GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,
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2
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The Sign of the Radiative Forcing from Marine
Cloud Brightening Depends on Both Particle Size
and Injection Amount
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1
K. Alterskjær, J. E. Kristjánsson
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.
1
Department of Geosciences, Meteorology
and Oceanography Section, University of
Oslo, Oslo, NORWAY.
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Marine cloud brightening (MCB) is a proposed technique to limit global
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warming through injections of sea spray into the marine boundary layer. Us-
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ing the Norwegian Earth System Model, the sensitivity of MCB to sea salt
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amount and particle size was studied by running a set of simulations in which
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Aitken (re =0.04µm), accumulation (re =0.22µm) or coarse (re =2.46µm) mode
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sea salt emissions were increased uniformly by 10−11 to 10−8 kg m−2 s−1 . As
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desired, accumulation mode particles had a negative radiative effect of down
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to -3.3 W m−2 . Conversely, for Aitken mode particles, injections of 10−10 kg
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m−2 s−1 or greater led to a positive forcing of up to 8.4 W m−2 , caused by
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a strong competition effect combined with the high critical supersaturation
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of Aitken mode sea salt. The coarse mode particles gave a positive forcing
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of up to 1.2 W m−2 because of a decrease in activation of background aerosols.
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Sensitivity experiments show that the competition effect dominated our re-
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sults. MCB may have a cooling effect, but if the wrong size or injection amount
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is used, our simulations show a warming effect on the climate system.
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1. Introduction
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Deliberate engineered cooling of the global climate has received increased scientific
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interest over the last decade as mitigation strategies to limit global warming are yet to be
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of significance. One climate engineering strategy involves enhancing the albedo of marine
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clouds and thus increasing the reflection of solar radiation from the Earth-atmosphere
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system [Latham, 1990]. The idea is that spraying sea water into the marine boundary layer
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will increase the number of sea salt particles that ascend into overlying clouds and increase
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their albedo through the aerosol indirect effect [Twomey, 1974]. This may significantly
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affect the global radiation budget because of the low albedo of the underlying ocean surface
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in the subtropics, where extensive low clouds are found.
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Early estimates of the global radiative effect of marine cloud brightening (MCB) as-
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sumed a certain change in cloud droplet number concentration (CDNC) in seeded clouds
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and found that MCB could wholly or partially cancel the positive forcing associated with
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a doubling of CO2 from preindustrial times [Latham et al., 2008; Jones et al., 2009; Rasch
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et al., 2009]. Korhonen et al. [2010] used a global aerosol transport model, while Pringle
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et al. [2012] used three independent global aerosol models and a box model and Wang
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et al. [2011] used a cloud-system-resolving model to investigate what changes in CDNC
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were achievable from sea salt injections, but these studies did not include estimates of the
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radiative effect of the cloud seeding.
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To our knowledge only three global studies have so far included estimates of the forcing
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from MCB using fully prognostic treatments of sea salt. Jones and Haywood [2012]
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studied the radiative impact and climate effects of wind speed dependent MCB while
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Alterskjær et al. [2012] injected sea salt uniformly over the ocean to study the geographical
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distribution of clouds susceptible to seeding. Partanen et al. [2012] used wind speed
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dependent emissions of sea salt and estimated that injecting 20.6 Tg yr−1 into the most
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sensitive stratocumulus regions led to a forcing of -0.8 W m−2 . Decreasing the size of the
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particles by 60 % or multiplying the injections by five led to a forcing of -2.1 W m−2 and
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-2.2 W m−2 , respectively.
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In this study, we have investigated further the importance of particle size and injection
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strength. This is necessary to understand the outcome of a potential seeding measure -
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what sea salt size category would be most effective and therefore least costly?; is the same
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size category most effective for all injection mass fluxes?; and is there a simple linear
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relation between injection strength and forcing? We have used the Norwegian Earth
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System Model (NorESM) [Bentsen et al., 2012] to investigate the global sensitivity to
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particle size and injection strength. We have looked at how injections of sea salt affect
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the cloud radiative properties and how the increased competition effect resulting from the
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added sea salt affects our global estimates. In section 2 we describe the model used and
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the experimental design, while in section 3 we go through the results of our experiments,
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including a presentation and discussion of sensitivity experiments performed to test the
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robustness of our findings. We summarize and conclude in section 4.
