Introduction
During the eruption of Mt. Pinatubo in June 1991, a large amount of sulfur
dioxide was emitted into the stratosphere, leading to an enhancement of the
stratospheric aerosol burden. The aerosol layer perturbed the Earth's
radiative balance, resulting in a top-of-the-atmosphere global mean radiative
forcing up to -3 Wm-2 , a global
surface cooling of ∼ 0.4–0.5 K , and a temperature increase of ∼ 2.5–3.5 K in the
tropical lower stratosphere . During the past decades it was shown that these observed
temperature perturbations are connected to many feedbacks in the Earth system
such as alteration of the stratospheric circulation with consequences for the
troposphere e.g.,,
dynamical and chemical effects on stratospheric ozone
, drying of the
troposphere causing significant changes in the regional hydrological cycle
, modulation of the global monsoon , and
even modulation of the ocean circulation e.g.,.
The distribution and evolution of stratospheric sulfate can therefore
be considered as the main forcing constraint for these and many other
processes following large volcanic eruptions , and proper information about the aerosol
layer is crucial for the characterization and understanding of numerous
inherent feedbacks.
Modeling studies help to synthesize our knowledge of how the Mt. Pinatubo and
other big eruptions impact the climate system. Global three-dimensional
general circulation models (GCMs) or chemistry–climate models (CCMs) used for
studying volcanic effects on climate can be mainly separated into two
groups: those using prescribed aerosol distributions and those using online
aerosol microphysical modules e.g.,. Models of
the first type can use aerosol composites derived from satellite and
ground-based observations e.g.,, but for studies
of the pre-satellite era and the future such models have to rely on estimates
provided by models of the second type or derived by simple reconstruction
methods e.g.,. They have only limited ability to
reproduce the climate response to volcanic eruptions, as the aerosols are
prescribed and therefore the feedbacks between aerosols and the stratosphere
are completely missed, resulting in biased aerosol radiative forcing depending
on concrete circumstances. Such models also inherit either all instrumental
uncertainties (see Sect. 3) or uncertainties from the second type of models
and reconstruction models. Models with aerosol microphysics can also be
grouped depending on how they treat the aerosol size distribution: a first
class of so-called “modal” and “bulk” (unimodal) schemes and a second
class of size-resolving (also called “sectional”) aerosol modules.
Currently, there are more than a dozen active global models with aerosol
microphysics see review by, a smaller part of which
employ sectional aerosol schemes.
Both modal and size-resolving schemes have their benefits and problems. Modal
aerosol schemes prescribe some basic parameters characterizing size
distribution (e.g., size distribution function) and therefore have high
computational efficiency. Size-resolving schemes simulate an evolution of
size distribution and can better describe gravitational sedimentation, which
crucially affects the stratospheric aerosol lifetime.
argued that bulk schemes are less
satisfactory in reproducing volcanic aerosol distributions, which cast doubts
on the success of such approaches. For 2-D models,
have shown that size-resolving aerosol models
are superior to modal approaches in accurately representing the
time-dependent aerosol size distribution after large volcanic eruptions.
Further progress using a CCM coupled with a size-resolving microphysical
aerosol module to simulate Pinatubo-like eruptions has been achieved by
; however, the decline of the simulated
aerosol burden was too fast compared with observations, which they attributed
to the lack of heating as the aerosol radiative feedback remained decoupled
in their model. This highlighted the fact that the fine resolution of aerosol sizes is
not a universal solution and the performance of any model, even with highly
resolved aerosol sizes, depends on representation of relevant chemical,
microphysical, and radiative processes, large-scale transport, and
gravitational sedimentation, as well as their interactions.
The Pinatubo eruption is the strongest volcanic event since the beginning of
the satellite era and is therefore often used as a model performance test.
Modeling studies of the Pinatubo eruption using models with an assumed
lognormal size distribution e.g., and sectional distribution generally agree reasonably well with observations of
atmospheric longwave and shortwave extinctions, aerosol burden, and other
integral parameters. However, an intercomparison of different Pinatubo
studies is hampered by the fact that models make different assumptions of how
much sulfur was initially emitted and how the plume was distributed as
a function of altitude. Models that reported good agreement with observations
used a variety of emission estimates ranging from 10 to 17 teragrams (Tg) of
SO2 and SO2 plume altitudes in the lower stratosphere
differing by a few kilometers. This hints at large differences in how models
treat important microphysical and transport processes and significantly
increases the uncertainty of the overall aerosol layer understanding.
An ongoing model intercomparison project on the climatic response to volcanic
forcing VOLMIP; aims to address the existing
intermodel uncertainties. So far only the Tambora eruption in 1815 has been
considered for global models with interactive aerosol microphysics.
evaluated the performance of four state-of-the-art
models (WACCM, UM-UKCA, SOCOL-AER, and ECHAM5-HAM) using mostly the same
settings of the initial emission and compared the results to the available
observations of ice core sulfate. The focus of that study was on sulfate
deposition in polar areas, as ice cores are the best available source of
information about historical eruptions .
