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Atmospheric aerosols affect the Earth's climate in many ways, including acting as the seeds on which cloud droplets form. Since a large fraction of these particles is anthropogenic, the clouds' microphysical and radiative characteristics are influenced by human activity on a global scale leading to important climatic effects. The respective change in the energy budget at the top of the atmosphere is defined as the effective radiative forcing due to aerosol‐cloud interaction (ERFaci). It is estimated that the ERFaci offsets presently nearly 1/4 of the greenhouse‐induced warming, but the uncertainty is within a factor of two. A common method to calculate the ERFaci is by the multiplication of the susceptibility of the cloud radiative effect to changes in aerosols by the anthropogenic change of the aerosol concentration. This has to be done by integrating it over all cloud regimes. Here we review the various methods of the ERFaci estimation. Global measurements require satellites' global coverage. The challenge of quantifying aerosol amounts in cloudy atmospheres are met with the rapid development of novel methodologies reviewed here. The aerosol characteristics can be retrieved from space based on their optical properties, including polarization. The concentrations of the aerosols that serve as cloud drop condensation nuclei can be also estimated from their impact on the satellite‐retrieved cloud drop number concentrations. These observations are critical for reducing the uncertainty in the ERFaci calculated from global climate models (GCMs), but further development is required to allow GCMs to properly simulate and benefit these novel observables. Plain Language Summary: Aerosols affect the climate in many ways, including serving as the basis for the formation of cloud droplets. Therefore, aerosols have profound impacts on clouds and through that on the Earth's energy budget. Increasing aerosols leads to additional cloud droplets, changing cloud properties such that they reflect more solar radiation back to space and offset nearly 1/4 of the warming induced by greenhouse gases, but with a large uncertainty within a factor of two. Here we review the ways by which aerosols and their radiative effects are retrieved from space. A major challenge is to reduce the uncertainty by better retrieval methods of atmospheric aerosol and cloud properties. This challenge is met by rapid satellite retrieval methodological developments and numerous new satellite missions, which are described here. These new methodologies have to be matched with parallel development in the global circulation models for the improved estimation of the actual climatic impacts. Key Points: Calculating the susceptibility of clouds to aerosols has to include the cleanest conditions where measuring the aerosols is challengingThe definition and use of regimes to group clouds with similar responses to aerosol is vital for future observation‐based effective radiative forcing due to aerosol‐cloud interaction (ERFaci) estimatesNew observational tools are vital for reducing ERFaci uncertainty but require further global climate model development to simulate these novel observables [ABSTRACT FROM AUTHOR]

The estimation of cloud radiative forcing due to aerosol‐cloud interactions, RFaci (also known as the first indirect effect), relies on approximating the cloud albedo susceptibility to changes in droplet concentration, β. β depends on the cloud albedo and droplet concentration, both of which can be observed by satellites. Satellite observations are often spatially aggregated to coarser resolutions, typically 1 × 1° scenes. However, on such spatial scales, the cloud albedo tends to be heterogeneous, whereas the β approximation assumes homogeneity. Here, we demonstrate that the common practice of aggregating satellite data and neglecting cloud albedo heterogeneity results in an average overestimation of 10% in previous estimates of the RFaci. Additionally, we establish a relationship between the magnitude of the bias in β and Stratocumulus morphologies, providing a physical context for cloud heterogeneity and the associated bias. Lastly, we propose a correction method that can be applied to cloud albedo gridded data. Plain Language Summary: This paper explores the effect of cloud albedo morphology, which is a reflection of cloud heterogeneity, on radiative forcing due to aerosol‐cloud interactions (RFaci). The RFaci is estimated from satellite observations based on the assumption that clouds are homogeneous within a given scene. However, when satellite data is spatially aggregated to reduce the amount of data to a user‐friendly gridded format—a common practice—this assumption is no longer valid. Consequently, an overestimation of the RFaci occurs, particularly in heterogeneous scenes, where the overestimation can reach up to 50%. This means that the RFaci is lower than previously estimated. Our results also suggest that cloud albedo enhancement due to an increase in droplet concentrations would be most effective in homogeneous scenes. Therefore, marine cloud brightening strategies should take cloud albedo homogeneity into account to achieve the most effective albedo enhancement. Key Points: The common practice of spatial aggregation of satellite data into 1 × 1° scenes leads to an average 10% overestimation of the RFaciThe overestimation is due to neglecting cloud albedo heterogeneity, and is associated to different types of Stratocumulus morphologiesA correction is proposed, which calls for the incorporation of cloud reflectance statistics in Level 3 data [ABSTRACT FROM AUTHOR]