2. Model and Methods
2.1. NorESM
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Simulations were performed using the NorESM, which is based on the NCAR (National
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Center for Atmospheric Research) CCSM4 (Community Climate System Model version
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4), but includes new treatments of clouds, aerosols, aerosol-radiation and aerosol-cloud
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interactions and chemistry [Kirkevåg et al., 2012], along with a new ocean model com-
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ponent. The aerosol module accounts for prognostic sea salt, sulfate (SO4 ), particulate
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organic matter, black carbon and mineral dust as well as two gaseous aerosol precursors
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producing sulfate (DMS and SO2 ). The model uses the Mårtensson et al. [2003] scheme
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for wind speed and temperature dependent sea salt emissions [Struthers et al., 2011]. It
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includes sea salt particles with dry number modal radii of 0.022 µm (Aitken mode), 0.13
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µm (accumulation mode) and 0.74 µm (coarse mode) and geometric standard deviations
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of 1.59, 1.59 and 2.0, respectively, corresponding to dry effective radii of 0.04 µm, 0.22
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µm and 2.46 µm.
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The aerosol indirect effect is accounted for as described in Hoose et al. [2009] and has
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a magnitude of -0.91 W m−2 in the current set up. The model uses the Abdul-Razzak and
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Ghan [2000] cloud droplet nucleation scheme and parametrized updraft velocities follow-
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ing Morrison and Gettelman [2008], with annually averaged in-cloud velocities ranging
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between about 10 cm s−1 and 100 cm s−1 at a model hybrid level at ∼945 hPa over ocean
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(see auxiliary material). For an overview of typically observed updrafts we refer the reader
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to Pringle et al. [2012]. Model cloud properties were compared to satellite retrievals in
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e. g. Alterskjær et al. [2012] and Jiang et al. [2012] indicating a general underestimation
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of marine cloud CDNC and an overestimation of the simulated cloud liquid water path
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(LWP). The low CDNC makes the model more susceptible to MCB, while the high LWP
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has the opposite effect. Alterskjær et al. [2012] also showed that the simulated cloud
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fraction below 700 hPa is generally overestimated, except for a slight underestimation in
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the sub-tropical stratocumulus regions.
2.2. Experimental Design
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Cloud brightening simulations were performed by artificially increasing the emissions of
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each of the three fully prognostic NorESM sea salt modes, meaning that the injected sea
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salt was treated in a similar manner to natural sea salt and that no explicit assumptions
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were made on their ability to act as cloud condensation nuclei (CCN). The emissions
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were uniform and confined between 30◦ S and 30◦ N based on the findings of Alterskjær
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et al. [2012]. Most earlier studies have used wind speed dependent emissions due to the
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experimental design proposed by Salter et al. [2008]. For simplicity, this dependency is
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not included in our idealized study as we believe that the emission technology may change
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prior to possible implementation. Injection fluxes ranged from 10−11 kg m−2 s−1 to 10−8
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kg m−2 s−1 (Table 1(a)), which corresponded to a global sea salt emission increase of from
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0.9 % to 913 %. A list of all simulations performed is shown in Table 1(b).
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The model was run offline, meaning that the meteorological evolution remains un-
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changed between simulations, so that the simulated change in radiative forcing is due to
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indirect effects only. The control run uses year 2000 greenhouse gas concentrations and
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year 2000 CMIP 5 aerosol emissions and the model resolution is 1.9◦ x 2.5◦ . It runs with
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26 vertical levels, with a top at about 2 hPa. All simulations include one year of spin up
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and results presented are averaged over the four following years.
3. Results
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3.1. Reference Simulations
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The radiative effect of sea salt injections was studied by investigating the resulting
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change in shortwave cloud forcing (SWCF) at the top of the atmosphere. Note that the
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longwave effect was negligible. Figure 1(a) shows the resulting radiative effect, where ref-
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erence model runs are marked by Umin = 10 cm/s as this is the minimum sub-grid scale
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in-cloud vertical velocity used in the model parametrisation [Morrison and Gettelman,
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2008]. The accumulation mode sea salt is closest to the particle size suggested fit for
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cloud brightening by Latham [2002] (0.13 µm dry radius), and all simulations with injec-
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tions of this mode (dark green) show the negative radiative effect desired from climate
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engineering, with a maximum negative forcing of -3.3 W m−2 . By comparison, a dou-
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bling of atmospheric CO2 yields a positive forcing of about 3.7 W m−2 . Figures 1(b) and
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1(c) show that the negative forcing associated with MCB is directly linked to an increase
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in the column integrated CDNC, which in turn leads to smaller droplets, a decrease in
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precipitation release and therefore to an increase in the LWP.