The comparison revealed that modeled volcanic sulfate deposition varies
substantially in timing, spatial pattern, and magnitude between the models.
The ratio of the hemispheric atmospheric sulfate aerosol burden after the
eruption to the average amount of sulfate deposited on ice sheets varied
among models by up to a factor of 15. The burden-to-deposition ratio is to a
large extent determined by the treatment of deposition processes, which are
simplified in models. Furthermore, it also depends on sulfur species that
never entered the stratosphere but were transported through the (upper)
troposphere, oxidized, and removed by wet or dry deposition. Moreover, it
depends on how fast aerosols grow and sediment from the stratosphere back to
the troposphere. The analysis of the stratospheric burdens of SO2 and
liquid H2SO4 as well as the polar winds also revealed large
intermodel differences. Therefore there is still no clear understanding of
which model is closer to reality in describing the stratospheric aerosol
distribution, since direct stratospheric observations are missing and the ice
core estimates could be strongly modulated by the tropospheric deposition
schemes.
List of experiments.
QBO
Aerosol
Sedimentation
Coagulation
Emission
Name
nudged
feedback
scheme
efficiency α
rate (Tg SO2)
REF
Yes
Yes
Walcek
α=1 everywhere
14
REF12
Yes
Yes
Walcek
α=1 everywhere
12
UPWIND
Yes
Yes
Upwind
α=1 everywhere
14
noRAD
Yes
No
Walcek
α=1 everywhere
14
noQBO
No
Yes
Walcek
α=1 everywhere
14
noRADnoQBO
No
No
Walcek
α=1 everywhere
14
COAG
Yes
Yes
Walcek
α based on a Lennard–Jones potential:
14
α∼1 in continuum regime (Kn≫1);
α∼ 1–3 in transition regime (Kn ∼ 1–10);
α≪1 in free molecular regime (Kn≪1).
The representation of aerosol evolution in the stratosphere requires
treatment of many processes, which can substantially differ among models.
Previous studies e.g., illustrated the importance of the quasi-biennial
oscillation (QBO) and radiative heating of volcanic aerosols in the models,
as these processes affect the transport and thus the lifetime and climate
impact of aerosols. found that numerical
diffusion induced by an inaccurate sedimentation scheme may lead to excessive
transport of the aerosol to the middle and upper stratosphere. So, even with
a fine aerosol size resolution, resulting sedimentation can be biased due to
the model's numerical scheme. suggested that attractive van der Waals forces
may lead to an enhanced coagulation efficiency and should be taken into
account (in the transition and free molecular regimes). However, such
interactions led to an even faster decay in their simulated global aerosol
burden after the Pinatubo eruption. and
also investigated the role of this process and
reported significant effects on aerosol parameters. Interactive chemistry was
also shown to be important for aerosol formation, as hydroxyl radical (OH)
can become depleted after big eruptions, which prolongs the time needed for
the conversion of volcanic SO2 to H2SO4 .
The coupled size-resolving stratospheric aerosol–chemistry–climate SOCOL-AER
model has been evaluated in detail for volcanically quiescent conditions
. In this study, we employ it for the Pinatubo
eruption of 1991 and aim to characterize its performance by comparing our
results against satellite observations and in situ measurements. By means of
this model we also attempt to illustrate the roles of the aerosol radiative
heating, sedimentation scheme, coagulation efficiency, and the QBO in the
evolution of the aerosol burdens, aerosol optical properties, and particle
size distributions, which may also help to better understand differences
between various models.
Methods
The coupled aerosol–chemistry–climate model SOCOL-AERv1.0 (SOCOL-AER
hereafter) has been introduced by , who applied the
model to analyze the global atmospheric sulfur budget under volcanically
quiescent conditions and its sensitivity to anthropogenic emissions.
SOCOL-AER is a CCM SOCOLv3 GCM ECHAM5 plus chemical module
MEZON; combined with an aerosol module AER
. AER includes a comprehensive description of sulfur
chemistry and microphysics, in which the particles are size-resolved by 40
size bins spanning radii from 0.39 nm to 3.2 µm, which allows us to
consider all relevant microphysical processes (e.g., nucleation and
coagulation). The influence of the aerosol on radiation fluxes at all
wavelengths is also taken into account. SOCOL-AER uses 6-band shortwave
and 16-band longwave radiation
schemes. Extinction coefficients, single-scattering albedos, and asymmetry
factors required by the radiation codes are calculated online from aerosol
physical properties using Mie theory for actual H2SO4 weight percent
and temperature using refraction indices from . The
aerosol surface area density and composition is used to calculate
heterogeneous reaction rates in a chemical module. In this study, the spatial
resolution of SOCOL-AER is set to T42 horizontal truncation
(2.8∘ × 2.8∘ latitude–longitude resolution) and 39
vertical hybrid sigma-pressure levels from the surface to 80 km (about
1–1.5 km per level in the upper troposphere and lower stratosphere,
2–3 km above). The QBO in the model is nudged to observed wind profiles.
