Sulfur cycle and sulfate radiative forcing simulated from a coupled global climate-chemistry model

Abstract. The sulfur cycle and radiative effects of sulfate aerosol on climate are studied with a Global tropospheric Climate-Chemistry Model in which chemistry, radiation and dynamics are fully coupled. Production and removal mechanisms of sulfate are analyzed for the conditions of natural and anthropogenic sulfur emissions. Results show that the 1985 anthropogenic emission tripled the global SO2 and sulfate loadings from its natural value of 0.16 and 0.10 Tg S, respectively. Under natural conditions, the fraction of sulfate produced in-cloud is 74%; whereas with anthropogenic emissions, the fraction of in-cloud sulfate production slightly increased to 76%. Lifetimes of SO2 and sulfate under polluted conditions are estimated to be 1.7 and 2.0 days, respectively. The tripling of sulfate results in a direct radiative forcing of −0.43 W m−2 (clear-sky) or −0.24 W m−2 (all-sky), and a significant first indirect forcing of −1.85 W m−2, leading to a mean global cooling of about 0.1 K. Regional forcing and responses are significantly stronger than the global values. The first indirect forcing is sensitive to the relationship between aerosol concentration and cloud droplet number concentration which requires further investigation. Two aspects of chemistry-climate interaction are addressed. Firstly, the coupling effects lead to a slight decrease of 1% in global sulfate loading for both the cases of natural and anthropogenic added sulfur emissions. Secondly, only the indirect effect of sulfate aerosols yields significantly stronger signals in changes of near surface temperature and sulfate loading than changes due to intrinsic climate variability, while other responses to the indirect effect and all responses to the direct effect are below noise level.

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Sulfur cycle and sulfate radiative forcing simulated from a coupled global climate-chemistry model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-22365-2009 ◽

2009 ◽

Vol 9 (5) ◽

pp. 22365-22406

Author(s):

I.-C. Tsai ◽

J.-P. Chen ◽

P.-Y. Lin ◽

W.-C. Wang ◽

I. S. A. Isaksen

Keyword(s):

Indirect Effect ◽

Radiative Forcing ◽

Global Climate ◽

Cloud Droplet ◽

Sulfur Cycle ◽

Near Surface ◽

Sulfate Production ◽

Sky Conditions ◽

Sulfur Emissions ◽

Indirect Forcing

Abstract. The sulfur cycle and radiative effects of sulfate aerosol on climate are studied with a Global tropospheric Climate-Chemistry Model in which chemistry, radiation and dynamics are fully coupled. Production and removal mechanisms of sulfate are analyzed for the conditions of natural and anthropogenic sulfur emissions. Results show that the 1985 anthropogenic emission doubled the global SO2 and sulfate loadings from its natural value of 0.15 and 0.27 Tg S, respectively. Under natural conditions, the fraction of sulfate produced in-cloud is 87%, and the lifetime of SO2 and sulfate are 1.8 and 4.0 days, respectively; whereas with anthropogenic emissions, changes in in-cloud sulfate production are small, while SO2 and sulfate lifetimes are significant reduced (1.0 and 2.4 days, respectively). The doubling of sulfate results in a direct radiative forcing of −0.32 and −0.14 W m−2 under clear-sky and all-sky conditions, respectively, and a significant first indirect forcing of −1.69 W m−2. The first indirect forcing is sensitive to the relationship between aerosol concentration and cloud droplet number concentration. Two aspects of chemistry-climate interaction are addressed. Firstly, the coupling effects lead to 10% and 2% decreases in sulfate loading, respectively, for the cases of natural and anthropogenic added sulfur emissions. Secondly, only the indirect effect of sulfate aerosols yields significantly stronger signals in changes of near surface temperature and sulfate loading than changes due to intrinsic climate variability, while other responses to the indirect effect and all responses to the direct effect are weak.

