PNNL

The Science

Aerosol particles have a large impact on climate, but they are difficult to represent in climate models. Particles are tremendously diverse, and their properties change as they move through the atmosphere. However, these particle-level details must be simplified in climate models, and errors from these simplifications are not well understood. To address this need, researchers at Pacific Northwest National Laboratory developed a framework for evaluating the simplified aerosol scheme, the 4-mode version of the Modal Aerosol Module (MAM4) that is used in the U.S. Department of Energy’s Energy Exascale Earth System Model (E3SM), against a detailed particle-resolved aerosol model. They found that the approximations of aerosol sizes in MAM4 introduce large errors in predictions of aerosol particles’ ability to form cloud droplets, errors that were greatest in areas with more air pollution, where the size distribution evolves rapidly though aging.

The Impact

Climate models cannot simulate individual aerosol particles and errors from approximations of particle properties are not known. In this study, researchers used a detailed aerosol model that resolves complex aerosol physical and chemical processes to evaluate how approximations of aerosol particle properties in E3SM influence predictions of aerosol properties that impact climate. They found that errors from approximations in particle sizes contribute to inaccurate cloud droplet nucleation, resulting in errors in the estimated aerosol effects on climate. 

Summary

Aerosol effects on clouds and radiation are a large source of uncertainty in our understanding of human impacts on the climate system. Uncertainty from the numerical representation of particle properties in climate models has not been well quantified. In this study, researchers introduced a framework for quantifying errors from the numerical approximation of aerosol sizes in MAM4, the aerosol model in E3SM. This analysis revealed large errors in simulation of aerosol particle sizes that lead to large errors in predictions of particles’ ability to form cloud droplets. These structural deficiencies lead to uncertainties in its Earth system predictions. These findings provide insights into future model development directions for a better aerosol representation in Earth system models.

PNNL Contact

Po-Lun Ma, Pacific Northwest National Laboratory, Po-Lun.Ma@pnnl.gov

Funding

This study was supported by the U.S. Department of Energy Office of Science’s Biological and Environmental Research program as part of the Earth System Model Development program area as part of the Enabling Aerosol-cloud interactions at GLobal convection-permitting scalES (EAGLES) project.