Monthly mean transient sea surface temperatures (SSTs) and sea ice coverage
(SIC) are prescribed from the Hadley Centre Sea Ice and SST data set
. Comprehensive sulfur surface emissions are also fully
taken into account. A more detailed description of the SOCOL-AER modules can be
found in .
Observational estimates of the total SO2 mass emitted by Pinatubo and its
vertical distribution are still very uncertain
. We use an estimate of
, who used a 2-D sulfate aerosol model with the
same microphysical module as in SOCOL-AER to identify the optimized emission
parameters by running ∼ 300 sensitivity experiments spanning the observational uncertainty and
comparing the results to observations of different aerosol parameters. Based
on , the Pinatubo eruption is introduced here by
an injection of 14 Tg SO2 in the region 97–112∘ E and
1.8∘ S–12∘ N. SO2 is continuously released from 14 to
15 June 1991 and spread between 16 and 30 km with a vertical mass
distribution skewed to low altitudes with the mass peak between 18 and
21 km. All experiments are summarized in Table . The reference
run subsequently termed REF represents the standard setup of SOCOL-AER,
including nudged QBO, interactive aerosol radiative and chemical effects, and
coagulation efficiency uniformly set to one. In terms of module versions it
replicates the model configuration used for the Tambora study
.
By means of the experiment REF12 we estimate the model sensitivity to
uncertainty in the emission amount by lowering it to 12 Tg SO2. In the
experiment termed noRAD, the radiative fluxes are calculated using the
SAGE-4λ data set averaged over the
period 1995–2002 instead of the interactively simulated aerosol
distribution, which eliminates the radiative effects of volcanic aerosols.
The experiment termed noQBO is carried out without QBO, which leads to a weak
easterly zonal wind in the tropical stratosphere. Both QBO nudging and
interactive radiation are switched off in the experiment termed noRADnoQBO.
These three experiments allow us to identify the impact of QBO and the radiative
heating of volcanic aerosols on the evolution of the stratospheric aerosol
burden after the Pinatubo eruption. We also carry out an exploratory experiment
concerning the coagulation efficiency; the experiment termed COAG
represents the coagulation efficiency as Lennard–Jones potential, i.e., a
smooth function of the Knudsen number retrieved from the results in Fig. 3 of
with a Hamaker constant of
5×10-19 J. As an approximation of attractive van der Waals forces
it enhances the coagulation efficiency in the transition regime (maximum
enhancement larger than 2), but decreases it rapidly (less than 1) as the
Knudsen number increases in the free molecular regime. The experiment termed
UPWIND employs the upwind sedimentation scheme
, while all other simulations use the more
elaborate Walcek method with minimal numerical diffusion
. This is sufficient to clarify the impact of
different sedimentation schemes, though work by
presented a further improved modified
Walcek method.
Each of these experiments consists of five ensemble members. In the figures
we show the ensemble spread for the REF experiment and only ensemble means for
other experiments to keep figures as uncomplicated as possible. In addition
to Pinatubo, for all runs we considered the smaller eruption of Cerro Hudson
in Chile in August 1991. We used the latest estimate of 2.3 Tg total
SO2 emitted with 75 % of mass injected between
16 and 18 km. Sensitivity studies with and without this event showed that
its contribution is minor, since it is located at higher latitudes
(45.5∘ S), but we keep it for completeness.
(a, b) Evolution of model-calculated global (pole to pole,
left) and tropical (20∘ S–20∘ N, right) stratospheric
aerosol burden (Tg of S) compared with the HIRS- and SAGE-II-derived data
(SAGE-3,4λ). HIRS-derived aerosol sulfur burden assumes 75 %
sulfuric acid by weight. Light blue shaded area: uncertainties of HIRS. Grey
shaded area: 2σ ensemble spread of the REF experiment. All other
experiments are shown as ensemble means. (c, d) Same as (a)
and (b), but deviations of all the numerical experiments from the
REF in %.
Results and discussion
Aerosol burden
Figure shows the evolution of observation-derived and
model-calculated stratospheric aerosol burdens in units of Tg of sulfur
globally integrated (panel a) and in the tropics (panel b). The High-Resolution Infrared Radiation
Sounder (HIRS) measured the aerosol vertical column and derived total aerosol
mass with about 10 % uncertainties. HIRS includes tropospheric and
stratospheric aerosols together . In contrast, the
limb-occultation measurements of SAGE II allow aerosols in the troposphere
and stratosphere to be distinguished from one another. In this work the SAGE-II-derived total aerosol mass is represented by two data sets. The first one,
the SAGE-4λ method used within the
Chemistry–Climate Model Initiative (CCMI), employs all four SAGE wavelengths
with overall about 30 % uncertainties for non-gap-filled data and higher
uncertainties in data gaps filled by lidar station data. The second data set
was recently compiled for phase 6 of the Coupled Model Intercomparison
Project (CMIP6; ) using the SAGE-3λ method,
which is similar to SAGE-4λ but refrains from employing the less
reliable channel at 385 nm, thus considering only three SAGE wavelengths.
Directly after Pinatubo the SAGE-3λ data set uses additional
satellite and ground-based data for gap filling. More information about the
SAGE-3,4λ composites can be found in a recent paper by
.
