The Sign Of The Radiative Forcing From Marine Cloud Brightening
Depends On Both Particle Size And Injection Amount
1
J. E. Kristjánsson
1
Accepted Article
K. Alterskjær,
K. Alterskjær, Department of Geosciences, Meteorology and Oceanography Section,
University of Oslo, P.O.Box 1022, 0315 Oslo, NORWAY. (karialt@geo.uio.no)
J. E. Kristjánsson, Department of Geosciences, Meteorology and Oceanography Section,
University of Oslo, P.O.Box 1022, 0315 Oslo, NORWAY.
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process which may
lead to differences between this version and the Version of Record. Please cite this article
as doi: 10.1002/grl.50117
© 2013 American Geophysical Union. All rights reserved.
Abstract
Marine cloud brightening (MCB) is a proposed technique to limit global warming through
injections of sea spray into the marine boundary layer. Using the Norwegian Earth System
Model, the sensitivity of MCB to sea salt amount and particle size was studied by running a
set of simulations in which Aitken (r e =0.04 µ m), accumulation (r e =0.22 µ m) or coarse
(r e =2.46 µ m) mode sea salt emissions were increased uniformly by 10 −11 to 10 −8 kg m −2
Accepted Article
s −1 . As desired, accumulation mode particles had a negative radiative effect of down to -3.3
W m −2 . Conversely, for Aitken mode particles, injections of 10 −10 kg m −2 s −1 or greater led
to a positive forcing of up to 8.4 W m −2 , caused by a strong competition effect combined
with the high critical supersaturation of Aitken mode sea salt. The coarse mode particles gave
a positive forcing of up to 1.2 W m −2 because of a decrease in activation of background
aerosols. Sensitivity experiments show that the competition effect dominated our results.
MCB may have a cooling effect, but if the wrong size or injection amount is used, our
simulations show a warming effect on the climate system.
1. Introduction
Deliberate engineered cooling of the global climate has received increased scientific interest
over the last decade as mitigation strategies to limit global warming are yet to be of
significance. One climate engineering strategy involves enhancing the albedo of marine
clouds and thus increasing the reflection of solar radiation from the Earth-atmosphere system
[Latham, 1990]. The idea is that spraying sea water into the marine boundary layer will
increase the number of sea salt particles that ascend into overlying clouds and increase their
albedo through the aerosol indirect effect [Twomey, 1974]. This may significantly affect the
global radiation budget because of the low albedo of the underlying ocean surface in the
subtropics, where extensive low clouds are found.
Early estimates of the global radiative effect of marine cloud brightening (MCB) assumed a
certain change in cloud droplet number concentration (CDNC) in seeded clouds and found
that MCB could wholly or partially cancel the positive forcing associated with a doubling of
CO 2 from preindustrial times [Latham et al., 2008; Jones et al., 2009; Rasch
et al., 2009]. Korhonen et al. [2010] used a global aerosol transport model, while Pringle
et al. [2012] used three independent global aerosol models and a box model and Wang et al.
[2011] used a cloud-system-resolving model to investigate what changes in CDNC were
achievable from sea salt injections, but these studies did not include estimates of the radiative
effect of the cloud seeding.
To our knowledge only three global studies have so far included estimates of the forcing from
MCB using fully prognostic treatments of sea salt. Jones and Haywood [2012] studied the
radiative impact and climate effects of wind speed dependent MCB while [Alterskj_r et al.
[2012] injected sea salt uniformly over the ocean to study the geographical distribution of
clouds susceptible to seeding. Partanen et al. [2012] used wind speed dependent emissions
of sea salt and estimated that injecting 20.6 Tg yr −1 into the most sensitive stratocumulus
regions led to a forcing of -0.8 W m −2 . Decreasing the size of the particles by 60 % or
multiplying the injections by five led to a forcing of -2.1 W m −2 and -2.2 W m −2 ,
respectively.
