Atmospheric Chemistry and Physics, vol. 13, issue 6 (2013) pp. 3027-3044 Published by European Geosciences Union
The Hadley Centre Global Environmental Model (HadGEM) includes two aerosol schemes: the Coupled Large-scale Aerosol Simulator for Studies in Climate (CLASSIC), and the new Global Model of Aerosol Processes (GLOMAP-mode). GLOMAP-mode is a modal aerosol mi-crophysics scheme that simulates not only aerosol mass but also aerosol number, represents internally-mixed particles, and includes aerosol microphysical processes such as nucle-ation. In this study, both schemes provide hindcast simula-tions of natural and anthropogenic aerosol species for the pe-riod 2000–2006. HadGEM simulations of the aerosol optical depth using GLOMAP-mode compare better than CLASSIC against a data-assimilated aerosol re-analysis and aerosol ground-based observations. Because of differences in wet de-position rates, GLOMAP-mode sulphate aerosol residence time is two days longer than CLASSIC sulphate aerosols, whereas black carbon residence time is much shorter. As a result, CLASSIC underestimates aerosol optical depths in continental regions of the Northern Hemisphere and likely overestimates absorption in remote regions. Aerosol direct and first indirect radiative forcings are computed from sim-ulations of aerosols with emissions for the year 1850 and 2000. In 1850, GLOMAP-mode predicts lower aerosol op-tical depths and higher cloud droplet number concentra-tions than CLASSIC. Consequently, simulated clouds are much less susceptible to natural and anthropogenic aerosol changes when the microphysical scheme is used. In particu-lar, the response of cloud condensation nuclei to an increase in dimethyl sulphide emissions becomes a factor of four smaller. The combined effect of different 1850 baselines, res-idence times, and abilities to affect cloud droplet number, leads to substantial differences in the aerosol forcings simu-lated by the two schemes. GLOMAP-mode finds a present-day direct aerosol forcing of −0.49 W m −2 on a global av-erage, 72 % stronger than the corresponding forcing from CLASSIC. This difference is compensated by changes in first indirect aerosol forcing: the forcing of −1.17 W m −2 obtained with GLOMAP-mode is 20 % weaker than with CLASSIC. Results suggest that mass-based schemes such as CLASSIC lack the necessary sophistication to provide realis-tic input to aerosol-cloud interaction schemes. Furthermore, the importance of the 1850 baseline highlights how model skill in predicting present-day aerosol does not guarantee re-liable forcing estimates. Those findings suggest that the more complex representation of aerosol processes in microphysi-cal schemes improves the fidelity of simulated aerosol forc-ings.
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