During the first year after the Pinatubo eruption, the aerosol mass in both SAGE-II-based data sets
is noticeably lower than in HIRS data. This is likely
related to the saturation effects of SAGE II as a limb-occultation
instrument during this period . The
SAGE-3λ composite provides significantly larger burdens than its
predecessor due to additional data used in a gap-filling procedure
, but still much lower than HIRS. After this period, when
the atmosphere becomes sufficiently transparent, SAGE II measurements are
expected to provide more accurate aerosol extinctions. In contrast, the
HIRS-derived mass becomes less reliable with time, when the aerosol cloud
spreads to higher latitudes with lower values that are close to the noise
level of the technique . This suggests trusting the
HIRS data up to mid-1992 and the SAGE data afterwards
. Note, however, that the updated SAGE-II-based
data set now also provides values closer to HIRS from mid-1992 to early 1993
and considerably larger values later in 1993.
Comparison of in situ measurements of particle size (at Laramie,
Wyoming; ) with SOCOL-AER simulations.
(a) Stratospheric effective particle radius averaged for 14–30 km
of altitude. Thin blue whiskers reflect measurement uncertainty (taken from
). (b–d) Profiles of cumulative number
densities for two size channels with radii R>0.15 µm (right
group of curves) and R>0.5 µm (left group of curves) in
August 1991, May 1992, and March 1993, respectively. SOCOL-AER results are
monthly means, while OPC data are discrete measurements within chosen
months.
The global stratospheric aerosol burden calculated by REF (grey shaded area
representing 2σ ensemble spread) agrees well with the HIRS data
peaking around 5.4 Tg at the end of 1991. Later, REF agrees well with the
SAGE-4λ composite, while the updated SAGE-3λ has a generally
larger burden. Qualitatively similar results are found for the tropics.
Recent modeling studies by and
showed very similar time series of the global aerosol burden with initial
emissions of 10 and 14 Tg of SO2, respectively. These studies, another
work by , and the present study fail to reproduce
the pronounced step-like evolution of the burden seen in HIRS and
SAGE-3λ, showing a smoother decrease instead.
overestimated the HIRS peak burden even with 10 Tg of SO2 emitted, but
obtained this step-like behavior. explained it by
variability in the background aerosols related to the summer increase in
photolysis, but they also noted that their background values are
significantly larger than in the other models and observations. Besides
instrumental uncertainty, another reason for the complicated shape of the
observational curves seen in Fig. could be the seasonal
variability of the stratospheric circulation that is known to be
underestimated in ECHAM5 and LMDZ
, which are core GCMs of the sectional models
SOCOL-AER and LMDZ-S3A, respectively.
Panels (c) and (d) of Fig. show deviations (%) from REF
in all experiments. Our experiment with lower emission (REF12, 12 Tg of
SO2 instead of 14 in REF, but otherwise unchanged plume
characteristics) shows lower burdens of up to 14 % globally and 17 %
in the tropics, which is therefore even farther than REF from the
latest SAGE-II-derived estimates after mid-1992. The results of the sensitivity runs
noQBO, noRAD, and noRADnoQBO are presented by the red curves. During the first
few months after the Pinatubo eruption, the aerosol mass loading in the
tropical reservoir is maintained by the competition between sedimentation and
enhanced tropical upwelling due to the radiative heating of volcanic aerosols
with the QBO in a strongly descending easterly phase
. Therefore, deactivation
of each of these processes leads to a stratospheric burden decrease mostly
located in the tropics. About 1 year after the eruption the global aerosol
burden in noQBO and noRAD is approximately 15 % lower than in REF. The
experiment noRADnoQBO shows a cumulative effect up to -30 % around
1993. Gravitational sedimentation becomes a dominant removal process when
particles grow sufficiently large after the Pinatubo eruption. With effective
radii of 0.5 µm or more these particles
sediment efficiently. The burden calculated by UPWIND mostly lies within
±10 % with respect to REF. This upwind scheme was shown to have a
large numerical diffusion smearing the aerosol layer out in both the up and
down directions . This results in a
slightly lower mass during 1–1.5 years after the eruption (effect of the
downward diffusion) and a slightly larger mass later on (upward transported
aerosols stay longer in the stratosphere). Although this diffusion effect is
of numerical origin, for our model it increases the stratospheric lifetime of
aerosols and leads to a better agreement with SAGE-3λ after 1993. The
aerosol burden calculated by COAG, which differs from REF by a higher
coagulation efficiency, shows a more rapid decay rate of the global volcanic
aerosol burden compared to REF and the measurements. The difference to REF
maximizes in late 1993 at approximately -33 %, which is in agreement
with other studies also looking at the van der Waals forces effects
.
Aerosol size distribution
Figure shows
comparisons of the optical particle counter (OPC) measurements operated above
Laramie, Wyoming 41∘ N,
105∘ W; against our
model experiments. The model was sampled as monthly mean values averaged over a
region of ±5∘ latitude and longitude around Laramie. Panel (a)
shows the effective aerosol radius averaged over 14–30 km. The effective
radius calculated by REF generally lies within the observational uncertainty.
