Representing the effects of giant aerosol in droplet nucleation in E3SMv2

Giant aerosol, i.e., those with diameters larger than 1 μm, can form large droplets via condensational growth to sizes similar to drizzle particles without being activated. In this study, we assess the impacts of giant aerosol on clouds, precipitation, and radiation when activated giant aerosol are directly categorized as raindrops using the U.S. Department of Energy’s Energy Exascale Earth System Model version 2 (E3SMv2). We find that categorizing activated giant aerosol as raindrops reduces cloud liquid water path by 11.38 % globally, with most pronounced reduction in the mid-latitudes. We also find that this approach improves model's ability to simulate the positive correlation between surface rain rate and coarse mode aerosol concentration in regions of low precipitation. The effective radiative forcing associated with aerosol-cloud interactions (ERFaci) reduces from -1.37 to between -0.94 and -1.23 W m-2, depending on the size of giant aerosol. Our results highlight the importance of a better representation of giant aerosol in Earth system models to provide better predictions of cloud, precipitation, and the climate.