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Figure 1(a) also reveals a weaker negative forcing for the largest compared to the second
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largest injections of the accumulation mode sea salt. As described in Alterskjær et al.
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[2012] this can be caused by an increased competition effect, because the added sea salt
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particles swell, creating a moisture sink which lowers the maximum supersaturation (S)
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and therefore increases the critical size that particles must have to activate [Ghan et al.,
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1998; Korhonen et al., 2010]. This competition effect may bring the S below that necessary
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to activate background aerosols and in some cases below that necessary to activate the
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added sea salt itself. Figure 1(d) shows that as the injection flux goes up, the S goes
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down monotonically, as found in Korhonen et al. [2010] and Wang et al. [2011], and for
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the accumulation mode 10−8 kg m−2 s−1 case, the globally and annually averaged S is
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brought down to only 0.078 %. Based on Köhler theory, this is well above the critical S
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of 0.013 % needed to activate accumulation mode sea salt, but very close to the critical
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S of the model’s accumulation mode droplets of pure H2 SO4 of 0.07 % (Fig. 1(d), lower
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purple dashed line). Our results show that regionally and temporarily the S goes below
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the value necessary for activation of accumulation mode sulfate, and there is a reduction in
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activation of background aerosols leading to a drop in both the CDNC and the LWP (Fig.
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1(b) and 1(c)), so that the efficiency of the seeding goes down for the largest injection
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mass.
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Figures 1(e) and 1(f) show the average aerosol number concentration and CDNC ob-
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tained in the latitudinal band where sea salt is injected for the model level where the
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subtropical stratocumulus base is found most frequently (945 hPa). The aerosol number
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concentration is on the order of 103 cm−3 to 106 cm−3 . For comparison, polluted urban
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areas have particle concentrations of the order of 105 cm−3 [Pruppacher and Klett, 1997].
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Only one simulation gives an averaged CDNC above 375 cm−3 , a value suggested to offset
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the radiative forcing of a doubling of CO2 [Latham et al., 2008].
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The change in SWCF resulting from injections of Aitken mode sea salt is shown as dark
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blue columns in Fig. 1(a). For the smallest injection amount the cooling effect of Aitken
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mode sea salt is larger than for accumulation mode sea salt. This is because we add a
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larger number of particles per injection mass for the Aitken mode, and more particles
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are activated to form cloud droplets (Fig. 1(b)). A large increase in CDNC leads to an
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increase in LWP that is larger for Aitken mode than for accumulation mode injections for
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this mass flux.
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Figure 1(d) shows that the suppression of S is greater for the Aitken mode particles
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than for accumulation or coarse mode particles, which is due to the larger surface area
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for water vapor to condense on. Also, as the particles themselves are small in the Aitken
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mode injections, their critical S (0.16 %, Fig. 1(d), upper purple dashed line) is higher
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than for accumulation mode sea salt. Figure 1(d) shows that the S is above this value
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only for the smallest injection mass flux of Aitken mode sea salt. For injection strengths
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of 10−10 kg m−2 s−1 or greater, the water vapor condensing on the added sea salt brings
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the globally averaged S below that necessary to activate the Aitken mode sea salt itself.
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Thus the added particles may not contribute as CCN and the lowered S instead suppresses
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activation of background aerosols. As a result, the simulated change in SWCF is positive
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and increasingly more so as the injection mass flux increases, reaching a maximum forcing
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in our simulations of +8.4 W m−2 . This is the opposite effect of what one would seek to
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achieve if performing climate engineering.
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The radiative effect of injecting coarse mode sea salt is positive in our simulations,
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albeit much less so than for the Aitken mode particles (Fig. 1(a), dark red bars). The
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reason for this warming effect is twofold. Firstly, the number of particles per mass is small
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for the coarse mode case, so that while the sea salt particles are always large enough to
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activate, in number they do not contribute much to the CDNC. Secondly, while the added
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surface area is low for this mode, water vapor does condense on the sea salt and the S is
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decreased (Fig. 1(d)). This keeps some of the background aerosols from activating, and
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as long as this number is greater than the number of new droplets created on the coarse
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mode sea salt, there will be a warming effect. Figure 1 shows that both the CDNC and
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the LWP decrease with increasing injection strength. The radiative effect of this sea salt
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mode increases substantially for the largest injection flux, for which a positive forcing of
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1.2 W m−2 is reached (Fig. 1(a)). In this case, the globally averaged S is brought below
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0.16 % (Fig. 1(d)), the critical S of Aitken mode sea salt. This indicates that injecting
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enough coarse mode sea salt to shut off the activation of natural Aitken mode sea salt
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may lead to a warming of the climate system.