In this study, we have investigated further the importance of particle size and injection
strength. This is necessary to understand the outcome of a potential seeding measure - what
sea salt size category would be most effective and therefore least costly?; is the same size
© 2013 American Geophysical Union. All rights reserved.
Accepted Article
category most effective for all injection mass fluxes?; and is there a simple linear relation
between injection strength and forcing? We have used the Norwegian Earth System Model
(NorESM) [Bentsen et al., 2012] to investigate the global sensitivity to particle size and
injection strength. We have looked at how injections of sea salt affect the cloud radiative
properties and how the increased competition effect resulting from the added sea salt affects
our global estimates. In section 2 we describe the model used and the experimental design,
while in section 3 we go through the results of our experiments, including a presentation and
discussion of sensitivity experiments performed to test the robustness of our findings. We
summarize and conclude in section 4.
2. Model and Methods
2.1. NorESM
Simulations were performed using the NorESM, which is based on the NCAR (National
Center for Atmospheric Research) CCSM4 (Community Climate System Model version 4),
but includes new treatments of clouds, aerosols, aerosol-radiation and aerosol-cloud
interactions and chemistry [Bentsen et al., 2012], along with a new ocean model component.
The aerosol module accounts for prognostic sea salt, sulfate (SO 4 ), particulate organic
matter, black carbon and mineral dust as well as two gaseous aerosol precursors producing
sulfate (DMS and SO 2 ). The model uses the M_artensson et al. [2003] scheme for wind
speed and temperature dependent sea salt emissions [Struthers et al., 2011]. It includes sea
salt particles with dry number modal radii of 0.022 µ m (Aitken mode), 0.13 µ m
(accumulation mode) and 0.74 µ m (coarse mode) and geometric standard deviations of 1.59,
1.59 and 2.0, respectively, corresponding to dry effective radii of 0.04 µ m, 0.22 µ m and
2.46 µ m.
The aerosol indirect effect is accounted for as described in Hoose et al. [2009] and has a
magnitude of -0.91 W m −2 in the current set up. The model uses the Abdul-Razzak and Ghan
[2000] cloud droplet nucleation scheme and parametrized updraft velocitiesfollowing
Morrison and Gettelman [2008], with annually averaged in-cloud velocities ranging between
about 10 cm s −1 and 100 cm s −1 at a model hybrid level at  945 hPa over ocean (see
auxiliary material). For an overview of typically observedupdrafts we refer the reader to
Pringle et al. [2012]. Model cloud properties were compared to satellite retrievals in e. g.
Alterskj_r et al. [2012] and Jiang et al. [2012] indicating a general underestimation of
marine cloud CDNC and an overestimation of the simulated cloud liquid water path (LWP).
The low CDNC makes the model more susceptible to MCB, while the high LWP has the
opposite effect. Alterskj_r et al. [2012] also showed that the simulated cloud fraction below
700 hPa is generally overestimated, except for a slight underestimation in the sub-tropical
stratocumulus regions.
2.2. Experimental Design
Cloud brightening simulations were performed by artificially increasing the emissions of
each of the three fully prognostic NorESM sea salt modes, meaning that the injected sea salt
was treated in a similar manner to natural sea salt and that no explicit assumptions were made
on their ability to act as cloud condensation nuclei (CCN). The emissions were uniform and
confined between 30  S and 30  N based on the findings of Alterskj_r et al. [2012]. Most
earlier studies have used wind speed dependent emissions due to the experimental design
© 2013 American Geophysical Union. All rights reserved.
Accepted Article
proposed by Salter et al. [2008]. For simplicity, this dependency is not included in our
idealized study as we believe that the emission technology may change prior to possible
implementation. Injection fluxes ranged from 10 −11 kg m −2 s −1 to 10 −8 kg m −2 s −1
(Table 1(a)), which corresponded to a global sea salt emission increase of from 0.9 % to 913
% . A list of all simulations performed is shown in Table 1(b).
