Abstract
Abstract. This paper introduces and evaluates the second version of the global aerosol-climate model ECHAM-HAM. Major changes have been brought into the model, including new parameterizations for aerosol nucleation and water uptake, an explicit treatment of secondary organic aerosols, modified emission calculations for sea salt and mineral dust, the coupling of aerosol microphysics to a two-moment stratiform cloud microphysics scheme, and alternative wet scavenging parameterizations. These revisions extend the model's capability to represent details of the aerosol lifecycle and its interaction with climate. Nudged simulations of the year 2000 are carried out to compare the aerosol properties and global distribution in HAM1 and HAM2, and to evaluate them against various observations. Sensitivity experiments are performed to help identify the impact of each individual update in model formulation. Results indicate that from HAM1 to HAM2 there is a marked weakening of aerosol water uptake in the lower troposphere, reducing the total aerosol water burden from 75 Tg to 51 Tg. The main reason is the newly introduced κ-Köhler-theory-based water uptake scheme uses a lower value for the maximum relative humidity cutoff. Particulate organic matter loading in HAM2 is considerably higher in the upper troposphere, because the explicit treatment of secondary organic aerosols allows highly volatile oxidation products of the precursors to be vertically transported to regions of very low temperature and to form aerosols there. Sulfate, black carbon, particulate organic matter and mineral dust in HAM2 have longer lifetimes than in HAM1 because of weaker in-cloud scavenging, which is in turn related to lower autoconversion efficiency in the newly introduced two-moment cloud microphysics scheme. Modification in the sea salt emission scheme causes a significant increase in the ratio (from 1.6 to 7.7) between accumulation mode and coarse mode emission fluxes of aerosol number concentration. This leads to a general increase in the number concentration of smaller particles over the oceans in HAM2, as reflected by the higher Ångström parameters. Evaluation against observation reveals that in terms of model performance, main improvements in HAM2 include a marked decrease of the systematic negative bias in the absorption aerosol optical depth, as well as smaller biases over the oceans in Ångström parameter and in the accumulation mode number concentration. The simulated geographical distribution of aerosol optical depth (AOD) is better correlated with the MODIS data, while the surface aerosol mass concentrations are very similar to those in the old version. The total aerosol water content in HAM2 is considerably closer to the multi-model average from Phase I of the AeroCom intercomparison project. Model deficiencies that require further efforts in the future include (i) positive biases in AOD over the ocean, (ii) negative biases in AOD and aerosol mass concentration in high-latitude regions, and (iii) negative biases in particle number concentration, especially that of the Aitken mode, in the lower troposphere in heavily polluted regions.
Highlights
It is widely believed that natural and anthropogenic aerosols play an important role in determining the current state and future changes of the Earth’s climate, various physical processes in the aerosol lifecycle are not yet understood with certainty
The ECHAM5-HAM model (Stier et al, 2005) developed at the Max Planck Institute for Meteorology was one of the earlier examples of a global atmospheric general circulation model that can dynamically predict the composition and size distribution of aerosols by taking into account the most important chemical and physical processes ranging from the micro- to the global scale, and in return, calculate the radiative effects of aerosols on the atmospheric dynamics
In addition to the evaluation presented by Stier et al (2005), the model has been compared with other models and with observations through participation in the AeroCom (Aerosol Comparisons between Observations and Models) model intercomparison project, as well as the EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality interactions) model intercomparison (Kulmala et al, 2011)
Summary
It is widely believed that natural and anthropogenic aerosols play an important role in determining the current state and future changes of the Earth’s climate, various physical processes in the aerosol lifecycle are not yet understood with certainty. Quantifications of the climatic effects of aerosols, through their impacts on clouds, remain insufficient. The newer models have included more detailed descriptions of aerosol composition and size distribution, using different approaches (Wilson et al, 2001; Jacobson, 2001; Vignati et al, 2004; Easter et al, 2004; Stier et al, 2005; Spracklen et al, 2005; Liu et al, 2005; Bauer et al, 2008; Liu et al, 2012). The ECHAM5-HAM model (Stier et al, 2005) developed at the Max Planck Institute for Meteorology was one of the earlier examples of a global atmospheric general circulation model that can dynamically predict the composition and size distribution of aerosols by taking into account the most important chemical and physical processes ranging from the micro- to the global scale, and in return, calculate the radiative effects of aerosols on the atmospheric dynamics. The estimated direct forcing of aerosols is close to the multi-model mean (Schulz et al, 2006)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.