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A large part of the uncertainty in climate projections comes from uncertain aerosol properties and aerosol-cloud interactions as well as the difficulty in remotely sensing them. The southeastern Atlantic functions as a natural laboratory to study biomass-burning smoke and to constrain this uncertainty. We address these gaps by comparing the Weather Research and Forecasting with Chemistry Community Atmosphere Model (WRF-CAM5) to the multi-campaign observations ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS), CLARIFY (CLoud-Aerosol-Radiation Interaction and Forcing), and LASIC (Layered Atlantic Smoke Interactions with Clouds) in the southeastern Atlantic in August 2017 to evaluate a large range of the model's aerosol chemical properties, size distributions, processes, and transport, as well as aerosol-cloud interactions. Overall, while WRF-CAM5 is able to represent smoke properties and transport, some key discrepancies highlight the need for further analysis. Observations of smoke composition show an overall decrease in aerosol mean diameter as smoke ages over 4-12 d, while the model lacks this trend. A decrease in the mass ratio of organic aerosol (OA) to black carbon (BC), OA:BC, and the OA mass to carbon monoxide (CO) mixing ratio, OA:CO, suggests that the model is missing processes that selectively remove OA from the particle phase, such as photolysis and heterogeneous aerosol chemistry. A large (factor of ∼2.5) enhancement in sulfate from the free troposphere (FT) to the boundary layer (BL) in observations is not present in the model, pointing to the importance of properly representing secondary sulfate aerosol formation from marine dimethyl sulfide and gaseous SO2 smoke emissions. The model shows a persistent overprediction of aerosols in the marine boundary layer (MBL), especially for clean conditions, which multiple pieces of evidence link to weaker aerosol removal in the modeled MBL than reality. This evidence includes several model features, such as not representing observed shifts towards smaller aerosol diameters, inaccurate concentration ratios of carbon monoxide and black carbon, underprediction of heavy rain events, and little evidence of persistent biases in modeled entrainment. The average below-cloud aerosol activation fraction (NCLD/NAER) remains relatively constant in WRF-CAM5 between field campaigns (∼0.65), while it decreases substantially in observations from ORACLES (∼0.78) to CLARIFY (∼0.5), which could be due to the model misrepresentation of clean aerosol conditions. WRF-CAM5 also overshoots an observed upper limit on liquid cloud droplet concentration around NCLDCombining double low line 400-500 cm-3 and overpredicts the spread in NCLD. This could be related to the model often drastically overestimating the strength of boundary layer vertical turbulence by up to a factor of 10. We expect these results to motivate similar evaluations of other modeling systems and promote model development to reduce critical uncertainties in climate simulations. Copyright:

Aerosol effective radiative forcing (ERF) has persisted as the most uncertain aspect of anthropogenic forcing over the industrial period, limiting our ability to constrain estimates of climate sensitivity and to confidently predict 21st century climate change. Aerosol–cloud interactions are the most uncertain component of aerosol ERF. The 2014–2015 Holuhraun volcanic eruption acted as a large source of sulfur dioxide, providing an opportunistic experiment for studying aerosol–cloud interactions at a climatically relevant scale. We evaluate the observed aerosol-induced perturbation to marine liquid cloud properties inside the volcanic plume in the first month of the eruption and compare the results to those from UKESM1 (UK Earth System Model). In the first 2 weeks, as expected, we find an in-plume shift to smaller and more numerous cloud droplets in both the observations and the simulations. We find an observed increase in liquid water path (LWP) values inside the plume that is not captured in UKESM1. However, in the third week, the in-plume shift to smaller and more numerous cloud droplets is neither observed nor modelled, and there are discrepancies between the observed and modelled response in the fourth week. An analysis of the model simulations and trajectory modelling reveals that air mass history and background meteorological factors can strongly influence aerosol–cloud interactions between the weeks of our analysis. Overall, our study supports the findings of many previous studies: the aerosol impact on cloud effective radius is significant, with differences in the observed and modelled response for in-cloud LWP. [ABSTRACT FROM AUTHOR]