The model was run offline, meaning that the meteorological evolution remains unchanged
between simulations, so that the simulated change in radiative forcing is due to indirect
effects only. The control run uses year 2000 greenhouse gas concentrations and year 2000
CMIP 5 aerosol emissions and the model resolution is 1.9  x 2.5  . It runs with 26 vertical
levels, with a top at about 2 hPa. All simulations include one year of spin up and results
presented are averaged over the four following years.
3. Results
3.1. Reference Simulations
The radiative effect of sea salt injections was studied by investigating the resulting change in
shortwave cloud forcing (SWCF) at the top of the atmosphere. Note that the longwave effect
was negligible. Figure 1(a) shows the resulting radiative effect,where reference model runs
are marked by Umin = 10 cm/s as this is the minimum sub-grid scale in-cloud vertical
velocity used in the model parametrisation [Morrison and Gettelman, 2008]. The
accumulation mode sea salt is closest to the particle size suggested fit for cloud brightening
by Latham [2002] (0.13 µ m dry radius), and all simulations with injections of this mode
(dark green) show the negative radiative effect desired from climate engineering, with a
maximum negative forcing of -3.3 W m −2 . By comparison, a doubling of atmospheric CO 2
yields a positive forcing of about 3.7 W m −2 . Figures 1(b) and 1(c) show that the negative
forcing associated with MCB is directly linked to an increase in the column integrated
CDNC, which in turn leads to smaller droplets, a decrease in precipitation release and
therefore to an increase in the LWP.
Figure 1(a) also reveals a weaker negative forcing for the largest compared to the second
largest injections of the accumulation mode sea salt. As described in Alterskj_r et al. [2012]
this can be caused by an increased competition effect, because the added sea salt particles
swell, creating a moisture sink which lowers the maximum supersaturation (S) and therefore
increases the critical size that particles must have to activate [Ghan et al., 1998; Korhonen et
al., 2010]. This competition effect may bring the S below that necessary to activate
background aerosols and in some cases below that necessary to activate the added sea salt
itself. Figure 1(d) shows that as the injection flux goes up, the S goes down monotonically, as
found in Korhonen et al. [2010] and Wang et al. [2011], and for the accumulation mode
10 −8 kg m −2 s −1 case, the globally and annually averaged S is brought down to only 0.078 %.
Based on Köhler theory, this is well above the critical S of 0.013 % needed to activate
accumulation mode sea salt, but very close to the critical S of the model’s accumulation mode
droplets of pure H 2 SO 4 of 0.07 % (Fig. 1(d), lower purple dashed line). Our results show that
regionally and temporarily the Sgoes below the value necessary for activation of
accumulation mode sulfate, and there is a reduction in activation of background aerosols
leading to a drop in both the CDNC and the LWP (Fig. 1(b) and 1(c)), so that the efficiency
of the seedinggoes down for the largest injection mass.
Figures 1(e) and 1(f) show the average aerosol number concentration and CDNC obtained in
the latitudinal band where sea salt is injected for the model level where the subtropical
© 2013 American Geophysical Union. All rights reserved.
Accepted Article
stratocumulus base is found most frequently (945 hPa). The aerosol number concentration is
on the order of 10 3 cm −3 to 10 6 cm −3 . For comparison, polluted urban areas have particle
concentrations of the order of 10 5 cm −3 [Pruppacher and Klett , 1997]. Only one simulation
gives an averaged CDNC above 375 cm −3 , a value suggested to offset the radiative forcing of
a doubling of CO 2 [Latham et al., 2008].
The change in SWCF resulting from injections of Aitken mode sea salt is shown as dark blue
columns in Fig. 1(a). For the smallest injection amount the cooling effect of Aitken mode sea
salt is larger than for accumulation mode sea salt. This is because we add a larger number of
particles per injection mass for the Aitken mode, and more particles are activated to form
cloud droplets (Fig. 1(b)). A large increase in CDNC leads to an increase in LWP that is
larger for Aitken mode than for accumulation mode injections for this mass flux.