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Predicting decadal trends in cloud droplet number concentration using reanalysis and satellite data

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-2035-2018 ◽

2018 ◽

Vol 18 (3) ◽

pp. 2035-2047 ◽

Cited By ~ 16

Author(s):

Daniel T. McCoy ◽

Frida A.-M. Bender ◽

Daniel P. Grosvenor ◽

Johannes K. Mohrmann ◽

Dennis L. Hartmann ◽

...

Keyword(s):

North America ◽

Radiative Forcing ◽

Cloud Droplet ◽

Number Concentration ◽

Ozone Monitoring Instrument ◽

Anthropogenic Aerosol ◽

Near Surface ◽

Cloud Properties ◽

Cloud Droplet Number Concentration ◽

Moderate Resolution Imaging Spectroradiometer

Abstract. Cloud droplet number concentration (CDNC) is the key state variable that moderates the relationship between aerosol and the radiative forcing arising from aerosol–cloud interactions. Uncertainty related to the effect of anthropogenic aerosol on cloud properties represents the largest uncertainty in total anthropogenic radiative forcing. Here we show that regionally averaged time series of the Moderate-Resolution Imaging Spectroradiometer (MODIS) observed CDNC of low, liquid-topped clouds is well predicted by the MERRA2 reanalysis near-surface sulfate mass concentration over decadal timescales. A multiple linear regression between MERRA2 reanalyses masses of sulfate (SO4), black carbon (BC), organic carbon (OC), sea salt (SS), and dust (DU) shows that CDNC across many different regimes can be reproduced by a simple power-law fit to near-surface SO4, with smaller contributions from BC, OC, SS, and DU. This confirms previous work using a less sophisticated retrieval of CDNC on monthly timescales. The analysis is supported by an examination of remotely sensed sulfur dioxide (SO2) over maritime volcanoes and the east coasts of North America and Asia, revealing that maritime CDNC responds to changes in SO2 as observed by the ozone monitoring instrument (OMI). This investigation of aerosol reanalysis and top-down remote-sensing observations reveals that emission controls in Asia and North America have decreased CDNC in their maritime outflow on a decadal timescale.

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Study of cloud droplet number concentration using the A-Train satellites

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-29035-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 29035-29058

Author(s):

S. Zeng ◽

J. Riedi ◽

C. R. Trepte ◽

D. M. Winker ◽

Y.-X. Hu

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Global Climate ◽

Cloud Droplet ◽

Number Concentration ◽

Effective Radius ◽

Global Climate Models ◽

Atmospheric Conditions ◽

Cloud Droplet Number Concentration ◽

Passive Sensors

Abstract. Cloud droplet number concentration (CDNC) is an important microphysical property of liquid clouds that impacts radiative forcing, precipitation and it is pivotal for understanding of cloud-aerosols interactions. Current studies of this parameter at global scales with satellite observations are still challenging, especially because retrieval algorithms developed for passive sensors (i.e. MODIS/Aqua) have to rely on the assumption of cloud adiabatic growth. The active sensor component of the A-Train constellation (i.e., CALIOP/CALIPSO) allows retrievals of CDNC from depolarization measurements at 532 nm. For that case, the retrieval does not rely on the adiabatic assumption but instead must use a priori information on effective radius (re), which can be obtained from other passive sensors. In this paper, re values obtained from MODIS/Aqua and POLDER/PARASOL (two passive sensors, conponents of the A-Train) are used to constrain CDNC retrievals from CALIOP. Intercomparison of CDNC products retrieved from MODIS and CALIOP sensors is performed, and the impacts of cloud entrainment, drizzling, horizontal heterogeneity, and effective radius are discussed. By analyzing the strengths and weaknesses of different retrieval techniques, this study aims to better understand global CDNC distribution, and eventually determine cloud structure and atmospheric conditions in which they develop. The improved understanding of CDNC should help contribute to future studies of global cloud-aerosol-precipitation interaction and parameterization of clouds in global climate models (GCMs).

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Predicting decadal trends in cloud droplet number concentration using reanalysis and satellite data

10.5194/acp-2017-811 ◽

2017 ◽

Cited By ~ 2

Author(s):

Daniel T. McCoy ◽

Frida A.-M. Bender ◽

Daniel P. Grosvenor ◽

Johannes K. Mohrmann ◽

Dennis L. Hartmann ◽

...

