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Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the hadley centre global environmental model

by N. Bellouin, G. W. Mann, M. T. Woodhouse, C. Johnson, K. S. Carslaw, M. Dalvi
Atmospheric Chemistry and Physics ()
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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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