Figure 1(d) shows that the suppression of S is greater for the Aitken mode particles than for
accumulation or coarse mode particles, which is due to the larger surface area for water vapor
to condense on. Also, as the particles themselves are small in the Aitken mode injections,
their critical S (0.16 %, Fig. 1(d), upper purple dashed line) is higher than for accumulation
mode sea salt. Figure 1(d) shows that the S is above this value only for the smallest injection
mass flux of Aitken mode sea salt.For injection strengths of 10 −10 kg m −2 s −1 or greater, the
water vapor condensing on the added sea salt brings the globally averaged S below that
necessary to activate the Aitken mode sea salt itself. Thus the added particles may not
contribute as CCN and the lowered S instead suppresses activation of background aerosols.
As a result, the simulated change in SWCF is positive and increasingly more so as the
injection mass flux increases, reaching a maximum forcing in our simulations of +8.4 W
m −2 . This is the opposite effect of what one would seek to achieve if performing climate
engineering.
The radiative effect of injecting coarse mode sea salt is positive in our simulations, albeit
much less so than for the Aitken mode particles (Fig. 1(a), dark red bars). The reason for this
warming effect is twofold. Firstly, the number of particles permass is small for the coarse
mode case, so that while the sea salt particles are always large enough to activate, in number
they do not contribute much to the CDNC. Secondly, while the added surface area is low for
this mode, water vapor does condense on the sea salt and the S is decreased (Fig. 1(d)). This
keeps some of the background aerosols from activating, and as long as this number is greater
than the number of new droplets created on the coarse mode sea salt, there will be a warming
effect. Figure 1 shows that both the CDNC and the LWP decrease with increasing injection
strength. The radiative effect of this sea salt mode increases substantially for the largest
injection flux, for which a positive forcing of 1.2 W m −2 is reached (Fig. 1(a)). In this case,
the globally averaged S is brought below 0.16 % (Fig. 1(d)), the critical S of Aitken mode sea
salt. This indicates that injecting enough coarse mode sea salt to shut off the activation of
natural Aitken mode sea salt may leadto a warming of the climate system.
In agreement with our findings, Pringle et al. [2012] found in their box model investigation
that a decrease in CDNC was possible under certain conditions. However, these conditions
were not met in their global study, possibly due to their moderate injection amounts relative
to our values.
3.2. Sensitivity Tests
The maximum simulated negative forcing of -3.3 W m −2 was achieved when injecting 10 −9
kg m −2 s −1 of sea salt between 30  S and 30  N, corresponding to injecting 5936 Tg of sea
salt per year. By comparison Partanen et al. [2012] simulated a forcing of -5.1 W m −2 when
injecting only 443.9 Tg yr −1 of sea salt of a comparable size distributed over all ocean
© 2013 American Geophysical Union. All rights reserved.
Accepted Article
regions. A direct comparison of these two studies is difficult due to the differences in
experimental design, but the findings indicate that NorESM may be less sensitive to sea salt
seeding than the ECHAM5.5-HAM2 used by Partanen et al. [2012]. Firstly, the forcing
estimate from Partanen et al. [2012] was based on a simulation without “ultrafine” seasalt
(dry diameter < 100 nm), which was included in our study. Including these particles in their
model weakened their forcing estimate to -4.5 W m −2 . Secondly, the updraft vertical velocity
in the Partanen et al. [2012] study was very high, ranging between 1.0 and 1.4 m s −1 , which
according to the authors eliminated the competition effect from their study. Our results
indicate that the competition effect significantly reduces the maximum in-cloud
supersaturation. If overestimated, this competition effect will influence our results
substantially and may lead to an underestimation of the effectiveness of MCB.
