Uncertainties persist regarding cloud responses to global warming in climate models. Some research highlighted the impact of uncertainty in boundary layer schemes on marine low cloud feedback. This study reveals that both deep convection and cloud microphysics also significantly influence this feedback. We examine the weakened positive cloud feedback in the Department of Energy's Energy Exascale Earth System Model (E3SM) from version 1 to version 2 using present-day atmosphere-only experiments with uniform 4 K sea surface temperature warming. By isolating individual model modifications, we find that the reduced cloud feedback primarily arises from changes in the tropical marine low cloud regime. The reduction is attributed to a new trigger function for the deep convection scheme and modifications in the cloud microphysics scheme.

The new trigger function helps weaken the low cloud reduction by increasing the cloud water detrainment at low levels from deep convection under warming. Changes to the formula of autoconversion rate from liquid to rain and an introduced minimum cloud droplet number concentration threshold in cloud microphysical calculations help sustain clouds against dissipation by suppressing precipitation generation with warming. In the midlatitudes, the increased Wegener-Bergeron-Findeisen (WBF) efficiency strongly reduces present-day liquid water and leads to a stronger negative cloud optical depth feedback. The reduced trade cumulus cloud feedback in v2 is closer to estimates from recent observational and large-eddy modeling studies but might not be due to the right physical reasons. The reduced mid-latitude cloud feedback may be more plausible because more realistic present-day mixed-phase clouds are produced through the change in the WBF efficiency.