The Geoengineering Model Intercomparison Project (GeoMIP) has proposed multiple model experiments during phases 5 and 6 of the Climate Model Intercomparison Project (CMIP), with the latest set of model experiments proposed in 2015. With phase 7 of CMIP in preparation and with multiple efforts ongoing to better explore the potential space of outcomes for different solar radiation modifications (SRMs) both in terms of deployment strategies and scenarios and in terms of potential impacts, the GeoMIP community has identified the need to propose and conduct a new experiment that could serve as a bridge between past iterations and future CMIP7 experiments. Here we report the details of such a proposed experiment, named G6-1.5K-SAI, to be conducted with the current generation of scenarios and models from CMIP6 and clarify the reasoning behind many of the new choices introduced. Namely, compared to the CMIP6 GeoMIP scenario G6sulfur, we decided on (1) an intermediate emission scenario as a baseline (the Shared Socioeconomic Pathway 2-4.5), (2) a start date set in the future that includes both considerations for the likelihood of exceeding 1.5 °C above preindustrial levels and some considerations for a likely start date for an SRM implementation, and (3) a deployment strategy for stratospheric aerosol injection that does not inject in the tropical pipe in order to obtain a more latitudinally uniform aerosol distribution. We also offer more details regarding the preferred experiment length and number of ensemble members and include potential options for second-tier experiments that some modeling groups might want to run. The specifics of the proposed experiment will further allow for a more direct comparison between results obtained from CMIP6 models and those obtained from future scenarios for CMIP7. [ABSTRACT FROM AUTHOR]

Aerosol effective radiative forcing (ERF) has persisted as the most uncertain aspect of anthropogenic forcing over the industrial period, limiting our ability to constrain estimates of climate sensitivity and to confidently predict 21st century climate change. Aerosol-cloud interactions are the most uncertain component of aerosol ERF. The 2014–15 Holuhraun volcanic eruption acted as large source of sulphur dioxide, providing an opportunistic experiment for studying aerosol-cloud interactions at a climatically relevant scale. We evaluate the observed aerosol-induced perturbation to cloud properties inside the volcanic plume in the first month of the eruption and compare the results to those from UKESM1 (UK Earth System Model). In the first two weeks, as expected, we find an in-plume shift to smaller and more numerous cloud droplets in both the observations and the simulations, as well as an observed change in the distribution of liquid water path (LWP) values inside the plume. However, in the third week, the in-plume shift to smaller and more numerous cloud droplets is neither observed nor modelled, and there are discrepancies between the observed and modelled response in the fourth week. Analysis of the model simulations and trajectory modelling reveals that airmass history and background meteorological factors can strongly influence aerosol-cloud interactions between the weeks of our analysis. Overall, our study supports the findings of many previous studies; that the aerosol impact on cloud effective radius is significant, with a less significant effect on in-cloud LWP. [ABSTRACT FROM AUTHOR]

Aerosol effective radiative forcing (ERF) has persisted as the most uncertain aspect of anthropogenic forcing over the industrial period, limiting our ability to constrain estimates of climate sensitivity and to confidently predict 21st century climate change. Aerosol-cloud interactions are the most uncertain component of aerosol ERF. The 2014–15 Holuhraun volcanic eruption acted as large source of sulphur dioxide, providing an opportunistic experiment for studying aerosol-cloud interactions at a climatically relevant scale. We evaluate the observed aerosol-induced perturbation to cloud properties inside the volcanic plume in the first month of the eruption and compare the results to those from UKESM1 (UK Earth System Model). In the first two weeks, as expected, we find an in-plume shift to smaller and more numerous cloud droplets in both the observations and the simulations, as well as an observed change in the distribution of liquid water path (LWP) values inside the plume. However, in the third week, the in-plume shift to smaller and more numerous cloud droplets is neither observed nor modelled, and there are discrepancies between the observed and modelled response in the fourth week. Analysis of the model simulations and trajectory modelling reveals that airmass history and background meteorological factors can strongly influence aerosol-cloud interactions between the weeks of our analysis. Overall, our study supports the findings of many previous studies; that the aerosol impact on cloud effective radius is significant, with a less significant effect on in-cloud LWP. [ABSTRACT FROM AUTHOR]

