scholarly journals Marine cloud brightening – as effective without clouds

2017 ◽  
Author(s):  
Lars Ahlm ◽  
Andy Jones ◽  
Camilla W. Stjern ◽  
Helene Muri ◽  
Ben Kravitz ◽  
...  
Keyword(s):  
Direct Effect ◽  
Radiative Forcing ◽  
Cloud Droplet ◽  
Droplet Radius ◽  
Sea Spray ◽  
Climate Engineering ◽  
Cloud Optical Depth ◽  
Clear Sky ◽  
Aerosol Direct Effect ◽  
Global Mean

Abstract. Marine cloud brightening through sea spray injection has been proposed as a climate engineering method for avoiding the most severe consequences of global warming. A limitation of most of the previous modelling studies on marine cloud brightening is that they have either considered individual models, or only investigated the effects of a specific increase in the number of cloud droplets. Here we present results from coordinated simulations with three Earth system models (ESMs) participating in the Geoengineering Model Intercomparison Project (GeoMIP) G4sea-salt experiment. Injection rates of accumulation mode sea spray aerosol particles over ocean between 30° N and 30° S are set in each model to generate a global-mean effective radiative forcing (ERF) of −2.0 W m−2 at the top of atmosphere. We find that the injection increases the cloud droplet number concentration in lower layers, reduces the cloud-top effective droplet radius, and increases the cloud optical depth over the injection area. We also find, however, that the global-mean clear-sky ERF by the injected particles is as large as the corresponding total ERF in all three ESMs, indicating a large potential of the aerosol direct effect in regions of low cloudiness. The largest enhancement in ERF due to the presence of clouds occur as expected in the subtropical stratocumulus regions off the west coasts of the American and African continents. However, outside these regions, the ERF is in general equally large in cloudy and clear-sky conditions. These findings suggest a more important role of the aerosol direct effect in sea spray climate engineering than previously thought.

scholarly journals Marine cloud brightening – as effective without clouds

2017 ◽  
Vol 17 (21) ◽  
pp. 13071-13087 ◽  
Author(s):  
Lars Ahlm ◽  
Andy Jones ◽  
Camilla W. Stjern ◽  
Helene Muri ◽  
Ben Kravitz ◽  
...  
Keyword(s):  
Direct Effect ◽  
Radiative Forcing ◽  
Cloud Droplet ◽  
Droplet Radius ◽  
Sea Spray ◽  
Climate Engineering ◽  
Cloud Optical Depth ◽  
Clear Sky ◽  
Aerosol Direct Effect ◽  
Global Mean

Abstract. Marine cloud brightening through sea spray injection has been proposed as a climate engineering method for avoiding the most severe consequences of global warming. A limitation of most of the previous modelling studies on marine cloud brightening is that they have either considered individual models or only investigated the effects of a specific increase in the number of cloud droplets. Here we present results from coordinated simulations with three Earth system models (ESMs) participating in the Geoengineering Model Intercomparison Project (GeoMIP) G4sea-salt experiment. Injection rates of accumulation-mode sea spray aerosol particles over ocean between 30° N and 30° S are set in each model to generate a global-mean effective radiative forcing (ERF) of −2.0 W m−2 at the top of the atmosphere. We find that the injection increases the cloud droplet number concentration in lower layers, reduces the cloud-top effective droplet radius, and increases the cloud optical depth over the injection area. We also find, however, that the global-mean clear-sky ERF by the injected particles is as large as the corresponding total ERF in all three ESMs, indicating a large potential of the aerosol direct effect in regions of low cloudiness. The largest enhancement in ERF due to the presence of clouds occur as expected in the subtropical stratocumulus regions off the west coasts of the American and African continents. However, outside these regions, the ERF is in general equally large in cloudy and clear-sky conditions. These findings suggest a more important role of the aerosol direct effect in sea spray climate engineering than previously thought.

scholarly journals Direct Aerosol Radiative Forcing Uncertainty Based on a Radiative Perturbation Analysis