Satellite images show solid marine stratocumulus cloud decks (Sc) that break up over the remote oceans. The Sc breakup is initiated by precipitation and is accompanied by a strong reduction in the cloud radiative effect. Aerosol has been shown to delay the Sc breakup by postponing the onset of precipitation, however its climatic effect is uncertain. Here we introduce a new approach that allows us to re‐cast currently observed cloud cover and albedo to their counterfactual cleaner world, enabling the first estimate of the radiative effect due to delayed cloud breakup. Using simple radiative approximation, the radiative forcing with respect to pre‐industrial times due to delayed Sc breakup is −0.39 W m−2. The radiative effect changes nearly linearly with aerosol due to the droplet concentration control on the cloud cover, suggesting a potentially accelerated warming if the current trend of reduction in aerosol emissions continues. Plain Language Summary: The response of cloud cover to aerosol is a climatologically important quantity that has been extremely difficult to estimate. The challenge is that one would need to estimate the fractional area that is currently overcast, but which would have been partly overcast in a cleaner atmosphere. Global climate models (GCMs) are one tool to address such a problem. They allow one to change aerosol levels and to evaluate the cloud response. However, representation of warm, low‐level cloud processes, and in particular aerosol‐cloud interactions in GCMs, is inadequate. Here we introduce an observational method that allows us to re‐cast the currently observed cloud cover and albedo of oceanic warm clouds to their counterfactual state in a cleaner world. We find a linear relationship between the cloud radiative effect and droplet concentration. If we continue to experience a decrease in aerosol emissions then we anticipate a reduction in the aerosol‐cloud radiative effect. The global annual radiative forcing associated with anthropogenic aerosol delaying closed cell breakup is found to be −0.39 W m−2. Key Points: A simple model is presented to describe closed cell breakup by initiation of precipitationThe model demonstrates that the global annual radiative effect due to delayed closed cells breakup changes nearly linearly with emissionsThe linearity emerges from the nearly linear relationship between cloud cover and albedo [ABSTRACT FROM AUTHOR]

The Arctic climate changes at a faster rate than the rest of the globe. Boundary‐layer clouds may play an important role in this change. At temperatures below 0°C, mixed‐phase clouds exist and their phase and longevity is influenced by the abundance of ice crystals, which in turn is a function of aerosols serving as ice nucleating particles (INPs). Previous in situ studies suggested a local source of INPs due to biological activity over open ocean. Here we investigate ice crystal concentrations in clouds below 2 km at a large scale, by exploiting a newly developed data set—DARDAR‐Nice—retrieved from active satellite remote sensing. The data set spans from 2006 to 2016. Contrary to previous expectation, we find that at a given latitude and temperature, there are more ice crystals over sea ice than over open ocean. This enhancement is particularly found in clouds south of 70°N, and also at temperatures between 0°C and −10°C. Plain Language Summary: The Arctic region is particularly affected by climate change, its warming is 2–3 times larger than global average during recent decades. One of the contributors to this "Arctic Amplification" may be the Arctic clouds and in particular the mixed phase type, where ice and supercooled liquid coexist at temperatures lower than 0°C. Aerosols play a significant role in cloud formation, since without the presence of some effective particles, the ice crystals could not form at all at temperatures between 0°C and roughly −40°C. In this study, we use a new satellite data set which provides an important cloud quantity, the amount of ice crystals in the clouds. Although this data set is limited to pure ice clouds, it can prove useful for understanding the behavior of Arctic clouds. What we find here is that Arctic low‐level clouds show larger quantities of ice crystals over sea ice than over ocean and we think that this can be attributed to the amount and type of aerosols related to each surface. This finding contradicts a previous hypothesis, which stated that more ice crystals would possibly form over ocean because of the presence of highly ice effective aerosols there. Key Points: New ice microphysics from active satellite instruments enable large‐scale analysis of Arctic boundary‐layer cloudsIce crystal numbers are enhanced over sea ice compared to open ocean at temperatures above −10°CThis difference is most pronounced in clouds south of 70°N through all the temperature range [ABSTRACT FROM AUTHOR]