However, compared to the observational mean, it is biased high under
quiescent conditions and biased low during the volcanically perturbed period.
COAG shifts the effective radius up compared to REF, which improves the
agreement with observations after 1992, but worsens it earlier. Differences
of other experiments reflect the burden of the aerosol behavior shown in
Fig. , illustrating that less mass present in the
stratosphere generally also leads to smaller sizes.
Panels (b)–(c) of Fig. show cumulative number distributions
for two OPC size channels (R>0.15 µm and R>0.5 µm) in
August 1991, May 1992, and March 1993 representing different stages of the
volcanic aerosol cloud. We use months with at least two soundings to obtain a
useful approximation of day-to-day variability. Aerosol number densities at
the altitudes of the maximum concentrations are well reproduced by REF for
both large and small particles. Higher altitudes, however, are not so well
reproduced, with modeled number densities being up to 3 orders of
magnitude too high for bigger particles at certain levels. However, at these
high altitudes OPC measurements are themselves uncertain, often having to
rely on only one or two channels (plus the concomitant condensation counter
measurement). Even larger deviations from the OPC measurements are found for
the UPWIND experiment, which clearly has too many particles, especially large
ones, in the middle stratosphere all the way to the upper edge of the
stratospheric aerosol layer, highlighting the importance of a sedimentation
scheme with low numerical diffusivity. Experiments with radiatively decoupled
aerosols, noRAD and noRADnoQBO, illustrate the importance of the enhanced
upwelling, even in midlatitudes, by showing more large particles staying at
the lower levels.
Comparison of remote measurements of aerosol optical depth (AOD)
ratios at two wavelengths, a proxy of particle size, with SOCOL-AER
simulations. Lines: SAGE-II-derived (SAGE-3,4λ) and modeled global
AOD (>18 km) ratios 565 nm / 940 nm. Grey shaded area: 2σ
ensemble spread of the REF experiment. All other experiments are shown as
ensemble means.
To analyze the global mean size distributions, in Fig. we
show the ratio of aerosol optical depth (AOD) at 565 and 940 nm for the
column above 18 km calculated from SAGE-II-derived composites (blue curves)
and from model results. These ratios are inversely related to the particle
size: a smaller ratio corresponds to larger particles. In the early phase of
the Pinatubo eruption, a large number of small particles are formed, which
coalesce very quickly as shown by the very sharp drop in the AOD ratio,
falling below 1.25 in observations. Afterwards, the small AOD ratio stays
almost constant for approximately 1 year. Around late 1993 the ratio starts
to return to higher values because the large particles continuously sediment
out of the stratosphere and smaller particles nucleate in the air entering
the stratosphere in the tropics. REF predicts smaller particles than derived
from SAGE II during the early phase after the eruption, and only in 1993 does it
start to agree well with the satellite observations. In contrast, due to the
enhanced coagulation, COAG produces larger particles (smaller AOD ratios)
than REF and shows better agreement with SAGE II during the 4 years
following the eruption. The model run UPWIND with a simplified upwind scheme
for sedimentation is initially close to REF but reveals overestimation of the
particle sizes compared to REF after 1993. This is related to a larger
aerosol burden (Fig. ), which enables further coagulation.
Our experiment with the reduced emission (REF12) further illustrates this
effect by showing that weaker emission leads to slightly smaller sizes over
the whole lifetime of the volcanic aerosol cloud.
In general, the model in its reference configuration slightly underestimates the
mean particle radius. The agreement with observations is much better if a
more detailed coagulation is used. However, as seen in panels (b)–(c) of
Fig. , the model in all configurations also has problems in
reproducing altitudes higher than 25 km. Comparison of the model to both
in situ OPC measurements and the satellite-based global composites
SAGE-3λ and SAGE-4λ reveals the same effects of individual
experiments. The main difference between the two comparisons is seen during the
pre-eruption time, as the model shows larger particles than OPC and smaller
particles than SAGE, which can be attributed to the local bias of model
parameters.
Aerosol optical depth
Monthly zonal average total AOD measured at 0.63 µm by
AVHRR and calculated at 0.56 µm above the tropopause by SOCOL-AER and
provided by SAGE-3,4λ composites. Since AVHRR measurements were
performed over oceans, we applied the same selection for the model here.
SAGE-3,4λ data were, however, initially provided as zonal means.
Background values are subtracted from all data sets (which may result in
slightly negative values). All panels are masked at winter high latitudes
where AVHRR data are missing.
Same as in Fig. but averaged over non-masked regions
globally (80∘ S–80∘ N) and in the tropics
(20∘ S–20∘ N). Grey shaded area: 2σ ensemble spread
of the REF experiment. All other experiments are shown as ensemble means.
Figure shows the latitudinal evolution of volcanic material as
modeled and measured AOD in the visible part of the solar spectrum, which
also represents the main direct climate forcing, since it defines the amount
scattered back to space solar irradiance responsible for global cooling.