Extreme precipitation events are linked to severe economic losses and casualties in China every year; hence, exploring the potential mitigation strategies to minimize these events and their changes in frequency and intensity under global warming is of importance, particularly for the populous subregions. In addition to global warming scenarios, this study examines the effects of the potential deployment of stratospheric aerosol injection (SAI) on hydrological extremes in China based on the SAI simulations (G6sulfur) of the Geoengineering Model Intercomparison Project (GeoMIP) from UKESM1 (The UK Earth System Model) simulations. The simulated SAI deployment is compared with simulations of the future climate under two different emission scenarios (SSP5-8.5 and SSP2-4.5) and reduction in the solar constant (G6solar) to understand the effect of SAI on extreme precipitation patterns. The results show that, under future global warming scenarios, precipitation and extreme wet climate events are projected to increase by 2100 relative to the present day across all the subregions in China. Additionally, analyses of extreme drought events show a projected increase in southern China. The G6sulfur and G6solar experiments ameliorate the increases in extreme rainfall intensities, especially for the eastern subregions of China. The impacts of SAI in decreasing extreme precipitation events and in consecutive wet days are more pronounced than in G6solar. While the results from both G6sulfur and G6solar show encouraging abatement of many of the impacts on detrimental extreme events that are evident in SSP5-8.5 there are some exceptions. Both G6sulfur and G6solar show drying trendsat high latitudes within the region, which is consistent with our understanding of the spin-down of the hydrological cycle under SRM. For instance, the projected dry days increase for G6sulfur compared to SSP5-8.5. These side effects imply that a cautionary approach and further optimization may be required should any future SRM deployment be considered. [ABSTRACT FROM AUTHOR]

Extreme precipitation events are linked to severe economic losses and casualties in China every year; hence, exploring the potential mitigation strategies to minimize these events and their changes in frequency and intensity under global 20 warming is of importance, particularly for the populous subregions. In addition to global warming scenarios, this study examines the effects of the potential deployment of stratospheric aerosol injection (SAI) on hydrological extremes in China based on the SAI simulations (G6sulfur) of the Geoengineering Model Intercomparison Project (GeoMIP) from UKESM1 (The UK Earth System Model) simulations. The simulated SAI deployment is compared with simulations of the future climate under two different emission scenarios (SSP5-8.5 and SSP2-4.5) and reduction in the solar constant (G6solar) to understand 25 the effect of SAI on extreme precipitation patterns. The results show that, under future global warming scenarios, precipitation and extreme wet climate events are projected to increase by 2100 relative to the present day across all the subregions in China. Additionally, analyses of extreme drought events show a projected increase in southern China. The G6sulfur and G6solar experiments ameliorate the increases in extreme rainfall intensities, especially for the eastern subregions of China. The impacts of SAI in decreasing extreme precipitation events and in consecutive wet days are more pronounced than in G6solar. While 30 the results from both G6sulfur and G6solar show encouraging abatement of many of the impacts on detrimental extreme events that are evident in SSP5-8.5 there are some exceptions. Both G6sulfur and G6solar show drying trends at high latitudes within the region, which is consistent with our understanding of the spin-down of the hydrological cycle under SRM. For instance, the projected dry days increase for G6sulfur compared to SSP5-8.5. These side effects imply that a cautionary approach and further optimization may be required should any future SRM deployment be considered. [ABSTRACT FROM AUTHOR]

The Geoengineering Model Intercomparison Project (GeoMIP) has proposed multiple model experiments during the phases 5 and 6 of the Climate Model Intercomparison Project (CMIP), with the latest set of model experiment proposed in 2015. With phase 7 of CMIP in preparation, and with multiple efforts ongoing to better explore the potential space of outcomes for different Solar Radiation Modification (SRM) both in terms of deployment strategies and scenarios and in terms of potential impacts, the GeoMIP community has identified the need to propose and conduct a new experiment that could serve as a bridge between past iterations and future CMIP7 experiments. Here we report the details of such a proposed experiment, named G6-1.5K-SAI, to be conducted with the current generation of scenarios and models from CMIP6, and clarify the reasoning behind many of the new choices introduced. Namely, compared to the CMIP6 GeoMIP scenario G6sulfur, here we decided on: 1) an intermediate emission scenario as baseline (the Shared Socioeconomic Pathway 2-4.5); 2) a start date set in the future that includes both considerations around the likelihood of exceeding 1.5 ºC above preindustrial and some considerations around a likely start date for an SRM implementation; 3) a deployment strategy for Stratospheric Aerosol Injection that does not inject in the tropical pipe in order to obtain a more latitudinally uniform aerosol distribution. We also offer more details over the preferred experiment length and number of ensemble members, and include potential options for second-tier experiments some modeling groups might want to run. The specifics of the proposed experiment will further allow for a more direct comparison between results obtained with CMIP6 models and those obtained with future scenarios for CMIP7. [ABSTRACT FROM AUTHOR]

