02 May 2012

Evaluating Aerosol-Cloud Interaction: Scientists Show Climate Model Reasonably Simulates The Relationship Between Aerosol And Cloud Droplet Size


The indirect effect of aerosols on climate change is one of the biggest uncertainties in current climate models. Evaluating distinct components such as the impact of aerosols on cloud droplet size (first indirect effect) can help identify where problems occur and where model representations are more robust, which could lead to a narrowing of this uncertainty. Scientists at Lawrence Livermore National Laboratory (LLNL) and Pacific Northwest National Laboratory (PNNL) have achieved this identification of strengths (first indirect effect) and potential weaknesses (other indirect effects) in the Community Atmospheric Model Version 5 (CAM5) by taking advantage of process-level data and comparing these directly to CAM5 model output. Their study is published in Geophysical Research Letters.


Researchers examine the aerosol first indirect effects (FIE) for non-precipitating low-level single-layer liquid-phase clouds simulated by CAM5 by running them at three DOE Atmospheric Radiation Measurement (ARM) sites. In this study, CAM5 runs in a forecast mode with the Cloud-Associated Parameterizations Testbed (CAPT). The scientists quantify FIE in terms of a relative change in cloud droplet effective radius for a relative change in aerosol amount of fixed liquid water content (LWC). They use profiles of aerosol and cloud properties simulated from the modal aerosol module (MAM) and two-moment cloud microphysics scheme in CAM5. This study shows CAM5 simulates statistically significant aerosol first indirect effects with values between 0.05 and 0.25 for examined clouds and locations (see Figure 1). It also shows that simulated FIE is generally larger for using cloud condensation nuclei (CCN) than for using aerosol accumulation mode number concentration (Na), and generally decreases with LWC at ARM coastal sites (Figure 2). Moreover, FIE shows weak sensitivity to examined location and time (Figure 2). All of these findings are consistent with numerous observational studies, particularly those long-term observations at examined locations.


All the results shown in this study suggest reasonable CAM5 simulations in aerosol first indirect effects for examined cloud type at selected locations. If this study has general applicability for the FIE in other cloud types and locations, it suggests the overestimation of aerosol indirect effects in CAM5 found by other studies may be a problem from other indirect effects (second or semi) rather than FIE. This work was performed by Drs. Chuanfeng Zhao, Stephen A. Klein, Shaocheng Xie, James S. Boyle and Yuying Zhang of LLNL and Dr. Xiaohong Liu of PNNL.

Chuanfeng Zhao
Zhao, C, SA Klein, S Xie, X Liu, JS 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 Letter L08806, doi:10.1029/2012GL051213.