New Community Atmospheric Model Passes ARM Test for Aerosol Effects on Cloud Droplet Size


Using measurements to evaluate the impacts of aerosols on cloud properties can help narrow climate model uncertainties by identifying where model problems occur and where model representations are robust for aerosol-cloud interactions. DOE scientists at Lawrence Livermore and Pacific Northwest National Laboratories have quantified the aerosol impacts on cloud droplet effective radius (aerosol first indirect effect, FIE) for non-precipitating, low-level, single-layer liquid phase clouds simulated in the Community Atmospheric Model version 5 (CAM5) at three Atmospheric Radiation Measurement (ARM) sites. The aerosol FIE is quantified in terms of a relative change in cloud droplet effective radius for a relative change in aerosol amount under conditions of fixed liquid water amount. The study shows that CAM5 simulates aerosol-cloud interactions reasonably well for this specific cloud type and the simulated FIE is consistent with the long-term ARM observations at the examined locations. The high sensitivity of aerosol FIE to cloud liquid water amount and aerosol variable and low sensitivity to location and time are also consistent with observational studies. If this study has general applicability for other cloud types and locations, it suggests that the possible overestimation of aerosol climate impacts found by other studies may be a problem from other aerosol indirect effects, such as cloud lifetime effects, rather than the FIE.


Zhao, C., S. A. Klein, S. Xie, X. Liu, J. S. Boyle, and Y. Zhang. 2012. “Aerosol First Indirect Effects on Non-Precipitating Low-Level Liquid Cloud Properties as Simulated by CAM5 at ARM Sites,” Geophysical Research Letters 39, L08806. DOI:10.1029/2012GL051213.