Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 21-29, 2017
https://doi.org/10.5194/acp-17-21-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
02 Jan 2017
Why do general circulation models overestimate the aerosol cloud lifetime effect? A case study comparing CAM5 and a CRM
Cheng Zhou and Joyce E. Penner Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, Michigan, USA
Abstract. Observation-based studies have shown that the aerosol cloud lifetime effect or the increase of cloud liquid water path (LWP) with increased aerosol loading may have been overestimated in climate models. Here, we simulate shallow warm clouds on 27 May 2011 at the southern Great Plains (SGP) measurement site established by the Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) program using a single-column version of a global climate model (Community Atmosphere Model or CAM) and a cloud resolving model (CRM). The LWP simulated by CAM increases substantially with aerosol loading while that in the CRM does not. The increase of LWP in CAM is caused by a large decrease of the autoconversion rate when cloud droplet number increases. In the CRM, the autoconversion rate is also reduced, but this is offset or even outweighed by the increased evaporation of cloud droplets near the cloud top, resulting in an overall decrease in LWP. Our results suggest that climate models need to include the dependence of cloud top growth and the evaporation/condensation process on cloud droplet number concentrations.

Citation: Zhou, C. and Penner, J. E.: Why do general circulation models overestimate the aerosol cloud lifetime effect? A case study comparing CAM5 and a CRM, Atmos. Chem. Phys., 17, 21-29, https://doi.org/10.5194/acp-17-21-2017, 2017.
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Short summary
Observation-based studies have shown that the aerosol cloud lifetime effect or the increase of cloud liquid water with increased aerosol loading may have been overestimated in climate models. Here, by simulating the same shallow, warm clouds using a global climate model (CAM5) and a cloud resolving model (CRM) which has more complete and detailed cloud physics, we show how a climate model can overestimate the aerosol cloud lifetime effect due to its simplified representation of cloud processes.
Observation-based studies have shown that the aerosol cloud lifetime effect or the increase of...
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