm−2 K−1 for net, shortwave, and longwave componentsCloud particle‐size feedback is an underappreciated contributor to the spread of climate sensitivity in current models