The vast majority of reductions in aerosol emissions are projected to take place in the near future; however, associated impacts on the large-scale circulation over the populated Asian monsoon region remain uncertain. Using the state-of-the-art UK Earth System Model version 1 (UKESM1), this study examines the response of the South Asian summer monsoon and East Asian summer monsoon (SASM and EASM, respectively) to idealized reductions in anthropogenic emissions of carbonaceous aerosols and SO2. The analysis focuses on changes in the monsoon temporal extent and intensity of precipitation following decreases in either scattering (SCT) or absorbing (ABS) aerosols or decreases in both. For SCT, the combination of the early transition of land–sea thermal contrast and sea level pressure gradient during the pre-monsoon season, together with the late transition in the post-monsoon season associated with the tropospheric warming, advances the monsoon onset but delays its withdrawal, which leads to an extension of the summer rainy season across South Asia and East Asia. The northward shift of the upper-tropospheric Asian jet forced by the SCT reduction causes the anomalous convergence of tropospheric moisture and low-level ascent over northern India and eastern China. The intensification of the South Asian high (SAH) due to the warming over land also contributes to the dynamic instability over Asia. These changes enhance the rainy season of these regions in boreal summer. Reductions in absorbing aerosol act in the opposite sense, making the Asia's rainy season shorter and weaker due to the opposite impacts on land–sea contrast, Asian jet displacement and SAH intensity. With reductions in both SCT and ABS aerosol together the monsoon systems intensify, as the overall impact is dominated by aerosol scattering effects and results in the strengthening of monsoon precipitation and 850 hPa circulation. Although aerosol scattering and absorption play quite different roles in the radiation budget, their effects on the monsoon precipitation seem to add almost linearly. Specifically, the patterns of monsoon-related large-scale responses from reducing both SCT and ABS together are similar to the linear summation of separate effect of reducing SCT or ABS alone; this is despite the inherent nonlinearity of the atmospheric systems. The opposing adjustments of Asian rainy season forced by the ABS and SCT aerosol emission reductions suggest that emission controls that target factors like emissions of black carbon that warm the climate would have a different response to those that target overall aerosol emissions. [ABSTRACT FROM AUTHOR]

Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosolcloud interactions. Here we show that the massive 20142015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud dropletsconsistent with expectationsbut had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around 0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response., Author(s): Florent F. Malavelle (corresponding author) [1]; Jim M. Haywood [1, 2]; Andy Jones [2]; Andrew Gettelman [3]; Lieven Clarisse [4]; Sophie Bauduin [4]; Richard P. Allan [5, 6]; Inger [...]

The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinating framework, started in 2010, that includes a series of standardized climate model experiments aimed at understanding the physical processes and projected impacts of solar geoengineering. Numerous experiments have been conducted, and numerous more have been proposed as "test-bed" experiments, spanning a variety of geoengineering techniques aimed at modifying the planetary radiation budget: stratospheric aerosol injection, marine cloud brightening, surface albedo modification, cirrus cloud thinning, and sunshade mirrors. To date, more than 100 studies have been published that used results from GeoMIP simulations. Here we provide a critical assessment of GeoMIP and its experiments. We discuss its successes and missed opportunities, for instance in terms of which experiments elicited more interest from the scientific community and which did not, and the potential reasons why that happened. We also discuss the knowledge that GeoMIP has contributed to the field of geoengineering research and climate science as a whole: what have we learned in terms of intermodel differences, robustness of the projected outcomes for specific geoengineering methods, and future areas of model development that would be necessary in the future? We also offer multiple examples of cases where GeoMIP experiments were fundamental for international assessments of climate change. Finally, we provide a series of recommendations, regarding both future experiments and more general activities, with the goal of continuously deepening our understanding of the effects of potential geoengineering approaches and reducing uncertainties in climate outcomes, important for assessing wider impacts on societies and ecosystems. In doing so, we refine the purpose of GeoMIP and outline a series of criteria whereby GeoMIP can best serve its participants, stakeholders, and the broader science community. [ABSTRACT FROM AUTHOR]