In addition to SAGE II, we used data from the Advanced Very High Resolution
Radiometer (AVHRR) satellite instrument, which makes observations over global
oceans . Modeled and SAGE-3,4λ AODs are obtained by
vertically integrating the extinctions above the tropopause. We removed
latitudes not observed by AVHRR for each month from the other data sets and
subtracted background values from observations and calculations. Aerosol
optical depths derived from SAGE II and AVHRR significantly disagree both in
magnitude and spatial distribution. SAGE-3,4λ shows much smaller AOD
in the tropics in 1991 and not so strong a southward plume as seen in AVHRR at
the end of 1991, part of which is influenced by the high-latitude Cerro
Hudson eruption in August 1991. The northern hemispheric plume in 1992 is
also more pronounced in the AVHRR data. Figure shows the same
AOD values, but averaged over the non-masked regions in Fig.
over the globe and the tropics. The main difference between AVHRR and SAGE is
that AVHRR shows a higher AOD peak in 1991 (2 times higher in tropics)
similar to the faster increase in early aerosol burden of HIRS
(Fig. ). However, AVHRR reveals a much faster decay, so
starting from late 1992 SAGE-II-derived AOD is much larger than measured by
AVHRR.
Modeling results are closer to AVHRR before mid-1992 and to SAGE-II-derived
data later. REF shows weaker south- and northward plumes in
Fig. , but nicely captures the initial increase in the tropics
seen by AVHRR. The lifetime of the initial tropical cloud is also well
captured compared to AVHRR, except for a small increase in early 1992.
Similarly to the burden shown in Fig. , starting from
mid-1992 the model results are closer to SAGE-4λ than to
SAGE-3λ. The experiment REF12 shows lower AODs that are closer to
AVHRR globally but at the same time it also provides a weaker initial increase
in 1991. The experiment without QBO shows that less mass is maintained in the
tropics compared to REF, and therefore more mass is transported southward in
1991 following the Brewer–Dobson circulation. The experiment with increased
coagulation efficiency (COAG) shows a faster decay of initial AOD increase.
UPWIND has slight changes but mostly lies within the uncertainties of REF.
Similarly to the size evolution discussed in the previous section, in
general the details of all modeling experiments are also qualitatively consistent
with those shown for the burden in Fig. .
Stratospheric temperature response
Lower tropical stratospheric warming after major eruptions is one of the key
features of volcanic influence on climate e.g.,. It
is a forcing for the thermal wind balance, a mechanism through which
volcanoes can affect high-latitude tropospheric circulation. This warming is
also an important indicator for the aerosol mass distribution in the
stratosphere because it is mostly caused by the infrared absorption of
volcanic aerosols, which does not critically depend on aerosol particle size
. The difficulty of a correct representation of post-volcanic
stratospheric warmings is a known issue of global models. Key factors are,
besides uncertainties in aerosol distributions, model dynamics and radiative
transfer, which in turn also depend on many factors such as spatial and
spectral resolution, the presence and quality of interactive chemistry, and others
.
Zonal mean temperature anomalies from SOCOL-AER for the tropics
(20∘ S–20∘ N) at 30 hPa (a) and
70 hPa (b). Light and dark blue lines: MERRA and ERA-Interim
temperature reanalysis data. Anomalies are computed by subtracting the annual
cycle. Grey shaded area: 2σ ensemble spread of the REF experiment. All
other scenario curves are ensemble means.
Figure compares zonal mean tropical temperature anomalies
computed by SOCOL-AER in the lower stratosphere after the Pinatubo eruption
with the ERA-Interim and MERRA reanalyses. Anomalies are calculated by
subtracting the climatological annual cycle averaged over 1986–2013 for
reanalyses and over 1991–1995 of the noRADnoQBO experiment for all other
model experiments. Although the noRAD experiment has no aerosol radiative
effect, we have also added it to Fig. so that everything
between the noRAD line and other lines can be attributed to the radiative
effect of volcanic aerosols. By mid-1993 this effect is mostly gone and then
all model experiments are in line with reanalyses. Since the lower tropical
stratosphere is a dynamically very active region, the model also shows a large
ensemble spread in the stratospheric temperature signal so that all numerical
experiments and observations generally fall into this variability. While the
temperature anomalies in the reanalyses differ by up to 1 K, the ensemble
mean of REF (black curve) overestimates the warming both at 30 and 70 hPa by
1–2 K in late 1991 and mid-1992. The SOCOL-AER scenarios show some
differences with respect to each other. While the experiment with reduced
emissions (REF12) shows better agreement with reanalyses at both levels, this
apparent improvement comes with clear deteriorations in other quantities,
such as too-small particle sizes (Fig. 3). The scenario with enhanced
coagulation efficiency COAG is warmer at 70 hPa early in 1992, which is
related to the increased sedimentation of larger particles to lower altitudes.
The results of the UPWIND scenario show a smaller warming than REF, which
reflects the larger vertical spread of the aerosol mass due to enhanced
diffusion, leading to faster aerosol removal from the lowermost stratosphere.
Vertical distribution of liquid H2SO4 concentration
averaged over the tropics (20∘ S–20∘ N).