2010 ◽  
Vol 23 (19) ◽  
pp. 5288-5293 ◽  
Author(s):  
Norman G. Loeb ◽  
Wenying Su
Keyword(s):  
Radiative Forcing ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Aerosol Radiative Forcing ◽  
Clear Sky ◽  
Radiative Perturbation ◽  
Sky Conditions ◽  
Global Mean ◽  
Aerosol Properties ◽  
Aerosol Radiative

Abstract To provide a lower bound for the uncertainty in measurement-based clear- and all-sky direct aerosol radiative forcing (DARF), a radiative perturbation analysis is performed for the ideal case in which the perturbations in global mean aerosol properties are given by published values of systematic uncertainty in Aerosol Robotic Network (AERONET) aerosol measurements. DARF calculations for base-state climatological cloud and aerosol properties over ocean and land are performed, and then repeated after perturbing individual aerosol optical properties (aerosol optical depth, single-scattering albedo, asymmetry parameter, scale height, and anthropogenic fraction) from their base values, keeping all other parameters fixed. The total DARF uncertainty from all aerosol parameters combined is 0.5–1.0 W m−2, a factor of 2–4 greater than the value cited in the Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report. Most of the total DARF uncertainty in this analysis is associated with single-scattering albedo uncertainty. Owing to the greater sensitivity to single-scattering albedo in cloudy columns, DARF uncertainty in all-sky conditions is greater than in clear-sky conditions, even though the global mean clear-sky DARF is more than twice as large as the all-sky DARF.

scholarly journals Retrieval of Key Aerosol Optical Parameters from Spectral Direct and Diffuse Irradiances Observed by a Radiometer with Nonideal Cosine Response Characteristic

2012 ◽  
Vol 29 (5) ◽  
pp. 683-696 ◽  
Author(s):  
Pradeep Khatri ◽  
Tamio Takamura ◽  
Akihiro Yamazaki ◽  
Yutaka Kondo
Keyword(s):  
Direct Effect ◽  
Radiative Forcing ◽  
Response Characteristic ◽  
Single Scattering ◽  
Optical Parameters ◽  
Aerosol Radiative Forcing ◽  
Aerosol Direct Effect ◽  
Diffuse Irradiance ◽  
Isotropic Distribution ◽  
Sky Radiance

Abstract The spectral direct and diffuse irradiances observed by a radiometer with a horizontal surface detector have been frequently used to study aerosol optical parameters, such as aerosol optical thickness (τaer) and single scattering albedo (ω). Such radiometers more or less lack an ideal cosine response. Generally, either the cosine error of observed diffuse irradiance was corrected by assuming an isotropic distribution of sky radiance or it was neglected in the past studies. This study presents an algorithm to retrieve τaer and ω from direct and diffuse irradiances observed by a radiometer with a nonideal cosine response characteristic by taking into account the cosine errors of observed irradiances in detail. The proposed algorithm considers the anisotropic distribution of sky radiance while correcting the cosine error of observed diffuse irradiance. This algorithm can also be used to calculate the cosine error correction factor of diffuse irradiance. The results show that the aerosol optical parameters and the aerosol direct effect (aerosol radiative forcing and the heating rate) can be heavily affected by the cosine errors of observed direct and diffuse irradiances. The study further shows that assuming the isotropic distribution of sky radiance while correcting the cosine error of observed diffuse irradiance can affect the retrieved ω at small and large solar zenith angles; thus, the estimated aerosol direct effect can be quantitatively affected. Because of the cosine errors, this study found the actual values of diffuse irradiances at different wavelengths were underestimated by around 5%–11%.

scholarly journals Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces. Part II: Cloud Optical Depth and Shortwave Radiative Forcing from Pyranometer Measurements in the Southern Ocean

2005 ◽  
Vol 18 (22) ◽  
pp. 4637-4648 ◽  
Author(s):  
Melanie F. Fitzpatrick ◽  
Stephen G. Warren
Keyword(s):  
Solar Radiation ◽  
Southern Ocean ◽  
Sea Ice ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Cloud Droplet ◽  
Frequency Distributions ◽  
Cloud Optical Depth ◽  
Cloud Radiative Forcing

