The Impact of Process-Based Warm Rain Constraints on the Aerosol Indirect Effect

Abstract

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.