Abstract
AbstractGlobal climate models (GCMs) have been found to share the common too-frequent bias in the warm rain formation process. In this study, five different autoconversion schemes are incorporated into a single GCM, to systematically evaluate the warm rain formation processes in comparison with satellite observations and investigate their effects on the aerosol indirect effect (AIE). It is found that some schemes generate warm rain less efficiently under polluted conditions in the manner closer to satellite observations, while the others generate warm rain too frequently. Large differences in AIE are found among these schemes. It is remarkable that the schemes with more observation-like warm rain formation processes exhibit larger AIEs that far exceed the uncertainty range reported in IPCC AR5, to an extent that can cancel much of the warming trend in the past century, whereas schemes with too-frequent rain formations yield AIEs that are well bounded by the reported range. The power-law dependence of the autoconversion rate on the cloud droplet number concentration β is found to affect substantially the susceptibility of rain formation to aerosols: the more negative β is, the more difficult it is for rain to be triggered in polluted clouds, leading to larger AIE through substantial contributions from the wet scavenging feedback. The appropriate use of a droplet size threshold can mitigate the effect of a less negative β. The role of the warm rain formation process on AIE in this particular model has broad implications for others that share the too-frequent rain-formation bias.
Highlights
The aerosol indirect effect (AIE) is of great importance in regulating the global energy budget
This study extends the previous work to include a wide range of autoconversion representations in the literature, which will illustrate whether the findings by JS18 can generally apply to other autoconversion schemes, and allows for more systematic investigation of the key factors in the autoconversion formulations that determine the characteristics of warm rain intensity and frequency, and the consequent AIE
A common bias has been identified in many state-of-the-art Global climate models (GCMs): precipitation, especially in the form of warm rains, is triggered more frequently than revealed from observations
Summary
The aerosol indirect effect (AIE) is of great importance in regulating the global energy budget. One possible explanation for the negative Sacc is that the initial inhibition of autoconversion-based rain formation could result in an increased cloud water carried over to the following time steps, which may outweigh the autoconversion-induced decrease of the rainwater, leading to an enhanced Racc This explanation is supported by an additional investigation which shows that the LWP taken to the step becomes larger with increasing cloud number concentrations (see Fig. S1 in the online supplemental material). These differences in Swscv protrude in the differences in the AODs of the PD simulations (Fig. 1d and Table 2), in which the schemes with larger (more positive) Swscv have larger AODs, highlighting the mutual coupling between aerosol and cloud through rain formation and wet scavenging
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