To investigate this closer we conducted two sets of sensitivity simulations. In the first set, the
maximum in-cloud supersaturation was set to a fixed value of 0.2 % (annually averaged
control run S is 0.18 % globally around 945 hPa) and the model was run for an injection
strength of 10 −10 kg m −2 s −1 for the Aitken and accumulation modes and for a strength of
10 −8 kg m −2 s −1 for the coarse mode. The second set involved increasing the minimum subgridscale in-cloud vertical velocity in the Morrison and Gettelman [2008] parametrisation
from 10 cm s −1 to 30 cm s −1 . This increased the average updraft velocity between 30  S and
30  N from 30.4 cm s −1 to 41.6 cm s −1 around 945 hPa (see auxiliary material) and increased
the maximum in-cloud supersaturation. This setup was run for all injection strengths for the
Aitken and the accumulation mode injections and for the 10 −10 kg m −2 s −1 injections for the
coarse mode sea salt.
3.2.1. Fixed Supersaturation
Setting the in-cloud supersaturation to a fixed value increased the magnitude of the negative
forcing of the accumulation mode sea salt from -1.27 W m −2 to -1.43 W m −2 for an injection
strength of 10 −10 kg m −2 s −1 (Fig. 1(a); green dashed line). More strikingly, for the Aitken
mode, removing the competition effect led to a change in the sign of the resulting forcing;
going from +1.27 W m −2 to -5.91 W m −2 . The high number to mass ratio combined with an
S that is above the critical S of the Aitken mode sea salt (0.16 %), leads to a very large
cooling effect. These results confirm that the competition effect dramatically limits the
simulated cooling effect achieved from cloud seeding in the reference simulations.
Latham et al. [2008] suggested that seeding clouds with coarse mode sea salt particles might
lead to a warming because the “giant salt nuclei” may lead to an early onset of precipitation
and a decrease in LWP. We investigated this by running the 10 −8 kg m −2 s −1 coarse mode
case with a constant S of 0.2 %. This brought the change in SWCF from 1.2 W m −2 (Fig.
1(a)) to 0 W m −2 , indicating that there is no contribution to the simulated warming in our
coarse mode results from other sources than the competition effect.
3.2.2. Enhanced Minimum Vertical Velocity
Increasing the minimum in-cloud vertical velocity in the second set of sensitivity simulations
led to an increase in the globally and annually averaged control run S from 0.18 % to 0.26 %
around 945 hPa (Fig. 1(d); yellow bars). This led to activation of smaller background
particles in the increased updraft simulations (Umin30) than in the reference simulations (ref)
and therefore increased the CDNC (Fig. 1(b); yellow bars) and decreased the effective radius
around 945 hPa from 8.7 u m in ref to 8.2 µ m in Umin30. The LWP increased from 126
© 2013 American Geophysical Union. All rights reserved.
Accepted Article
gm −2 in ref to 137 gm −2 in the Umin30 simulation due to suppression of precipitation.
Combined, the influence on the effective radius and the LWP led to a control run that had a
higher cloud albedo in Umin30 than in the ref.
For accumulation mode sea salt injections, the radiative forcing achieved was smaller in
magnitude than in the ref simulations, except for the simulation of maximum sea salt seeding
mass (Fig. 1(a); green bars). The small magnitude was caused by (i) reduced precipitation
suppression due to high control run CDNC [Rasch and Kristj_ansson, 1998, Eq. 21] and (ii)
a reduced sensitivity for albedo to change with LWP due to high control run LWP; Assuming
an asymmetry parameter of 0.85 and an optical depth τ = ac3LWP 2 ρ L re gives a change in
cloud albedo, A [Hobbs, 1993], with LWP of:
4.67 ρ L re
δA
(1)
=
δ LWP ( LWP + 4.67 ρ L re ) 2
where re is the cloud droplet effective radius and ρ L the density of water. Combined, (i) and
(ii) led to a smaller radiative effect of MCB for Umin30 than for ref accumulation mode
seeding.
