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Assessing the effects of giant aerosols in E3SM

• Yu Yao

• yu.yao@pnnl.gov

Yu Yao, Po-Lun Ma, Yi Qin, Matthew W. Christensen, Hui Wan, Kai Zhang, Balwinder Singh, Meng Huang

‘Atmosphere'

EAGLES

The activation of giant aerosol particles alters the warm rain formation process and decreases the lifetime of liquid clouds as giant particles can form large cloud droplets and quickly initiate the warm rain process. Previous work applied cloud parcel models or large eddy simulations to study the impacts of these particles, but there is still limited research on assessing the effects of giant aerosols at global scale. In this study, we assessed the effects of giant aerosols using the U.S. Department of Energy’s Energy Exascale Earth System Model version 2(E3SMv2). We developed a GCCN parameterization by diagnosing the abundance and physical properties of giant particles in the 4-mode version of the Modal Aerosol Module (MAM4) and allowed these giant aerosol particles to be activated directly to form drizzle droplets with different sizes. Model simulations showed that GCCN accounts for less than 0.02% CCN. By activating this amount of GCCN to rain droplets with size of 25 um, liquid water path decreased -54.65% 26% globally. The effects are especially noticeable in storm track zones in the southern and northern oceans. We also found the GCCN effects on liquid water path are zonally different. The decrease is more pronounced over the mid-latitudes, while the changes over the tropics are comparatively smaller. This zonal variation change make the simulated liquid water path more consistent with the observations. Additional model evaluation shows GCCN behaves differently at regions with different precipitation rates. It improves model's ability to represent positive correlation between surface rain rates and coarse mode concentration better at regions with lower precipitation rates. The analysis of aerosol radiative forcing decomposition reveals that GCCN altered total effective radiative forcing, primarily attributed to the decrease radiative forcing related with aerosol-cloud interactions, with global mean decreasing from -1.56 to -1.44 W/m2, which brings the simulated value into closer alignment with the estimate presented in IPCC AR6. Our results implied GCCN can have substantial effects on hydrological changes and Earth's energy balance.

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https://docs.google.com/document/d/1otv62bfnkHo3tr7uXz3inlOY11NmF1m9nLHWWFs-qpw/edit?usp=sharing