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In agreement with our findings, Pringle et al. [2012] found in their box model investi-
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gation that a decrease in CDNC was possible under certain conditions. However, these
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conditions were not met in their global study, possibly due to their moderate injection
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amounts relative to our values.
3.2. Sensitivity Tests
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The maximum simulated negative forcing of -3.3 W m−2 was achieved when injecting
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10−9 kg m−2 s−1 of sea salt between 30◦ S and 30◦ N, corresponding to injecting 5936 Tg
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of sea salt per year. By comparison Partanen et al. [2012] simulated a forcing of -5.1
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W m−2 when injecting only 443.9 Tg yr−1 of sea salt of a comparable size distributed
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over all ocean regions. A direct comparison of these two studies is difficult due to the
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differences in experimental design, but the findings indicate that NorESM may be less
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sensitive to sea salt seeding than the ECHAM5.5-HAM2 used by Partanen et al. [2012].
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Firstly, the forcing estimate from Partanen et al. [2012] was based on a simulation without
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“ultrafine” sea salt (dry diameter < 100 nm), which was included in our study. Including
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these particles in their model weakened their forcing estimate to -4.5 W m−2 . Secondly, the
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updraft vertical velocity in the Partanen et al. [2012] study was very high, ranging between
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1.0 and 1.4 m s−1 , which according to the authors eliminated the competition effect from
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their study. Our results indicate that the competition effect significantly reduces the
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maximum in-cloud supersaturation. If overestimated, this competition effect will influence
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our results substantially and may lead to an underestimation of the effectiveness of MCB.
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To investigate this closer we conducted two sets of sensitivity simulations. In the first
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set, the maximum in-cloud supersaturation was set to a fixed value of 0.2 % (annually
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averaged control run S is 0.18 % globally around 945 hPa) and the model was run for an
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injection strength of 10−10 kg m−2 s−1 for the Aitken and accumulation modes and for a
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strength of 10−8 kg m−2 s−1 for the coarse mode. The second set involved increasing the
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minimum sub-grid scale in-cloud vertical velocity in the Morrison and Gettelman [2008]
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parametrisation from 10 cm s−1 to 30 cm s−1 . This increased the average updraft velocity
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between 30◦ S and 30◦ N from 30.4 cm s−1 to 41.6 cm s−1 around 945 hPa (see auxiliary
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material) and increased the maximum in-cloud supersaturation. This setup was run for
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all injection strengths for the Aitken and the accumulation mode injections and for the
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10−10 kg m−2 s−1 injections for the coarse mode sea salt.
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3.2.1. Fixed Supersaturation
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Setting the in-cloud supersaturation to a fixed value increased the magnitude of the
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negative forcing of the accumulation mode sea salt from -1.27 W m−2 to -1.43 W m−2 for
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an injection strength of 10−10 kg m−2 s−1 (Fig. 1(a); green dashed line). More strikingly,
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for the Aitken mode, removing the competition effect led to a change in the sign of
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the resulting forcing; going from +1.27 W m−2 to -5.91 W m−2 . The high number to
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mass ratio combined with an S that is above the critical S of the Aitken mode sea salt
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(0.16 %), leads to a very large cooling effect. These results confirm that the competition
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effect dramatically limits the simulated cooling effect achieved from cloud seeding in the
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reference simulations.
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Latham et al. [2008] suggested that seeding clouds with coarse mode sea salt particles
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might lead to a warming because the “giant salt nuclei” may lead to an early onset of
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precipitation and a decrease in LWP. We investigated this by running the 10−8 kg m−2 s−1
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coarse mode case with a constant S of 0.2 %. This brought the change in SWCF from 1.2
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W m−2 (Fig. 1(a)) to 0 W m−2 , indicating that there is no contribution to the simulated
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warming in our coarse mode results from other sources than the competition effect.