For over 6-months, the 2014-2015 effusive eruption at Holuhraun, Iceland injected considerable amounts of sulphur dioxide (SO2) into the lower troposphere with a daily rate of up to one-third of the global emission rate causing extensive air pollution across Europe. The large injection of SO2, which oxidises to form sulphate aerosol (SO42-), provides a natural experiment offering an ideal opportunity to scrutinise state-of-the-art general circulation models (GCMs) representation of aerosol-cloud interactions (ACIs). Here we present Part 1 of a two-part model inter-comparison using the Holuhraun eruption as a framework to analyse ACIs. We use SO2 retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) instrument and ground-based measurements of SO2 and SO42- mass concentrations across Europe in conjunction with trajectory analysis using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to assess the spatial and chemical evolution of the volcanic plume as simulated by 5 GCMs and a chemical transport model (CTM). IASI retrievals of plume altitude and SO2 column load reveal that the volcanic perturbation is largely contained within the lower troposphere and that the spatial evolution and vertical variability of the plume is reasonably well captured by the models, although the models underestimate the mean plume altitude. HYSPLIT trajectories are used to attribute to Holuhraun emissions 184 instances of elevated sulphurous surface mass concentrations recorded at 22 air monitoring stations across Europe. Comparisons with the simulated concentrations show that the models underestimate the elevated SO2 concentrations observed at stations closer to Holuhraun whilst overestimating those observed further away. Using a biexponential function to describe the decay of observed surface mass concentration ratios of SO2-to-SO42- with plume age, in-plume gas-phase and aqueous-phase oxidation rates are estimated as 0.031 ± 0.002 h-1 and 0.22 ± 0.16 h-1 respectively with a near-vent ratio of 31 ± 4 [μgm-3 of SO2 / ugm-3 of SO42-]. The derived gas-phase oxidation rates from the models are all lower than the observed estimate, whilst the majority of the aqueous-phase oxidation rates agree with the observed rate. This suggests that the simulated plumes capture the observed chemical behaviour in the young plume (when aqueous-phase oxidation is dominant), yet not in the mature plume (when gas-phase oxidation is dominant). Overall, despite their coarse resolution, the 6 models show reasonable skill in capturing the spatial and chemical evolution of the Holuhraun plume which is essential when exploring the eruption impact on ACIs in the second part of this study. [ABSTRACT FROM AUTHOR]

The vast majority of reductions in aerosol emissions are projected to take place in the near future; however, associated impacts on the large-scale circulation over the populated Asian monsoon region remain uncertain. Using the state-of-the-art UK Earth System Model version 1 (UKESM1), this study examines the response of the South Asian and East Asian summer monsoon (SASM and EASM) to idealized reductions in anthropogenic emissions of carbonaceous aerosols and SO2. The analysis focuses on changes in the monsoon temporal extent and intensity of precipitation following decreases in either scattering (SCT), absorbing (ABS) aerosols, or decreases in both. For SCT, the combination of the early transition of land-sea thermal contrast and sea level pressure gradient during the pre-monsoon season together with the late transition in the post-monsoon season associated with the tropospheric warming, advances the monsoon onset but delays its withdrawal, which leads to an extension of the summer rainy season across South and East Asia. The northward shift of the upper-tropospheric Asian jet forced by the SCT reduction causes the anomalous convergence of tropospheric moisture and low-level ascent over northern India and eastern China. The intensification of the South Asian High (SAH) due to the warming over land also contributes to the dynamic instability over Asia. These changes enhance the rainy season of these regions in boreal summer. Reductions in absorbing aerosol act in the opposite sense, making the Asia's rainy season shorter and weaker due to the opposite impacts on land-sea contrast, Asian jet displacement and SAH intensity. With reductions in both SCT and ABS aerosol together the monsoon systems intensify, as the overall impact is dominated by aerosol scattering effects and results in the strengthening of monsoon precipitation and 850-hPa circulation. Although aerosol scattering and absorption play quite different roles in the radiation budget, their effects on the monsoon precipitation seem to add almost linearly. Specifically, the patterns of monsoon-related large-scale responses from reducing both SCT and ABS together are similar to the linear summation of separate effect of reducing SCT or ABS alone, despite of the inherent nonlinearity of the atmospheric systems. Our findings suggest that emission controls that target e.g. emissions of black carbon that warm the climate would have a different response to those that target overall aerosol emissions. [ABSTRACT FROM AUTHOR]