Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environmental Model 1Met Office Hadley Centre, Exeter, UK
15 Mar 2013
2National Centre for Atmospheric Science, University of Leeds, Leeds, UK
3Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
*now at: Department of Meteorology, University of Reading, Reading, UK
Received: 31 Jul 2012 – Published in Atmos. Chem. Phys. Discuss.: 21 Aug 2012Abstract. 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 microphysics scheme that simulates not only
aerosol mass but also aerosol number, represents internally-mixed particles,
and includes aerosol microphysical processes such as nucleation.
In this study, both schemes provide hindcast simulations of natural and
anthropogenic aerosol species for the period 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 deposition 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
Aerosol direct and first indirect radiative forcings are computed from
simulations of aerosols with emissions for the year 1850 and 2000.
In 1850, GLOMAP-mode predicts lower aerosol optical depths and higher cloud
droplet number concentrations than CLASSIC.
Consequently, simulated clouds are much less susceptible to natural
and anthropogenic aerosol changes when the microphysical scheme is used.
In particular, 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, residence times, and abilities
to affect cloud droplet number, leads to substantial differences in the aerosol
forcings simulated by the two schemes.
GLOMAP-mode finds a present-day direct aerosol forcing of −0.49 W m−2 on
a global average, 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
Results suggest that mass-based schemes such as CLASSIC lack the necessary
sophistication to provide realistic 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 reliable forcing estimates.
Those findings suggest that the more complex representation of aerosol
processes in microphysical schemes improves the fidelity of simulated aerosol
Revised: 04 Feb 2013 – Accepted: 04 Mar 2013 – Published: 15 Mar 2013
Citation: Bellouin, N., Mann, G. W., Woodhouse, M. T., Johnson, C., Carslaw, K. S., and Dalvi, M.: Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environmental Model, Atmos. Chem. Phys., 13, 3027-3044, doi:10.5194/acp-13-3027-2013, 2013.