Section 5. Tropospheric aerosols
5.5. Increases in aerosol model complexity – 2nd generation models: 2010s
2328 2329
As aerosol modelling matured, further refinements of aerosol direct and indirect effects were
2330
included in GCMs. Further components of aerosol were included; Bellouin et al (2011) included
2331
nitrate aerosol and pointed out that as sulphur dioxide emissions decrease in the future owing to
2332
emission control, the radiative forcing of nitrate will likely increase owing to the availability of
2333
excess ammonia in the atmosphere. However, nitrate continues to remain a difficult aerosol to
2334
model owing to the dissociation to nitric acid and ammonia under ambient temperature and
2335
humidities.
2336 2337
The development of aerosol mass spectrometers and their location at surface sites and airborne
2338
platforms enabled, for the first time, a full appreciation of the complexity of optically active sub-
2339
micron aerosol composition as a function of location and altitude to be deduced (Jimenez et al.,
2340
2011) with sulphate, organic and nitrate highlighted as the dominant sub-micron components. The
2341
problems of the mutual exclusivity of satellite-retrievals of aerosol and cloud can be avoided using
2342
active satellite sensors such as CALIPSO lidar aerosol data collocated with MODIS cloud data
2343
(e.g. Costantino and Bréon, 2013). However,in-situ airborne platforms with dedicated
2344
instrumentation such as nephelometers, and aerosol optical particle counters continued to provide
2345
vital information on the aerosol vertical profiles at a level of detail and vertical resolution
2346
impossible to achieve with satellite mounted lidars.
2347 2348
2349 2350
In modelling, dual-moment schemes became more common, treating both aerosol number and
2351
aerosol mass prognostically (Stier et al., 2005) and both internal and external mixtures. This has
2352
particular relevance to estimates of aerosol-cloud-interactions because, for single-moment
2353
schemes with prognostic mass only, any increase in the aerosol mass (e.g. via condensation or
2354
coagulation), must artificially increase the aerosol number and hence CCN which then produces
2355
stronger aerosol indirect effects. The use of dual-moment state-of-the-art aerosol schemes in
2356
GCMs is now common-place.
2357 2358
Lohmann et al. (2010) examined the differences between i) the radiative flux perturbation (RFP;
2359
Haywood et al., 2009) which is calculated as the difference in the top-of-the-atmosphere
2360
radiation budget between a present-day simulation and a preindustrial simulation, both using the
2361
same sea surface temperatures and ii) the radiative forcing computed from two–calls to the
2362
radiative transfer code in GCMs holding the atmospheric state fixed. The RFP calculation allows
2363
for rapid responses (e.g. in clouds), that occur on a faster time-scale than the large-scale shifts in
2364
climate response thaare induced through SST responses. RFP has become more commonly
2365
known as the effective radiative forcing (ERF – see Section 2.3.6). Allowing rapid adjustment to
2366
occur in diagnosing the ERF allowed isolation of both the Twomey (1977) and the Albrecht
2367
(1989) aerosol indirect effects and also aerosol semi-direct effects.
2368 2369
2371
Myhre (2009) suggested that the discrepancy between observational (stronger) and modelled
2372
(weaker) estimates of aerosol direct forcing highlighted in IPCC (2007) was due to the lack of
2373
account of aerosol absorption above clouds and (ii) relatively larger fractional increase in BC
2374
containing absorbing than scattering aerosols since pre-industrial times. Analysis of models
2375
reported an aerosol direct radiative forcing of -0.3W m-2 which was found to be consistent with
2376
observational estimates. Aerosol absorption was again highlighted as a major uncertainty in
2377
accurate determination of aerosol direct radiative forcing (Bond et al., 2013) owing to aspects such
2378
as the morphology of the black carbon as a function of age and the impact of coatings of organic
2379
and inorganic components, the heating in the atmospheric column and subsequent rapid
2380
adjustment. Again this suggested that diagnosing the radiative forcing in a strict sense could not
2381
capture the rapid adjustment associated with atmospheric processes.
2382 2383
Boucher et al. (2013), Myhre et al (2013) and IPCC (2013) recognized that retaining the strict
2384
definition of radiative forcing as in previous IPCC reports was becoming untenable because it
2385
did not reflect the growing consensus that rapid responses can and should be isolated in any
2386
metric ofclimate change, but also because of the ease of application to GCM simulations. Hence
2387
the growth of ERF as the preferred metric for assessing potential climate impacts. Indeed, IPCC
2388
also chose the term aerosol-radiation-interactions over the aerosol direct forcing and aerosol-
2389
cloud interactions over aerosol indirect with rapid adjustment of aerosol-radiation interaction as a
2390
term for the semi-direct effect (Boucher et al., 2013). By this time there were many mature
2391
estimates of the impact of aerosol-radiation-interactions and aerosol-cloud-interactions from
sophisticated GCMs and satellite-based estimates (e.g. Fig 5.3; see also Table 7.4 and 7.5 of
2393
Boucher et al. , 2013) allowing Myhre et al (2013) to estimate the magnitude of pre-industrial to
2394
present-day aerosol-radiation-interactions (-0.45W m-2 with a 95% uncertainty range of -0.9 to
2395
+0.05W m-2)and aerosol-cloud-interactions (-0.45W m-2 with a 95% uncertainty range of -1.2
2396 to 0 W m-2). 2397 2398 2399 2400
2401 2402 2403