How Cloud and Particle Characteristics Affect Atmospheric Brightness (Radiation) in the Community Atmosphere Model
The Earth’s atmospheric system includes a complex and changing mix of clouds and gaseous and particulate emissions, which all interact with solar energy, or radiative fluxes, in complex ways that are difficult to predict. This complicates attempts to estimate how the Earth will warm or cool as greenhouse gases and particulates from fossil fuel combustion change. To better understand uncertainties in the current Community Atmosphere Model version 5 (CAM5), a research team led by U.S. Department of Energy scientists at Pacific Northwest National Laboratory developed and applied a sensitivity analysis framework to study the variance of the simulated radiative flux (FNET) at the top of atmosphere in the present-day climate. They found that the global mean FNET variance is dominated by the cloud forcing variance, given the assigned uncertain parameter ranges. They also found that most selected cloud microphysics- and emission-related parameters have statistically significant impacts on the global mean FNET. Three cloud microphysics parameters, associated with the fall speed of cloud ice and snow and assumed bounds on cloud droplet number, have a smaller impact than the size threshold required for ice to change to snow. Overall, these cloud microphysics-related parameters have a larger impact on high clouds than on low clouds. The team’s comprehensive approach not only estimates the contribution of each parameter to model sensitivity but also provides its statistical significance. This is an important quantification rarely obtained due to the limited sampled space of parameter uncertainty.
Zhao, C., X. Liu, Y. Qian, J. Yoon, Z. Hou, G. Lin, S. McFarlane, H. Wang, B. Yang, P.-L. Ma, H.Yan, and J. Bao. 2013. “A Sensitivity Study of Radiative Fluxes at the Top of Atmosphere to Cloud-Microphysics and Aerosol Parameters in the Community Atmosphere Model CAM5,” Atmospheric Chemistry and Physics 13, 10969-1098. DOI: 10.5194/acp-13-10969-2013.