Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environmental Model

Abstract

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 using GLOMAP-mode compare better than CLASSIC against a data-assimilated aerosol re-analysis and aerosol ground-based observations. 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 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 cloud susceptibilities, 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<sup>−2</sup> 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<sup>−2</sup> 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 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 forcings

The Impact of Changes in Cloud Water pH on Aerosol Radiative Forcing

“…Aerosol radiative forcing is defined here as the difference between ToA radiative perturbations over the period 1970 to 2009. Further details on HadGEM3‐UKCA are provided in the supporting information (Bellouin et al, ; Dee et al, ; Edwards & Slingo, ; Granier et al, ; Mann et al, ; O'Connor et al, ; Scott et al, ; Spracklen et al, ; Telford et al, ; Uppala et al, ; Woodward, ).…”

Small global effect on terrestrial net primary production due to increased fossil fuel aerosol emissions from East Asia since the turn of the century

“…We used the aerosol optical properties (scattering, absorption, and asymmetry coefficients) for each aerosol mode and spectral band based on Bellouin et al . []. The model is forced with monthly mean ECMWF climate (water vapor and temperature) and ozone reanalysis data together with cloud fields and surface albedo from the International Satellite Cloud Climatology Project (ISCCP‐D2) [ Rossow and Schiffer , ].…”

Anthropogenic contribution to cloud condensation nuclei and the first aerosol indirect climate effect

“…For several post-IPCC2007 studies that presented multiple values [4,5,10], either baseline case values or averaged values are given. Different FAIRF values based on different approaches, such as different meteorology fields [10], online versus off-line aerosol [9], and bulk versus modal approaches [8], are treated as separate studies because these approaches are quite different and have been used in other FAIRF studies.…”

“…The spread among model-calculated FAIRF values constitutes the largest uncertainty among the quantified radiative forcings [2]. FAIRF values based on post-IPCC2007 studies still vary significantly [3][4][5][6][7][8][9][10].…”

“…Among the 20 previous FAIRF studies listed in the table 2.7 of IPCC2007, at least 15 of them employed the prescribed (or bulk) aerosol scheme that assumes globally uniform size distributions. Bellouin et al [8] showed substantial differences in the aerosol forcings simulated by the bulk and modal schemes, and they pointed out that the bulk approach lacked the necessary sophistication to provide realistic aerosol input for aerosol-cloud-radiation calculations. More post-IPCC2007 FAIRF studies are based on the modal scheme which employs one or more log-normal modes to represent aerosols of various types.…”