Keyword(s):

North America ◽

Radiative Forcing ◽

Cloud Droplet ◽

Number Concentration ◽

Ozone Monitoring Instrument ◽

Anthropogenic Aerosol ◽

Decadal Time Scale ◽

Near Surface ◽

Cloud Properties ◽

Cloud Droplet Number Concentration

Abstract. Cloud droplet number concentration (CDNC) is the key state variable that moderates the relationship between aerosol and the radiative forcing arising from aerosol-cloud interactions. Uncertainty related to the effect of anthropogenic aerosol on cloud properties represents the largest uncertainty in total anthropogenic radiative forcing. Here we show that regionally-averaged time series of Moderate-Resolution Imaging Spectroradiometer (MODIS) observed CDNC are well-predicted by MERRA2 reanalysis near-surface sulfate mass concentration over decadel timescales. A multiple linear regression between MERRA2 reanalysis masses of sulfate (SO4), black carbon (BC), organic carbon (OC), sea salt (SS), and dust (DU) shows that CDNC across many different regimes can be reproduced by a simple power law fit to near-surface SO4, with smaller contributions from BC, OC, SS, and DU. This confirms previous work using a less-sophisticated retrieval of CDNC at monthly time scales. The analysis is supported by examination of remotely-sensed sulfur dioxide (SO2) over maritime volcanoes and the east coasts of North America and Asia, revealing that maritime CDNC responds to changes in SO2 as observed by the Ozone Monitoring Instrument (OMI). This investigation of aerosol reanalysis and top-down remote sensing observations reveals that emission controls in Asia and North America have decreased CDNC in their maritime outflow on a decadal time scale.

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The aerosol-cyclone indirect effect in observations and high-resolution simulations

10.5194/acp-2017-649 ◽

2017 ◽

Cited By ~ 1

Author(s):

Daniel T. McCoy ◽

Paul R. Field ◽

Anja Schmidt ◽

Daniel P. Grosvenor ◽

Frida A.-M. Bender ◽

...

Keyword(s):

High Resolution ◽

Indirect Effect ◽

Climate Models ◽

Climate Model ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

2014 Eruption ◽

Cloud Droplet Number Concentration ◽

Aerosol Cloud Interactions ◽

First Time

Abstract. Aerosol-cloud interactions are a major source of uncertainty in predicting 21st century climate change. Using high-resolution, convection-permitting global simulations we predict that increased cloud condensation nuclei (CCN) interacting with midlatitude cyclones will increase their cloud droplet number concentration (CDNC), liquid water (CLWP), and albedo. For the first time this effect is shown with 13 years of satellite observations. Causality between enhanced CCN and enhanced cyclone liquid content is supported by the 2014 eruption of Holuhraun. The change in midlatitude cyclone albedo due to enhanced CCN in a surrogate climate model is around 70 % of the change in a high-resolution convection-permitting model, indicating that climate models may underestimate this indirect effect.

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Response to marine cloud brightening in a multi-model ensemble

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-621-2018 ◽

2018 ◽

Vol 18 (2) ◽

pp. 621-634 ◽

Cited By ~ 11

Author(s):

Camilla W. Stjern ◽

Helene Muri ◽

Lars Ahlm ◽

Olivier Boucher ◽

Jason N. S. Cole ◽

...