Contrails can persist in cloud‐free supersaturated air, increasing high‐cloud cover, and inside natural cirrus cloud, modifying the microphysical properties of them. The latter effect is almost unknown, partly because of the lack of height‐resolved measurements and the capability of measurements to penetrate inside the cloud. New retrievals of the ice crystal number concentration from combined satellite cloud radar and lidar measurements (CloudSat/CALIPSO; DARDAR‐Nice algorithm) now allow for satellite‐based assessment inside the clouds. We investigate this issue at intersections between the aircraft flight tracks and these retrieval profiles. Regions behind the aircraft inside the flight track were compared to the adjacent regions and to ahead of the aircraft, along the satellites' profiles, where DARDAR‐Nice identify geometrically thin cirrus clouds. This comparison revealed a statistically significant increase of 25% and 54% in the concentration of ice crystals with the minimum size of 5 μm around 300–540‐m beneath an aircraft's flight altitude. Plain Language Summary: Aircraft emissions are the only human‐made source of pollution that is directly injected into the upper troposphere. Aircraft can form trails of ice crystals in a cloud‐free atmosphere and therefore change the high‐cloud cover. If they form inside the already existing cirrus clouds, they will modify the cloud microphysical properties, an effect that is not well quantified so far. In this study, we use a new retrieval of ice crystal number concentration from satellite lidar and radar, which provides height resolved information. We analyze intersections between the aircraft flight tracks and the swath of the satellite by comparing the ice crystal concentration in the regions affected by aviation with the neighboring regions and those ahead of the aircraft. The result of this work reveals a statistically significant increase in ice crystal concentration within clouds a few hundred meters beneath the flight height. Key Points: New satellite retrievals of ice crystal number concentration allow to assess microphysical effects of aviation on existing cirrus cloudsComparison of ICNC profiles on existing cirrus clouds shows increase after aircraft by 25% and 54% ICNCThe signal is the largest 300–540 m below the flight altitude [ABSTRACT FROM AUTHOR]

An analysis of the life cycle of shallow marine cumulus clouds is presented based on geostationary observations by the Spinning Enhanced Visible and InfraRed Imager aboard Meteosat Second Generation (MSG‐SEVIRI). Trajectories of about 250,000 individual shallow marine cumulus clouds have been derived by applying Particle Image Velocimetry to the Satellite Application Facility on Climate Monitoring CLoud property dAtAset using SEVIRI for a region in the trade wind zone centered around the Canary Islands in August 2015. The temporal evolution of the physical properties of these clouds allows to characterize cloud development and to infer the distribution of cloud life time and cloud extent. In the derived data set, the life time distribution follows a double power law with most clouds existing on a time scale of tens of minutes. The cloud physical properties, available during daytime, are analyzed along the cloud tracks. Relative time series of cloud extent, cloud water path, cloud droplet effective radius at cloud top, cloud optical thickness, and cloud droplet number concentration for clouds in two temporal ranges reveal conditions that can be attributed to long‐lasting clouds. Clouds of a certain horizontal extent and cloud top height as well as cloud droplet radius show longer life times if they are optically more dense, i.e., have a higher droplet number concentration. Furthermore, the investigation of the content of liquid cloud water regarding cloud life time and cloud extent shows that small short‐living clouds significantly contribute to cloud radiative effects. Plain Language Summary: A comprehensive analysis of the life cycle of shallow marine cumulus clouds is presented based on measurements of a specialized instrument, called SEVIRI, aboard Meteosat's Second Generation geostationary meteorological satellite. A new method is applied to derive the physic‐property temporal evolution of approximately 250,000 individual clouds in a region around the Canary Islands during August 2015. Several constraints are applied to infer the relationship between cloud life time and various cloud parameters. The study reveals that cloud life time is related to the optical thickness when constrained by horizontal extent, cloud top height, and droplet radius. The analysis further shows that small short‐living clouds significantly contribute to cloud radiative effects. Key Points: The life cycle of shallow marine cumulus clouds is inferred using a passive space‐based geostationary instrumentLife cycle is quantified by top temperature/height, cloud extent, cloud water path, optical thickness, and droplet radius/number concentrationCumulus clouds of a certain horizontal extent, cloud top height as well as droplet radius live longer if they are optically denser [ABSTRACT FROM AUTHOR]