Monthly mean midlatitude (30–60∘ north and south)
ozone anomalies from SOCOL-AER compared with observations. (a) Total
ozone column. (b) Ozone mixing ratio at 20–70 hPa. Observational
data sets SWOOSH and SBUVv8.6 are denoted by light and dark blue lines,
respectively. Anomalies are computed by subtracting the annual cycle. Grey
shaded area: 2σ ensemble spread of the REF experiment. All other
experiments are shown as ensemble means.
To further understand the model results, we plotted the vertical aerosol mass
distribution in the tropics for REF and the SAGE-3,4λ composites in
Fig. . We did not plot the vertical mass distributions from
other experiments because they are all very similar to REF relative to SAGE
data. There are small differences between experiments that are consistent
with the previous analysis; i.e., the UPWIND mass is more vertically diffused,
while COAG results show a faster decay of the whole aerosol cloud and therefore
slightly more mass present at lower levels. The main difference of all
SOCOL-AER experiments to the SAGE-II-derived data is the presence of a large
amount of aerosol mass in 1991 in the lowermost stratosphere, i.e., below
approximately 60 hPa, which is not consistent with the SAGE-II-based
composites, in particular SAGE-3λ. Potentially the SAGE-II-derived
data can be still influenced by the known problems in observing the lower
stratosphere, which became opaque for limb-occultation instruments in 1991
. This is partly confirmed by comparison of SAGE-II-derived data
with HIRS and AVHRR in previous sections. Recently,
analyzed the stratospheric warming after Pinatubo using
SOCOLv3, which has the same dynamical and chemical cores as SOCOL-AER, but
with prescribed aerosols from the SAGE-4λ and SAGE-3λ
composites (dashed and dotted blue curves in Fig. ). While our
SOCOL-AER-based REF simulation overestimates the temperature response at
70 hPa in 1991, both SOCOLv3 simulations driven by the SAGE-II-based data
sets are biased low compared to the reanalysis temperatures
(Fig. b). From a purely radiative point of view neglecting
dynamical feedbacks, this suggests that the correct aerosol mass loading
below 60 hPa lies between our REF simulation and SAGE-II-derived data.
At higher altitudes, i.e., around the level of maximum aerosol loading, all
SOCOL-AER simulations and the SOCOLv3–SAGE-4λ results show a
linear relation between aerosol mass and the resulting warming, while the
SOCOLv3–SAGE-3λ results show a different behavior: despite having the
largest aerosol mass among all considered cases, the simulated temperature
response is smallest (Fig. a). Even considering potential
dynamical effects e.g.,, the SAGE-3λ-based
results are surprising. This encouraged to reassess their
results, and they noticed an issue with the spectral integration of the
SAGE-3λ extinctions. The effect of this spectral integration issue on
the comparison between SAGE-3λ, SAGE-4λ, and SOCOL-AER will
be investigated and discussed further in an upcoming study.
Ozone response
The response of ozone to major volcanic eruptions is subject to a plethora
of dynamical and radiative stratospheric feedbacks, including changes in
heterogeneous chemistry. Previously, it was shown that even the sign of the
total ozone response after a volcanic eruption depends on the stratospheric
halogen loading e.g.,. Volcanic
eruptions can in principle also contribute to stratospheric chlorine,
which further affects ozone e.g.,, but there was no
significant increase in stratospheric chlorine observed after Pinatubo
. and pointed to
the hemispheric asymmetry of the midlatitude ozone response to Pinatubo due to
modified ozone transport from the tropics. Figure compares
monthly mean midlatitude ozone (30–60∘ for both hemispheres) from
SOCOL-AER simulations with the total ozone column from the combined record
SBUV Merged Ozone Data Set version 8.6; and
with the lower stratospheric (20–70 hPa) ozone mixing ratio from the merged
satellite composite SWOOSH Stratospheric Water and Ozone Satellite
Homogenized data set;. Anomalies are obtained by subtracting
monthly means for 1991–1995.
Compared to SBUV, SOCOL-AER does show a decline in ozone column; however, it
underestimates it in the Northern Hemisphere (NH) and overestimates it in the
Southern Hemisphere (SH). REF12, COAG, and UPWIND generally fit into the
ensemble spread of REF. REF also shows an ozone increase in 1991 in the
SH discussed by and , but in late 1991 a
similar increase is shown by the model in the NH, which is not seen in SBUV
data. The noRAD and noQBO results show that parts of these ozone increases in
the SH and NH are due to heating by aerosols in the tropics and due to QBO,
respectively, and both of these peaks disappear in the noRADnoQBO experiment.
This hints that SOCOL-AER dynamics do not adequately respond to perturbations
in the tropics given that QBO is prescribed and post-eruption warming is well
captured, at least by the REF12 experiment (Fig. ). In order
to check this, we performed another experiment with the dynamics nudged to
ERA-Interim reanalysis data with the rest of the settings kept as in REF. We
used the same nudging procedure as described in . The nudged
experiment also shows some differences compared to SBUV, but in general
reproduces its behavior much better, thus suggesting some problems in the
model's dynamics and good performance of the model's chemistry and aerosol
microphysics. Comparison of SOCOL-AER ozone with the SWOOSH composite of
stratospheric satellite measurements leads to the same conclusions for the
lower stratosphere.