Abstract Downward solar irradiance at the sea surface, measured on several voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident hourly sea ice reports, instantaneous cloud radiative forcing and effective cloud optical depth are obtained. Values of “raw cloud transmittance” (trc), the ratio of downward irradiance under cloud to downward irradiance measured under clear sky, vary from 0.1 to 1.0. Over sea ice, few values of trc were observed between 0.8 and 1.0, possibly due to the threshold nature of the aerosol-to-cloud-droplet transition. This sparsely populated region of transmittances is referred to as the Köhler gap. The instantaneous downward shortwave cloud radiative forcing is computed, as well as the time-averaged net forcing. The net forcing at a solar zenith angle of 60° is typically −250 W m−2 over open ocean, but only half this value over sea ice because of the higher surface albedo and less frequent occurrence of clouds. “Effective” optical depths τ (for a radiatively equivalent horizontally homogeneous cloud) are classified by season and surface type. The frequency distributions of τ are well fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47°S, increasing to 24 in the region of maximum cloud cover at 58°S, and decreasing to 11 at 67°S near the coast of Antarctica.

scholarly journals Quantification of DMS aerosol-cloud-climate interactions using the ECHAM5-HAMMOZ model in a current climate scenario

2010 ◽  
Vol 10 (15) ◽  
pp. 7425-7438 ◽  
Author(s):  
M. A. Thomas ◽  
P. Suntharalingam ◽  
L. Pozzoli ◽  
S. Rast ◽  
A. Devasthale ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Cloud Cover ◽  
Dimethyl Sulfide ◽  
Circulation Model ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Droplet Radius ◽  
Aerosol Cloud ◽  
Southern Oceans

Abstract. The contribution of ocean dimethyl sulfide (DMS) emissions to changes in cloud microphysical properties is quantified seasonally and globally for present day climate conditions using an aerosol-chemistry-climate general circulation model, ECHAM5-HAMMOZ, coupled to a cloud microphysics scheme. We evaluate DMS aerosol-cloud-climate linkages over the southern oceans where anthropogenic influence is minimal. The changes in the number of activated particles, cloud droplet number concentration (CDNC), cloud droplet effective radius, cloud cover and the radiative forcing are examined by analyzing two simulations: a baseline simulation with ocean DMS emissions derived from a prescribed climatology and one in which the ocean DMS emissions are switched off. Our simulations show that the model realistically simulates the seasonality in the number of activated particles and CDNC, peaking during Southern Hemisphere (SH) summer coincident with increased phytoplankton blooms and gradually declining with a minimum in SH winter. In comparison to a simulation with no DMS, the CDNC level over the southern oceans is 128% larger in the baseline simulation averaged over the austral summer months. Our results also show an increased number of smaller sized cloud droplets during this period. We estimate a maximum decrease of up to 15–18% in the droplet radius and a mean increase in cloud cover by around 2.5% over the southern oceans during SH summer in the simulation with ocean DMS compared to when the DMS emissions are switched off. The global annual mean top of the atmosphere DMS aerosol all sky radiative forcing is −2.03 W/m2, whereas, over the southern oceans during SH summer, the mean DMS aerosol radiative forcing reaches −9.32 W/m2.

scholarly journals Rate of non-linearity in DMS aerosol-cloud-climate interactions

2011 ◽  
Vol 11 (21) ◽  
pp. 11175-11183 ◽  
Author(s):  
M. A. Thomas ◽  
P. Suntharalingam ◽  
L. Pozzoli ◽  
A. Devasthale ◽  
S. Kloster ◽  
...  
Keyword(s):  
Liquid Water ◽  
Radiative Forcing ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Droplet Radius ◽  
Microphysical Properties ◽  
Non Linear ◽  
Climate Interactions ◽  
Warming World ◽  
Southern Oceans

