Decisive Role of Warm Rain Formation Process in Modulating Aerosol Indirect Effect in a Global Climate Model

The representation of precipitation process is one key source of biases that contributes largely to the uncertainty in predicted global energy budget. In particular, many GCMs have been found to share a common bias in warm rain formation process: rain is triggered much too frequently relative to observations. In this study, five distinguishing autoconversion schemes are incorporated into a single GCM, and the performances of the warm-rain formation processes under these schemes are evaluated in comparison with the A-Train observations. Moreover, the effects of the differing autoconversion schemes on preindustrial (PI) to present-day (PD) aerosol indirect radiative forcing (AIE) are investigated. It is found that the warm-rain formation process differs a lot among the various schemes, with some generating warm rain at a later timing that is close to observations (indicated by the similar cloud properties at the initial stage of warm rain) but some others generating warm rain much too readily compared with observations. In spite of the differences in warm rain formation behavior, similar PD climates are achieved for the various autoconversion schemes; however, large differences in the magnitudes of PI to PD AIE are seen for these schemes, to an extent large enough to cancel much of the historical greenhouse warming. Interestingly, among the five autoconversion schemes, the ones with the more realistic warm-rain formation processes exhibit the larger and hence unrealistic AIEs, and vice versa. The dependence of autoconversion rate on cloud droplet number concentration is likely to be the key factor that determines the timing of warm rain: the more negative the dependence is, the later warm rain is triggered, and consequently the larger the AIE is due to inhibited depletion of cloud water and aerosols. The decisive role of warm rain formation process on AIE in this particular model suggests the importance of process-level constraint of GCMs for climate simulations.