The integrated aerosol–climate model CIESM-PAMT: Part 2. Evaluation of clouds, precipitation, radiation, and aerosol–cloud interactions

Abstract. Aerosol–cloud interactions (ACI) remain one of the largest sources of uncertainty in current climate simulations, with their climatic effects strongly dependent on the coupling between aerosols, clouds, and radiation. To improve the physical representation of aerosol processes in the Community Integrated Earth System Model (CIESM), this study couples a new aerosol module, the Piecewise lognormal approximation Aerosol Model for Tsinghua University (PAMT), to construct the integrated aerosol–climate model CIESM-PAMT. In Part 1, we introduced the coupling framework and systematically evaluated its performance in simulating aerosols. Results showed that the new aerosol module PAMT substantially enhanced CIESM’s ability to simulate global aerosol optical properties and mass/number concentrations, providing a more reliable foundation for subsequent assessments of aerosol impacts on clouds and radiation.

Building on this, Part 2 uses multi-source observations to further evaluate CIESM-PAMT’s performance in simulating clouds, precipitation, radiation, and ACI, with systematic comparison to the original CIESM coupled with the Aerosol Module with seven lognormal modes (CIESM-MAM7). The results indicate that CIESM reproduces clouds, precipitation, and radiation with high fidelity, with simulated values largely falling within observational uncertainty ranges. Specifically, CIESM-PAMT shows better performance in simulating cloud condensation nuclei and cloud droplet number concentrations, while CIESM-MAM7 shows slightly better performance in cloud fraction. Both modules perform similarly for cloud droplet effective radius, liquid/ice water paths, and radiation-related variables. In terms of ACI representation, CIESM-PAMT provides a more realistic simulation of the Twomey effect, demonstrating an effective improvement in ACI modeling. These findings further suggest that aerosol impacts on atmospheric composition are primarily mediated through changes in cloud droplet number, with comparatively limited effects on other cloud properties. The study also identifies future avenues for model improvement, including enhanced representation of cloud fraction and hydrometeors, diagnostics and reduction of biases in cloud radiative effects and forcing, clarification of differences arising from alternative microphysical schemes, and expanded investigation of aerosol–ice cloud and aerosol–convective cloud interactions.

Overall, CIESM-PAMT exhibits robust aerosol–cloud–radiation coupling capabilities, providing a reliable platform for probing the physical mechanisms by which aerosols influence the climate system, and laying an important foundation for further model development and participation in international comparison projects.