Abstract. The degree of non-linearity in DMS-cloud-climate interactions is assessed using the ECHAM5-HAMMOZ model by taking into account end-to-end aerosol chemistry-cloud microphysics link. The evaluation is made over the Southern oceans in austral summer, a region of minimal anthropogenic influence. In this study, we compare the DMS-derived changes in the aerosol and cloud microphysical properties between a baseline simulation with the ocean DMS emissions from a prescribed climatology, and a scenario where the DMS emissions are doubled. Our results show that doubling the DMS emissions in the current climate results in a non-linear response in atmospheric DMS burden and subsequently, in SO2 and H2SO4 burdens due to inadequate OH oxidation. The aerosol optical depth increases by only ~20 % in the 30° S–75° S belt in the SH summer months. This increases the vertically integrated cloud droplet number concentrations (CDNC) by 25 %. Since the vertically integrated liquid water vapor is constant in our model simulations, an increase in CDNC leads to a reduction in cloud droplet radius of 3.4 % over the Southern oceans in summer. The equivalent increase in cloud liquid water path is 10.7 %. The above changes in cloud microphysical properties result in a change in global annual mean radiative forcing at the TOA of −1.4 W m−2. The results suggest that the DMS-cloud microphysics link is highly non-linear. This has implications for future studies investigating the DMS-cloud climate feedbacks in a warming world and for studies evaluating geoengineering options to counteract warming by modulating low level marine clouds.

scholarly journals Sea-spray geoengineering in the HadGEM2-ES earth-system model: radiative impact and climate response

2012 ◽  
Vol 12 (22) ◽  
pp. 10887-10898 ◽  
Author(s):  
A. Jones ◽  
J. M. Haywood
Keyword(s):  
Direct Effect ◽  
Indirect Effects ◽  
Earth System Model ◽  
System Model ◽  
Sea Spray ◽  
Earth System ◽  
Global Mean Temperature ◽  
Radiative Impact ◽  
The Impact ◽  
Global Mean

Abstract. The radiative impact and climate effects of geoengineering using sea-spray aerosols have been investigated in the HadGEM2-ES Earth system model using a fully prognostic treatment of the sea-spray aerosols and also including their direct radiative effect. Two different emission patterns were considered, one to maximise the direct effect in clear skies, the other to maximise the indirect effects of the sea-spray on low clouds; in both cases the emissions were limited to 10% of the ocean area. While the direct effect was found to be significant, the indirect effects on clouds were much more effective in reducing global mean temperature as well as having less of an impact on global mean precipitation per unit temperature reduction. The impact on the distribution of precipitation was found to be similar in character, but less in degree, to that simulated by a previous study using a much simpler treatment of this geoengineering process.

scholarly journals Sea-spray geoengineering in the HadGEM2-ES Earth-system model: radiative impact and climate response

2012 ◽  
Vol 12 (8) ◽  
pp. 20717-20743 ◽  
Author(s):  
A. Jones ◽  
J. M. Haywood
Keyword(s):  
Direct Effect ◽  
Indirect Effects ◽  
Earth System Model ◽  
System Model ◽  
Sea Spray ◽  
Earth System ◽  
Global Mean Temperature ◽  
Radiative Impact ◽  
The Impact ◽  
Global Mean

Abstract. The radiative impact and climate effects of geoengineering using sea-spray aerosols have been investigated in the HadGEM2-ES Earth system model using a fully prognostic treatment of the sea-spray aerosols and also including their direct raditive effect. Two different emission patterns were considered, one to maximise the direct effect in clear skies, the other to maximise the indirect effects of the sea-spray on low clouds; in both cases the emissions were limited to 10% of the ocean area. While the direct effect was found to be significant, the indirect effects on clouds were much more effective in reducing global mean temperature. Moreover, the impact on global mean precipitation per unit temperature reduction was found to be greatest when the emission pattern for maximising the direct effect was used, suggesting that targeting the direct effect of sea-spray is not a good strategy. The impact on the distribution of precipitation was found to be similar in character, but less in degree, than that simulated by a previous study using a much simpler treatment of this geoengineering process.

scholarly journals Reduced efficacy of marine cloud brightening geoengineering due to in-plume aerosol coagulation: parameterization and global implications