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3.2.2. Enhanced Minimum Vertical Velocity
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Increasing the minimum in-cloud vertical velocity in the second set of sensitivity simu-
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lations led to an increase in the globally and annually averaged control run S from 0.18
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% to 0.26 % around 945 hPa (Fig. 1(d); yellow bars). This led to activation of smaller
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background particles in the increased updraft simulations (Umin30) than in the reference
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simulations (ref) and therefore increased the CDNC (Fig. 1(b); yellow bars) and decreased
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the effective radius around 945 hPa from 8.7 µm in ref to 8.2 µm in Umin30. The LWP
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increased from 126 gm−2 in ref to 137 gm−2 in the Umin30 simulation due to suppression
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of precipitation. Combined, the influence on the effective radius and the LWP led to a
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control run that had a higher cloud albedo in Umin30 than in the ref.
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For accumulation mode sea salt injections, the radiative forcing achieved was smaller
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in magnitude than in the ref simulations, except for the simulation of maximum sea salt
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seeding mass (Fig. 1(a); green bars). The small magnitude was caused by (i) reduced
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precipitation suppression due to high control run CDNC [Rasch and Kristjánsson, 1998,
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Eq. 21] and (ii) a reduced sensitivity for albedo to change with LWP due to high control
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run LWP; Assuming an asymmetry parameter of 0.85 and an optical depth τ =
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gives a change in cloud albedo, A [Hobbs, 1993], with LWP of:
δA
4.67ρL re
=
δLW P
(LW P + 4.67ρL re )2
3LW P
2ρL re
(1)
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where re is the cloud droplet effective radius and ρL the density of water. Combined, (i)
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and (ii) led to a smaller radiative effect of MCB for Umin30 than for ref accumulation
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mode seeding.
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For the maximum injections of accumulation mode sea salt, the high Umin30 control
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run S needed a large reduction to bring it below that necessary to activate background
241
sulfate. Contrary to ref simulations, the Umin30 simulations resulted in an S that was well
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above the critical limit (Fig. 1(d); lower purple dashed line). The reduced competition
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effect resulted in a MCB that still served to increase the CDNC and LWP and therefore
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to increase the magnitude of the SW cloud forcing.
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For the Aitken mode, Fig. 1(d) (light blue columns) shows that both the 10−11 and
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the 10−10 kg m−2 s−1 injections have Umin30 S around 945 hPa that are above the
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critical limit to activate the added Aitken mode sea salt. The two cases therefore led to
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negative forcing, which for the smallest injections was stronger than the forcing produced
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in the ref simulation (Fig. 1(a)). The increased updraft led to increased activation of
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the added sea salt particles themselves and therefore to a more effective MCB, while
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for the corresponding accumulation mode case the increased updraft mainly increased
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activation of background particles, thus leading to a decreased the efficiency of MCB.
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The high Umin30 control run S leads to a “delay” in the competition effect, reducing the
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positive forcing for high Aitken mode injections because more sea salt is needed to remove
255
enough water vapor to bring the S below that necessary for activation of the Aitken mode
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particles.
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For the coarse mode, we saw no significant change between the ref and the Umin30
simulations.
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The sensitivity experiments greatly suppress the competition effect, but nevertheless do
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not lead to negative radiative flux perturbations of the same magnitude as that found by
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Partanen et al. [2012]. One possible reason is that the Umin30 simulations have average
262
updrafts of 41.6 cm s−1 around 945 hPa (see auxiliary material), which is still well below
263
that of Partanen et al. [2012]. The NorESM may also be less sensitive to the added sea
264
salt than the ECHAM5.5-HAM2 used by Partanen et al. [2012] due to the large model
265
LWP (Eq. 1). Additionally, Partanen et al. [2012] seeded all ocean areas, while we only
266
seeded between 30◦ S and 30◦ N. This may influence the forcing estimates because of the
267
nonlinear relation between added mass and forcing caused by e. g. differences in the
268
regional meteorological conditions - are there low clouds above?; Are there updrafts that
269
can carry the injected sea salt aloft?; Is the particle number in the region already high
270
rendering the clouds less sensitive to the injected sea salt?
