RESULTADOS Y DISCUSIÓN
4. Apoyo familiar como modo adaptativo de interdependencia
3606 3607 3608 3609
Stratospheric aerosols, both volcanic and background, scatter the incoming shortwave
3610
radiation depleting the direct and enhancing the diffuse downward solar fluxes; they also
3611
absorb shortwave near infrared, and absorb and emit outgoing terrestrial radiation. The
3612
cumulative radiative effect of stratospheric aerosols is to cool the Earth’s surface and heat
3613
the aerosol layer in the lower stratosphere.
3614 3615
Volcanic eruptions that have historically exerted the strongest radiative forcing, have i)
3616
significant SO2/H2S injected into the stratosphere (although there is growing evidence of
3617
non-linearity of injections strength and radiative forcing (e.g. Niemeier and Tilmes, 2017) ,
3618
ii) tend to occur in tropical regions where both hemispheres of the globe are impacted by the
3619
subsequent perturbation to the aerosol optical depth and iii) inject SO2 to sufficiently high
3620
altitudes within the stratosphere (e.g. Jones et al., 2017).
3621 3622
The large perturbations of the Earth’s radiative balance caused by explosive volcanic
3623
eruptions e.g., Pinatubo, are discernible in observations; however, this does not lend itself
3624
readily to quantifying their actual radiative forcing (Dutton and Christy, 1992; Minnis et al.,
3625
1993; Russell et al., 1993). The theoretical calculations of the radiative forcing of
3626
stratospheric aerosols were first attempted using conceptual models (Lacis et al., 1992;
3627
Harshvardhan, 1979; Toon and Pollack, 1976). Because aerosol microphysical and optical
characteristics, which have to be compiled from observations or calculated within the
3629
model, are the major input into the radiative forcing calculations, we discuss both these
3630
aspects together here.
3631 3632
The first generation of the atmospheric general circulation models simulated the impact of
3633
volcanic aerosols using simplified approaches, i.e., assuming a reduction of the solar
3634
constant, increase of planetary albedo, or representing stratospheric aerosols by a single
3635
reflecting layer (e.g., Broccoli et al., 2003; Soden et al., 2002).
3636 3637
The existing aerosol observations were used to build the global aerosol datasets with pre-
3638
calculated aerosol optical/microphysical characteristics that could be implemented in
3639
climate models (Stenchikov et al., 1998; Stenchikov, 2016; Ramachandran et al., 2000; Sato
3640
et al., 1993; Hansen et al., 2002; Schmidt et al., 2011; Tett et al., 2002; Ammann et al.,
3641
2003). One approach is to use the observed/reconstructed aerosol optical depth (usually in
3642
visible) and assume aerosol composition and size distribution to calculate aerosol extinction,
3643
single scattering albedo, and asymmetry parameter required for radiative transfer models as
3644
input (Stenchikov et al., 1998; Sato et al., 1993). Another approach uses the empirical
3645
estimates of SO2 emissions and a simplified model to distribute them globally and to obtain
3646
the aerosol optical parameters (Ammann et al., 2003; Gao et al., 2008). Ammann et al.
3647
(2003) and Sato et al. (1993) datasets have essentially provided the bases for
3648 3649
implementing volcanic aerosols in virtually all of the climate models that have performed the
3650
20th-century climate integrations within IPCC AR4 (Stenchikov et al., 2006; Forster et al.,
3651
2007).
3652 3653
For the IPCC AR5 and CMIP6, the improved “gap-filled” SAGE II Version 6 aerosol
3654
product from (Thomason and Peter, 2006) was employed (Arfeuille et al., 2013; Zanchettin
3655
et al., 2016). All three stratospheric optical depths (SATO, AMMAN, and CMIP6) in Figure
3656
9.1 vary by about 30%, with Amman’s optical depth being the largest and CMIP6 being the
3657
smallest.
3658 3659
Figure 9.2 and 9.3 compare the all-sky shortwave (SW), longwave (LW), and SW+LW
3660
instantaneous radiative forcing at the top of the atmosphere and perturbations of heating rates
3661
calculated using SATO and CMIP6 inputs within the GFDL CM2.1 (Delworth et al., 2006)
3662
employing a double radiation call. To calculate optical characteristics of stratospheric
3663
aerosols for the SATO case, it was assumed that the aerosol has lognormal distribution with
3664
the time-and-latitude-varying effective radii and a fixed geometric width of 1.8 µm
3665
(SATO1.8) or 2.0 µm (SATO2). Despite the differences in the input information and
3666
assumptions, the changes in total radiative balance for the three datasets appear to be quite
3667
close. Both SATO’s datasets slightly overestimate the SW radiative forcing in comparison
3668
with (Minnis et al. 1993). The CMIP6 heating rates appear to be higher than expected
3669
(Stenchikov et al., 1998) and shifted toward theSW heating. Typical stratospheric sulfate
3672
from 2.5 µm where solar flux is weak. This is why LW heating is expected to prevail
3673
contributing about 70% of the effect (Stenchikov et al., 1998). The stratospheric heating is
3674
important, as it controls stratospheric dynamic responses (Ramaswamy et al., 2006).
3675 3676
The complexity of radiative, microphysical, and transport processes forced by volcanic
3677
aerosols suggests that it is important to calculate aerosol radiative effects interactively with
3678
the aerosol plume development rather than use a pre-calculated set of aerosol optical
3679
parameters. To accomplish this, it is necessary to know the SO2 volcanic emissions (Krueger
3680
et al., 2000; Hopfner et al., 2013; 2015) and be able to calculate development, transport, and
3681
decay of a volcanic aerosol layer.
3682 3683
The “bulk” aerosol models calculate SO2 to H2SO4 conversion and transport their bulk
3684
concentrations. Sulfate aerosols are assumed to form instantaneously with the prescribed size
3685
distribution (Timmreck et al., 1999; Oman et al., 2006a; and Aquila et al., 2012) that defines
3686
aerosol optical properties and deposition rates. Modal aerosol models keep track of
3687
aerosol number-density approximating the aerosol size distribution by a few log-normal
3688
modes with the prescribed width and varying modal radii, accounting for coagulation,
3689
condensation growth, and size-dependent gravitational settling (Niemeier et al., 2009; Bruhl
3690
et al., 2015; Dhomze et al., 2014; LeGrande et al., 2016; Sekiya et al., 2016). The aerosol
3691
sectional microphysical models are the most accurate but computationally more demanding
3692
(English et al., 2013; Mills et al., 2016).
3694 3695
There are still significant discrepancies between models, and between the models and
3696
observations. This remains a challenging issue. The 1991 Pinatubo case-study is an important
3697
testbed where different approaches have been compared and could be further investigated.
3698
For example, in (Bruhl et al., 2015) the aerosol optical depth relaxes too fast, but in (Mills et
3699
al., 2016) the stratospheric aerosol plume decays too slowly and the initial SO2 loading has
3700
to be decreased by almost a factor of two to make the results consistent with observations.
3701 3702