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Many global climate models have been found to generate warm rain too frequently. This study, with a particular global climate model, found that the problem can be mitigated via altering the autoconversion process so as to inhibit rain formation under conditions of a large cloud number concentration and small droplet sizes. However, this improvement was found to cause an overly large aerosol indirect effect. This dichotomy between the constraint on warm-rain formation process and the energy-budget requirement on aerosol indirect effect was found to result from a pronounced cloud-water response to aerosol perturbations, which is amplified through the wet scavenging feedback to an extent depending on precipitation-formation parameterization. Thus, critical compensating errors exist between warm-rain formation and other key processes, and better constraint on these processes, particularly wet scavenging, is required to mitigate the dichotomy. The results have broad implication for models that suffer from the aforementioned too-frequent warm-rain formation bias.

Plain Language Summary Climate scientists rely on numerical climate models to project temperature changes in the coming decades. The credibility of the projections is argued to arise from the fact that models are able to reproduce well the temperature trend in the past century. However, success in reproducing the past does not necessarily mean credible projection for the future, because the former may be a result of compensating biases in the models, which potentially misrepresent the relationship between changes to the energy balance and aerosols/clouds. In this study, compensating biases were found to exist between rain formation and other processes in a model capable of simulating aerosol effects. Specifically, this model tends to generate rain too frequently against satellite observations, whereas satellite-based improvements with respect to the rain process yield an unrealistically strong aerosol indirect effect that can substantially cool the surface. This means that dual efforts are required to derive trustworthy projections of future climate changes: on the one hand, the rain-formation process should be represented realistically in accordance with observations, while on the other hand, errors in the interaction between rain and aerosol/cloud should be mitigated.