2013 ◽  
Vol 13 (7) ◽  
pp. 18679-18711 ◽  
Author(s):  
G. S. Stuart ◽  
R. G. Stevens ◽  
A.-I. Partanen ◽  
A. K. L. Jenkins ◽  
H. Korhonen ◽  
...  
Keyword(s):  
Climate Model ◽  
Climate Modeling ◽  
Cloud Droplet ◽  
Global Scale ◽  
Cloud Microphysics ◽  
Sea Spray ◽  
Computationally Efficient ◽  
Scale Models ◽  
Gaussian Plume ◽  
Global Mean

Abstract. The intentional enhancement of cloud albedo via controlled sea-spray injection from ships (Marine Cloud Brightening) has been proposed as a possible method to control anthropogenic global warming; however, there remains significant uncertainty in the efficacy of this method due to, amongst other factors, uncertainties in aerosol and cloud microphysics. A major assumption used in recent cloud- and climate-modeling studies is that all sea spray was emitted uniformly into some oceanic grid boxes, and thus these studies did not account for sub-grid aerosol coagulation within the sea-spray plumes. We explore the evolution of these sea-salt plumes using a multi-shelled Gaussian plume model with size-resolved aerosol coagulation. We determine how the final number of particles depends on meteorological conditions, including wind speed and boundary-layer stability, as well as the emission rate and size distribution of aerosol emitted. Under previously proposed injection rates and typical marine conditions, we find that the number of aerosol particles is reduced by over 50%, but this reduction varies from under 10% to over 90% depending on the conditions. We provide a computationally efficient parameterization for cloud-resolving and global-scale models to account for sub-grid scale coagulation, and we implement this parameterization in a global-scale aerosol-climate model. We find that accounting for this sub-grid scale coagulation reduces cloud droplet number concentrations by 46% over emission regions, and reduces the global mean radiative flux perturbation from −1.5 W m-2 to −0.8 W m-2.

scholarly journals Quantification of DMS aerosol-cloud-climate interactions using ECHAM5-HAMMOZ model in current climate scenario

2010 ◽  
Vol 10 (2) ◽  
pp. 3087-3127 ◽  
Author(s):  
M. A. Thomas ◽  
P. Suntharalingam ◽  
L. Pozzoli ◽  
S. Rast ◽  
A. Devasthale ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Cloud Cover ◽  
Dimethyl Sulfide ◽  
Circulation Model ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Droplet Radius ◽  
Aerosol Cloud ◽  
Southern Oceans

Abstract. The contribution of ocean dimethyl sulfide (DMS) emissions to changes in cloud microphysical properties is quantified seasonally and globally for present day climate conditions using an aerosol-chemistry-climate general circulation model, ECHAM5-HAMMOZ, coupled to a cloud microphysics scheme. We evaluate DMS aerosol-cloud-climate linkages over the southern oceans where anthropogenic influence is minimal. The changes in the number of activated particles, cloud droplet number concentration (CDNC), cloud droplet effective radius, cloud cover and the radiative forcing are examined by analyzing two simulations: a baseline simulation with ocean DMS emissions derived from a prescribed climatology and one in which the ocean DMS emissions are switched off. Our simulations show that the model realistically simulates the seasonality in the number of activated particles and CDNC, peaking during Southern Hemisphere (SH) summer coincident with increased phytoplankton blooms and gradually declining with a minimum in SH winter. In comparison to a simulation with no DMS, the CDNC level over the southern oceans is 128% larger in the baseline simulation averaged over the austral summer months. Our results also show an increased number of smaller sized cloud droplets during this period. We estimate a maximum decrease of up to 15–18% in the droplet radius and a mean increase in cloud cover by around 2.5% over the southern oceans during SH summer in the simulation with ocean DMS compared to when the DMS emissions are switched off. The global annual mean top of the atmosphere DMS aerosol all sky radiative forcing is −2.03 W/m2, whereas, over the southern oceans during SH summer, the mean DMS aerosol radiative forcing reaches −9.32 W/m2.

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