For the maximum injections of accumulation mode sea salt, the high Umin30 control run S
needed a large reduction to bring it below that necessary to activate background sulfate.
Contrary to ref simulations, the Umin30 simulations resulted in an S that was well above the
critical limit (Fig. 1(d); lower purple dashed line). The reduced competition effect resulted in
a MCB that still served to increase the CDNC and LWP and therefore to increase the
magnitude of the SW cloud forcing.
For the Aitken mode, Fig. 1(d) (light blue columns) shows that both the 10 −11 and the 10 −10
kg m −2 s −1 injections have Umin30 S around 945 hPa that are above the critical limit to
activate the added Aitken mode sea salt. The two cases therefore led to negative forcing,
which for the smallest injections was stronger than the forcing produced in the ref simulation
(Fig. 1(a)). The increased updraft led to increased activation of the added sea salt particles
themselves and therefore to a more effective MCB, while for the corresponding accumulation
mode case the increased updraft mainly increased activation of background particles, thus
leading to a decreased the efficiency of MCB. The high Umin30 control run S leads to a
“delay” in the competition effect, reducing the positive forcing for high Aitken mode
injections because more sea salt is needed to remove enough water vapor to bring the S below
that necessary for activation of the Aitken mode particles.
For the coarse mode, we saw no significant change between the ref and the Umin30
simulations.
The sensitivity experiments greatly suppress the competition effect, but nevertheless do not
lead to negative radiative flux perturbations of the same magnitude as that found by
Partanen et al. [2012]. One possible reason is that the Umin30 simulations have average
updrafts of 41.6 cm s −1 around 945 hPa (see auxiliary material), which is still well below that
of Partanen et al. [2012]. The NorESM may also be less sensitive to the added sea salt than
the ECHAM5.5-HAM2 used by Partanen et al. [2012] dueto the large model LWP (Eq. 1).
Additionally, [18] seeded all ocean areas, while we only seeded between 30  S and 30  N.
This may influence the forcing estimates because of the nonlinear relation between added
mass and forcing caused by e. g. differences in the regional meteorological conditions - are
there low clouds above?; Are there updrafts that can carry the injected sea salt aloft?; Is the
particle number in the region already high rendering the clouds less sensitive to the injected
sea salt?
This study does not account for kinetic limitations on the activation of giant CCN [Chuang et
al., 1997]. In regions of high S this may lead to an overestimated CDNC, while the opposite
© 2013 American Geophysical Union. All rights reserved.
may be true in regions of low S where an overestimated droplet activation may lead to an
exaggerated competition effect. Not accounting for this kinetic limitation may be especially
important for the coarse mode sea salt injections, for which we expect the simulated positive
radiative forcing to be an upper estimate.
4. Conclusions
Accepted Article
In this study we have investigated how deliberate injections of sea salt into the marine
boundary layer affect the global radiative budget as a function of both particle size category
and injection mass flux. Using the NorESM we find that injecting accumulation mode sea salt
between 30  S and 30  N leads to a desired negative radiative effect of down to -3.3 W m −2
which would almost cancel the positive forcing of a doubling of atmospheric CO 2 . On the
other hand, for Aitken mode injections greater or equal to 10 −10 kg m −2 s −1 , the simulated
net radiative effect is positive, reaching a maximum of 8.4 W m −2 . This is because the
competition effect reduces the maximum supersaturation and suppresses activationof both the
added Aitken mode sea salt and background particles, leading to a decrease in both CDNC
and LWP. When coarse mode sea salt is injected, the simulated net radiative effect is always
positive. While the coarse mode sea salt itself is large enough to activate, its small number to
mass ratio leads to a lower increase in CDNC due to the added sea salt than the decrease in
activation of background aerosols due to the competition effect.
