Probing Mechanisms Driving Model Resolution Dependence of Aerosol-Cloud Effects
Aerosols affect clouds in several ways, including influencing the number and size of cloud droplets, and therefore cloud radiative properties and lifetime (aerosol “indirect effects”). Although these effects occur at subgrid scale, increasing model resolution to 0.25° grid spacing, improvements in the cloud simulation may enable improved simulation of aerosol-cloud effects. A team of scientists led by U.S. Department of Energy researchers at Pacific Northwest National Laboratory quantified the resolution sensitivity of cloud and precipitation susceptibilities to aerosols, as well as aerosol indirect forcing in the Community Atmosphere Model Version 5 (CAM5). The team ran the model in a realistic climate at four different horizontal grid spacings with the model meteorology strongly nudged toward the very high-resolution Year Of Tropical Convection analysis. They found that aerosol effects on clouds vary with model resolution. A better characterization of aerosol-cloud interactions can be achieved by increasing model resolution as the CAM5 aerosol and cloud parameterizations are able to produce more realistic simulations with the higher-resolution model, despite the fact that most aerosol and cloud processes are still at subgrid scale even for the highest resolution explored in this study. The higher-resolution simulation has a stronger cloud brightness effect, but smaller cloud lifetime effect, with an overall 15% decrease in aerosol-cloud (cooling) effect.
Ma, P.-L., P. J. Rasch, M. Wang, H. Wang, S. J. Ghan, R. C. Easter, W. I. Gustafson Jr., X. Liu, Y. Zhang, and H.-Y. Ma. 2015. “How Does Increasing Horizontal Resolution in a Global Climate Model Improve the Simulation of Aerosol-Cloud Interactions?” Geophysical Research Letters 42, 5058–65. DOI: 10.1002/2015GL064183.