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This study does not account for kinetic limitations on the activation of giant CCN
272
[Chuang et al., 1997]. In regions of high S this may lead to an overestimated CDNC, while
273
the opposite may be true in regions of low S where an overestimated droplet activation
274
may lead to an exaggerated competition effect. Not accounting for this kinetic limitation
275
may be especially important for the coarse mode sea salt injections, for which we expect
276
the simulated positive radiative forcing to be an upper estimate.
4. Conclusions
277
In this study we have investigated how deliberate injections of sea salt into the marine
278
boundary layer affect the global radiative budget as a function of both particle size cat-
279
egory and injection mass flux. Using the NorESM we find that injecting accumulation
280
mode sea salt between 30◦ S and 30◦ N leads to a desired negative radiative effect of down
281
to -3.3 W m−2 which would almost cancel the positive forcing of a doubling of atmo-
282
spheric CO2 . On the other hand, for Aitken mode injections greater or equal to 10−10
283
kg m−2 s−1 , the simulated net radiative effect is positive, reaching a maximum of 8.4 W
284
m−2 . This is because the competition effect reduces the maximum supersaturation and
285
suppresses activation of both the added Aitken mode sea salt and background particles,
286
leading to a decrease in both CDNC and LWP. When coarse mode sea salt is injected,
287
the simulated net radiative effect is always positive. While the coarse mode sea salt itself
288
is large enough to activate, its small number to mass ratio leads to a lower increase in
289
CDNC due to the added sea salt than the decrease in activation of background aerosols
290
due to the competition effect.
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We also performed sensitivity tests which show that the size of the competition effect is
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crucial for the simulated forcing achieved. Omitting this effect led to a negative forcing of
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-5.9 W m−2 when injecting 10−10 kg m−2 s−1 of Aitken mode sea salt, whereas the same
294
injections gave a positive forcing of 1.3 W m−2 in the reference simulation. Increasing the
295
minimum in-cloud updraft velocity increased the S and led to a “delay” in the competition
296
effect. However, for accumulation mode injections, the increased updraft mainly led to
297
increased activation of background aerosols, leading to a reduced efficiency of the MCB.
298
The results presented in this study clearly show that the effectiveness of MCB is very
299
sensitive to the injection mass flux and particle size. While a cooling effect is simulated
300
for certain sea salt injections, emitting the wrong particle size or the wrong amount leads
301
to a simulated warming of the climate system, which is opposite to what one seeks to
302
achieve by climate engineering.
303
This study aims to be a first step in investigating the global effect of MCB of different
304
sized particles and injection amounts. Not all sea salt particle sizes are represented and
305
the model resolution is coarse. Wood [2007] showed that the cloud lifetime effect may
306
change sign depending on the vertical placement of the cloud base and Wang et al. [2011]
307
showed that the cloud albedo effect depends strongly on the fine scale atmospheric state,
308
neither of which are well represented in climate models. However, coarse models such as
309
the NorESM are currently the only tool available to study the global effects of MCB.
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Acknowledgments. This study was partly funded by the European Commission’s
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7th Framework Program through the IMPLICC project (FP7-ENV-2008-1-226567), and
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by the Norwegian Research Council through the EarthClim project (207711/E10) and
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its programme for supercomputing (NOTUR) through a grant for computing time. The
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authors are thankful to Philip Rasch and to the IMPLICC consortium for constructive
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discussions.
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Figure 1.
:
Dark colored bars present results from reference simulations (minimum updraft
velocity of 10 cm s−1 ) while light colored bars present results from simulations with a minimum
updraft velocity of 30 cm s−1 . 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 integrated cloud droplet
number concentration [cm−2 ]; (c) Change in liquid water path [g m−2 ]; (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 SO4
(lower); (e) Aerosol number concentration [cm−3 ] at ∼945 hPa; (f ) In-cloud CDNC
[cm−3 ] at ∼945 hPa.
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Table 1.
(a) Sea salt injections used to simulate MCB. (b) List of simulations in which Xs
indicate 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
−1
Total injections [Tg yr ]
48.2 595.2 1785.5 5935.6 59436
Increase in global sea salt emissions 0.9% 9.1% 27.4% 91.3% 913.1%
(b) List of Simulations
Control 10−11 10−10 3*10−10 10−9 10−8
Simulation name
Reference simulations (ref)
Aitken mode
Accumulation mode
Coarse mode
Fixed supersaturation
Aitken mode
Accumulation mode
Coarse mode
Enhanced vertical velocity (Umin30)
Aitken mode
Accumulation mode
Coarse mode
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