We also performed sensitivity tests which show that the size of the competition effect is
crucial for the simulated forcing achieved. Omitting this effect led to a negative forcing of 5.9 W m −2 when injecting 10 −10 kg m −2 s −1 of Aitken mode sea salt, whereas the same
injections gave a positive forcing of 1.3 W m −2 in the reference simulation. Increasing the
minimum in-cloud updraft velocity increased the S and led to a “delay” in the competition
effect. However,for accumulation mode injections, the increased updraft mainly led to
increased activation of background aerosols, leading to a reduced efficiency of the MCB.
The results presented in this study clearly show that the effectiveness of MCB is very
sensitive to the injection mass flux and particle size. While a cooling effect is simulated for
certain sea salt injections, emitting the wrong particle size or thewrong amount leads to a
simulated warming of the climate system, which is opposite to what one seeks to achieve by
climate engineering.
This study aims to be a first step in investigating the global effect of MCB of different sized
particles and injection amounts. Not all sea salt particle sizes are represented and the model
resolution is coarse. Wood [2007] showed that the cloud lifetime effect may change sign
depending on the vertical placement of the cloud base and Wang et al. [2011] showed that
the cloud albedo effect depends strongly on the fine scale atmospheric state, neither of which
are well represented in climate models. However, coarse models such as the NorESM are
currently the only tool available to study the global effects of MCB.
© 2013 American Geophysical Union. All rights reserved.
Accepted Article
Acknowledgments: This study was partly funded by the European Commission’s 7th
Framework Program through the IMPLICC project (FP7-ENV-2008-1-226567), and by the
Norwegian Research Council through the EarthClim project (207711/E10) and its programme
for supercomputing (NOTUR) through a grant for computing time. The authors are thankful
to Philip Rasch and to the IMPLICC consortium for constructive discussions.
© 2013 American Geophysical Union. All rights reserved.
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Accepted Article
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© 2013 American Geophysical Union. All rights reserved.
Accepted Article
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© 2013 American Geophysical Union. All rights reserved.
Accepted Article
Figure 1. Dark colored bars present results from reference simulations (minimum updraft velocity of 10
cm s
−1
−1
) while light colored bars present results from simulations with a minimum updraft velocity of 30
cm s . Yellow bars represent control run values for the reference run and the run of increased updraft,
respectively. Figures (a) through (d) show global annual averages, while (e) and (f) show annual results


averaged over the injection region between 30 S and 30 N. (a) Change in shortwave cloud forcing [W
m
−2
] with MCB. Stippled lines are results from simulations of fixed supersaturation; (b) Column
−2
−2
integrated cloud droplet number concentration [cm ]; (c) Change in liquid water path [g m ]; (d)
Maximum in-cloud supersaturation [%] at model hybrid layer  945 hPa over ocean. Dashed lines
indicate the critical supersaturation of Aitken mode sea salt (upper) and accumulation mode SO 4
(lower); (e) Aerosol number concentration [cm
−3
] at  945 hPa; (f) In-cloud CDNC [cm
−3
] at
 945 hPa.
© 2013 American Geophysical Union. All rights reserved.
Table 1. (a) Sea salt injections used to simulate MCB. (b) List of simulations in which Xs indicate
Accepted Article
simulations performed. The top row lists the names used to identify injection amounts in kg m
−2
s
−1
.
(a) Sea Salt Injections
Injections, 30  S to 30  N [kg m −2 s −1 ] 10 −11 10 −10 3*10 −10 10 −9
10 −8
48.2 595.2 1785.5 5935.6 59436
Total injections [Tg yr −1 ]
Increase in global sea salt emissions
0.9% 9.1% 27.4% 91.3% 913.1%
(b) List of Simulations
Simulation name
Control10 −11 10 −10 3*10 −10 10 −9 10 −8
Reference simulations (ref)
X
Aitken mode
Accumulation mode
Coarse mode
Fixed supersaturation
X
Aitken mode
Accumulation mode
Coarse mode
Enhanced vertical velocity (Umin30) X
Aitken mode
Accumulation mode
Coarse mode
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
© 2013 American Geophysical Union. All rights reserved.