Conclusions and discussion
We have simulated the temporal and spatial development of stratospheric
aerosols following the 1991 Pinatubo eruption, as well as temperature and
ozone responses, using SOCOL-AER, a free-running 3-D global chemistry–climate
model coupled with a size-resolving aerosol module. The simulations explore
the roles of the QBO, aerosol radiative heating, sedimentation scheme, and
coagulation efficiency in the evolution of the stratospheric aerosol after
Pinatubo.
The results show that QBO and interactive aerosol radiative heating play
important roles in maintaining the tropical stratospheric aerosol reservoir
over the whole course of volcanic aerosol cloud evolution, significantly
affecting volcanic aerosol lifetime. Furthermore, the results suggest that
an accurate sedimentation scheme helps to improve the model's ability to
reproduce stratospheric aerosol. Numerically diffusive methods, such as a
simple upwind method, must be avoided in modeling studies of large volcanic
eruptions to prevent artificially fast spreading of particles to high and low
altitudes. A more sophisticated coagulation scheme is capable of improving
the comparisons with in situ particle size measurements and
satellite-borne extinction ratios, which are a proxy for particle sizes. On
the other hand, the improved coagulation scheme leads to too-rapid
sedimentation and loss of stratospheric aerosol mass, which become noticeable
in the model about 1 year after the eruption.
There is significant uncertainty among the observational data of different
aerosol parameters. Observations differ by up to ±15 % in the global
aerosol burden, ±30 % in aerosol optical depth and spatiotemporal
aerosol distribution in the 2 years following the eruption, ±40 %
in the effective particle radii, and ±0.5 K in the lower stratospheric
temperature anomalies. This renders the exact determination of the required
emitted sulfur amount difficult. Thus, the vertically integrated tropical
mass simulated by the reference experiment in 1991 (Fig. b)
is in good agreement with HIRS, but later experiences faster decay that is
not consistent with HIRS and SAGE-3λ but closer to SAGE-4λ.
Considering this fact and relying on SAGE-3λ after 1991, we can
assume that our 14 Tg estimate of initial emissions was still sufficient for
our model, but the vertical distribution of the resulting aerosols could be
incorrectly shifted to the lowermost levels. This fact could be responsible
for one of the modeling deficiencies found, namely the 1–2 K larger warming
that is inconsistent with temperature reanalyses. It could also explain the
integrated modeled burden difference to SAGE-3λ since 1992
(Fig. ), as the mass located at lower levels also sediments
faster to the troposphere despite the increased buoyancy produced by additional
warming. The experiment with the reduced emissions revealed much better
representation of the post-eruption stratospheric warming, but at the same
time less optimal agreement with observations of other parameters. In terms
of AOD in the visible part of the spectrum, our model is also closer first to
AVHRR and later to SAGE-4λ than to SAGE-3λ. It is important
to note that the period when all aerosol burden and AOD observational data
overlap in 1992 is perfectly captured by the model. Observed features of the
ozone response appear to be problematic in being reproduced by the model in a
free-running mode, which can be overcome by using a nudged mode. Potentially,
both aerosol lifetime and ozone response can be improved with increased
horizontal and vertical resolution.
There is rising interest among the climate community in global models with
interactive aerosol microphysics. It is caused partly by widely discussed
climate geoengineering, namely a compensation for global warming by
artificial emissions of SO2 e.g.,, and
by the unclear role of major and smaller volcanoes in the future climate
e.g.,. Considering other modeling studies
of Pinatubo effects, our simulations corroborate the results of
who also used a sectional model (LDMZ-S3A) and the
same emission rate of 14 Tg of SO2. Their results also revealed
problems in reproducing aerosol sizes above 25 km and overestimation of
stratospheric warming; however, they attributed the latter to the fact that
aerosol composition is prescribed during the calculation of aerosol optical
properties in LDMZ-S3A (Christoph Kleinschmitt, personal communication,
2017), which is not the case for SOCOL-AERv1.0. The reasons for these and
other revealed problems are to be investigated, as SOCOL-AER is still
undergoing
further development.
The recent Tambora model intercomparison study by
demonstrated that SOCOL-AER has substantial problems in representing the
absolute values of sulfate deposition in the polar regions due to a
simplified tropospheric deposition scheme, but also that SOCOL-AER has the
closest agreement with ice core observations in terms of the timing of the start and
end of volcanic increases in deposition, which is defined by stratospheric
aerosol lifetime. A model intercomparison study for Pinatubo is planned
within the framework of the Stratospheric Sulfur and Its Role in Climate
activity SSiRC;, but, as was also shown here,
aerosol observational uncertainty concerning the eruption of Mt. Pinatubo is
high and will complicate the derivation of exact conclusions for certain
processes and models. Another strong eruption similar to Pinatubo could
significantly improve our understanding of the underlying microphysical and
transport processes given recent advances in measuring techniques
.