Keyword(s):

Radiative Forcing ◽

Cloud Droplet ◽

Model Ensemble ◽

Low Clouds ◽

Precipitation Decrease ◽

Intercomparison Project ◽

Cloud Droplet Number Concentration ◽

Significant Temperature ◽

Effective Radiative Forcing ◽

Subtropical Oceans

Abstract. Here we show results from Earth system model simulations from the marine cloud brightening experiment G4cdnc of the Geoengineering Model Intercomparison Project (GeoMIP). The nine contributing models prescribe a 50 % increase in the cloud droplet number concentration (CDNC) of low clouds over the global oceans in an experiment dubbed G4cdnc, with the purpose of counteracting the radiative forcing due to anthropogenic greenhouse gases under the RCP4.5 scenario. The model ensemble median effective radiative forcing (ERF) amounts to −1.9 W m−2, with a substantial inter-model spread of −0.6 to −2.5 W m−2. The large spread is partly related to the considerable differences in clouds and their representation between the models, with an underestimation of low clouds in several of the models. All models predict a statistically significant temperature decrease with a median of (for years 2020–2069) −0.96 [−0.17 to −1.21] K relative to the RCP4.5 scenario, with particularly strong cooling over low-latitude continents. Globally averaged there is a weak but significant precipitation decrease of −2.35 [−0.57 to −2.96] % due to a colder climate, but at low latitudes there is a 1.19 % increase over land. This increase is part of a circulation change where a strong negative top-of-atmosphere (TOA) shortwave forcing over subtropical oceans, caused by increased albedo associated with the increasing CDNC, is compensated for by rising motion and positive TOA longwave signals over adjacent land regions.

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The impact of sampling strategy on the cloud droplet number concentration estimated from satellite data

10.5194/amt-2021-371 ◽

2021 ◽

Author(s):

Edward Gryspeerdt ◽

Daniel T. McCoy ◽

Ewan Crosbie ◽

Richard H. Moore ◽

Graeme J. Nott ◽

...

Keyword(s):

Satellite Data ◽

Radiative Forcing ◽

Sampling Strategy ◽

Cloud Droplet ◽

Number Concentration ◽

Cloud Properties ◽

Systematic Biases ◽

Cloud Droplet Number Concentration ◽

Aerosol Cloud Interactions ◽

The Impact

Abstract. Cloud droplet number concentration (Nd) is of central importance to observation-based estimates of aerosol indirect effects, being used to quantify both the cloud sensitivity to aerosol and the base state of the cloud. However, the derivation of Nd from satellite data depends on a number of assumptions about the cloud and the accuracy of the retrievals of the cloud properties from which it is derived, making it prone to systematic biases. A number of sampling strategies have been proposed to address these biases by selecting the most accurate Nd retrievals in the satellite data. This work compares the impact of these strategies on the accuracy of the satellite retrieved Nd, using a selection of insitu measurements. In stratocumulus regions, the MODIS Nd retrieval is able to achieve a high precision (r2 of 0.5–0.8). This is lower in other cloud regimes, but can be increased by appropriate sampling choices. Although the Nd sampling can have significant effects on the Nd climatology, it produces only a 20 % variation in the implied radiative forcing from aerosol-cloud interactions, with the choice of aerosol proxy driving the overall uncertainty. The results are summarised into recommendations for using MODIS Nd products and appropriate sampling.

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Reducing uncertainties in satellite estimates of aerosol-cloud interactions over the subtropical ocean by integrating vertically resolved aerosol observations

10.5194/acp-2019-892 ◽

2019 ◽

Author(s):

David Painemal ◽

Fu-Lung Chang ◽

Richard Ferrare ◽

Sharon Burton ◽

Zhujun Li ◽

...

Keyword(s):

