Articles | Volume 5, issue 3
Geosci. Model Dev., 5, 709–739, 2012
Geosci. Model Dev., 5, 709–739, 2012

Model description paper 21 May 2012

Model description paper | 21 May 2012

Toward a minimal representation of aerosols in climate models: description and evaluation in the Community Atmosphere Model CAM5

X. Liu1, R. C. Easter1, S. J. Ghan1, R. Zaveri1, P. Rasch1, X. Shi1, J.-F. Lamarque2, A. Gettelman2, H. Morrison2, F. Vitt2, A. Conley2, S. Park2, R. Neale2, C. Hannay2, A. M. L. Ekman3, P. Hess4, N. Mahowald5, W. Collins6, M. J. Iacono7, C. S. Bretherton8, M. G. Flanner9, and D. Mitchell10 X. Liu et al.
  • 1Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA
  • 2National Center for Atmospheric Research, Boulder, Colorado, USA
  • 3Department of Meteorology and Bert Bolin Centre for Climate Research, Stockholm University, Sweden
  • 4Biological and Environmental Engineering, Cornell University, Ithaca, New York, USA
  • 5Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA
  • 6Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
  • 7Atmospheric and Environmental Research, Inc., Lexington, Massachusetts, USA
  • 8Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
  • 9Department of Atmospheric, Oceanic & Space Sciences, Univ. of Michigan, 2455 Hayward St., Ann Arbor, Michigan, USA
  • 10Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada, USA

Abstract. A modal aerosol module (MAM) has been developed for the Community Atmosphere Model version 5 (CAM5), the atmospheric component of the Community Earth System Model version 1 (CESM1). MAM is capable of simulating the aerosol size distribution and both internal and external mixing between aerosol components, treating numerous complicated aerosol processes and aerosol physical, chemical and optical properties in a physically-based manner. Two MAM versions were developed: a more complete version with seven lognormal modes (MAM7), and a version with three lognormal modes (MAM3) for the purpose of long-term (decades to centuries) simulations. In this paper a description and evaluation of the aerosol module and its two representations are provided. Sensitivity of the aerosol lifecycle to simplifications in the representation of aerosol is discussed.

Simulated sulfate and secondary organic aerosol (SOA) mass concentrations are remarkably similar between MAM3 and MAM7. Differences in primary organic matter (POM) and black carbon (BC) concentrations between MAM3 and MAM7 are also small (mostly within 10%). The mineral dust global burden differs by 10% and sea salt burden by 30–40% between MAM3 and MAM7, mainly due to the different size ranges for dust and sea salt modes and different standard deviations of the log-normal size distribution for sea salt modes between MAM3 and MAM7. The model is able to qualitatively capture the observed geographical and temporal variations of aerosol mass and number concentrations, size distributions, and aerosol optical properties. However, there are noticeable biases; e.g., simulated BC concentrations are significantly lower than measurements in the Arctic. There is a low bias in modeled aerosol optical depth on the global scale, especially in the developing countries. These biases in aerosol simulations clearly indicate the need for improvements of aerosol processes (e.g., emission fluxes of anthropogenic aerosols and precursor gases in developing countries, boundary layer nucleation) and properties (e.g., primary aerosol emission size, POM hygroscopicity). In addition, the critical role of cloud properties (e.g., liquid water content, cloud fraction) responsible for the wet scavenging of aerosol is highlighted.