Impact of changes in the formulation of cloud-related processes on model biases and climate feedbacks

To test the impact of modeling uncertainties and biases on the simulation of cloud feedbacks, several configurations of the EC-Earth climate model are built altering physical parameterizations. An overview of the various radiative feedbacks diagnosed from the reference EC-Earth configuration is documented for the first time. The cloud feedback is positive and small. While the total feedback parameter is almost insensitive to model configuration, the cloud feedback, in particular its shortwave (SW) component, can vary considerably depending on the model settings. The lateral mass exchange rate of penetrative convection and the conversion rate from condensed water to precipitation are leading uncertain parameters affecting the radiative feedbacks diagnosed. Consistent with other studies, we find a strong correlation between low-cloud model fidelity and low-cloud response under global warming. It is shown that this relationship holds only for stratocumulus regimes and is contributed by low-cloud cover, rather than low-cloud optical thickness. Model configurations simulating higher stratocumulus cover, which is closer to the observations, exhibit a stronger positive SW cloud feedback. This feedback is likely underestimated in the reference EC-Earth configuration, over the eastern basins of the tropical oceans. In addition, connections between simulated high-cloud top altitude in present-day climate and longwave cloud feedback are discussed