Climate Models ◽

Global Climate ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

Global Climate Models ◽

Orthogonal Polarization ◽

Near Surface ◽

Aerosol Cloud ◽

Error Characterization ◽

Aerosol Cloud Interactions

Abstract. Satellite quantification of aerosol effects on clouds relies on aerosol optical depth (AOD) as a proxy for aerosol concentration or cloud condensation nuclei (CCN). However, the lack of error characterization of satellite-based results hampers their use for the evaluation and improvement of global climate models. We show that the use of AOD for assessing aerosol-cloud interactions (ACI) is inadequate over vast oceanic areas in the subtropics. Instead, we postulate that a more physical approach that consists of matching vertically resolved aerosol data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite at the cloud-layer height with Aqua Moderate-resolution Imaging Spectroradiometer (MODIS) cloud retrievals reduces uncertainties in satellite-based ACI estimates. Combined aerosol extinction coefficients (σ) below cloud-top (σBC) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and cloud droplet number concentrations (Nd) from Aqua-MODIS yield high correlations across a broad range of σBC values, with σBC quartile correlations > 0.78. In contrast, CALIOP-based AOD yields correlations with MODIS Nd of less than 0.62 for the two lower AOD quartiles. Moreover, σBC explains 41 % of the spatial variance in MODIS Nd, whereas AOD only explains 17 %, primarily caused by the lack of spatial covariability in the eastern Pacific. Compared with σBC, near-surface σ weakly correlates in space with MODIS Nd, accounting for a 16 % variance. It is concluded that the linear regression calculated from ln(Nd)−ln(σBC) (the standard method for quantifying ACI) is more physically meaningful than that derived from the Nd−AOD pair.

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Impacts of solar-absorbing aerosol layers on the transition of stratocumulus to trade cumulus clouds

10.5194/acp-2017-255 ◽

2017 ◽

Author(s):

Xiaoli Zhou ◽

Andrew S. Ackerman ◽

Ann M. Fridlind ◽

Robert Wood ◽

Pavlos Kollias

Keyword(s):

Radiative Forcing ◽

Cloud Droplet ◽

Cloud Water ◽

Gradual Transition ◽

Liquid Water Path ◽

Diffusional Growth ◽

Cumulus Clouds ◽

Cloud Droplets ◽

Boundary Layer Height ◽

Cloud Droplet Number Concentration

Abstract. The effects of an initially overlying layer of solar-absorbing aerosol on the transition of stratocumulus to trade cumulus clouds are examined using large-eddy simulations. The transition of lightly drizzling cloud is generally hastened, resulting mainly from increased cloud droplet number concentration (Nc) induced by entrained aerosol. The increased Nc slows sedimentation of cloud droplets and shortens their relaxation time for diffusional growth, both of which accelerate entrainment of overlying air and thereby stratocumulus breakup. However, the decrease in albedo from cloud breakup is more than offset by redistributing cloud water over a greater number of droplets, such that the diurnal-average shortwave forcing at the top of atmosphere is negative. The negative radiative forcing is enhanced by sizable longwave contributions, which result from the greater cloud breakup and a reduced boundary layer height associated with aerosol heating. A perturbation of moisture instead of aerosol aloft leads to greater liquid water path and a more gradual transition. Adding absorbing aerosol to that atmosphere results in substantial reductions in LWP and cloud cover that lead to positive shortwave and negative longwave forcings on average canceling each other. Only for heavily drizzling clouds is the breakup delayed, as inhibition of precipitation overcomes cloud water loss from enhanced entrainment. Considering these simulations as an imperfect proxy for biomass burning plumes influencing Namibian stratocumulus, we expect regional indirect plus semi-direct forcings to be substantially negative to negligible at the top of atmosphere, with its magnitude sensitive to background and perturbation properties.

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The hemispheric contrast in cloud microphysical properties constrains aerosol forcing

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1922502117 ◽

2020 ◽

Vol 117 (32) ◽

pp. 18998-19006 ◽

Cited By ~ 2

Author(s):

Isabel L. McCoy ◽

Daniel T. McCoy ◽

Robert Wood ◽

Leighton Regayre ◽

Duncan Watson-Parris ◽

...

Keyword(s):

Southern Ocean ◽

Northern Hemisphere ◽

Radiative Forcing ◽

Climate Models ◽

Cloud Droplet ◽

Number Concentration ◽

Global Climate Models ◽

Aerosol Cloud ◽

Cloud Droplet Number Concentration ◽

Aerosol Cloud Interactions

The change in planetary albedo due to aerosol−cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth’s climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol−cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm−3and 24 cm−3. By extension, the radiative forcing since 1850 from aerosol−cloud interactions is constrained to be −1.2 W⋅m−2to −0.6 W⋅m−2. The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol−cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models.

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