Several regions worldwide have seen significant trends in anthropogenic aerosol emissions during the period of detailed satellite observations since 2001. Over Europe (EUR) and North America (NAM) there were strong declines, over China increases then declines and over India, strong increases. Regional trends in model‐simulated aerosol optical depth (AOD) and cloud radiative effects in both the Fifth and Sixth Coupled Model Intercomparison Projects (CMIP5 and CMIP6) are broadly consistent with the ones from satellite retrievals in most parts of EUR, NAM and India. CMIP6 models better match satellite‐derived AOD trend in western NAM (increasing) and eastern China (decreasing), where CMIP5 models failed, pointing to improved anthropogenic aerosol emissions. Drop concentration trends in both observations and models qualitatively match AOD trends. The result for solar cloud radiative effect in models, however, is due to compensating errors: Models fail to reproduce observed liquid water path trends and show, in turn, opposite trends in cloud fraction. Plain Language Summary: Historically, widespread changes in regional air pollution have occurred, with emission reductions over the United States and Europe in response to air quality controls since the mid‐1980s, and a general increase over Asia since the 1970s. In this study we found that current climate models with improved emission estimates match the regional aerosol and cloud trends derived from satellite observations over United States, Europe, China, and India regions. Further analysis revealed that good representation of regional solar cloud radiative effect trend is due to compensating errors in cloud trend in the models. Key Points: Strong trends in AOD in aerosol source regions during 2001 to 2017 reflected in cloud droplet number concentration trendsThe CMIP6 multimodel ensemble shows improved results compared to CMIP5 pointing to improved anthropogenic aerosol emissionsModel results in LWP inconsistent with data: trends in CRE match observations due to compensating errors in cloud fraction trends [ABSTRACT FROM AUTHOR]

Abstract: Rapid cloud adjustments are an important component of the atmosphere's total response to increased CO2 concentrations. Unfortunately, scientific understanding of rapid shortwave cloud adjustments is rather poor. State‐of‐the‐art 5th Coupled Model Intercomparison Project models showed large uncertainty in regard to rapid cloud adjustments. This study determines whether large‐eddy simulations may, in principle, be used as a reference, thanks to their ability to resolve cloud dynamics and thermodynamics, to constrain rapid shortwave cloud adjustments in general circulation models. This is an open question since large‐eddy models can only be run over limited domains, for a short period of time, and are influenced by boundary conditions. Using the Icosahedral Non‐hydrostatic global climate model—Atmospheric component (ICON‐A), we examine shortwave rapid cloud adjustments over central Europe, which is found to be representative of shortwave rapid cloud adjustments over Northern Hemispheric global continents in the 5th Coupled Model Intercomparison Project models. This work finds (i) a couple of days of simulation is sufficient to get a clear signal in the net top‐of‐atmosphere radiative balance to emerge after a 4xCO2 perturbation and (ii) use of present‐day meteorological and CO2 concentrations for boundary conditions in global simulations is not an issue for short lead times, up to ∼36 hr. We also found that atmospheric processes influencing shortwave rapid cloud adjustments over central Europe are largely thermodynamically driven changes in local cloud dynamics and are rather independent of the synoptic‐scale and circulation effects on short timescales (<2 days). These results imply that high‐resolved large‐eddy simulations over a limited area can be instructive for assessing and constraining global rapid cloud adjustments. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the relationships between marine stratocumulus clouds (MSC) coupling state with the ocean surface, their precipitation rate and fractional cloud cover (CF). This was possible by developing a novel methodology for satellite retrieval of the clouds coupling state. Decks of overcast MSC were reported in previous studies to break up often as their precipitation rate increases significantly, thus reducing CF and cloud radiative effect substantially. Here we show that decks of precipitating decoupled MSC have larger CF compared to similarly precipitating coupled MSC. The difference in CF between decoupled and coupled clouds was found to increase with precipitation rate, up to nearly doubling the CF of the heaviest precipitating decoupled MSC. This suggests that decoupling is a feature related to higher cloud radiative effect in precipitating MSC. [ABSTRACT FROM AUTHOR]

Radiation management ( RM), as an option to engineer the climate, is highly controversial and suffers from a number of ethical and regulatory concerns, usually studied in the context of the objective to mitigate the global mean temperature. In this article, we discuss the idea that RM can be differentiated and scaled in several dimensions with potential objectives being to influence a certain climate parameter in a specific region. Some short-lived climate forcers (e.g., tropospheric aerosols) exhibit strong geographical and temporal variability, potentially leading to limited-area climate responses. Marine cloud brightening and thinning or dissolution of cirrus clouds could be operated at a rather local scale. It is therefore conceivable that such schemes could be applied with the objective to influence the climate at a regional scale. From a governance perspective, it is desirable to avoid any substantial climate effects of regional RM outside the target region. This, however, could prove impossible for a sustained, long-term RM. In turn, regional RM during limited time periods could prove more feasible without effects beyond the target area. It may be attractive as it potentially provides the opportunity to target the suppression of some extreme events such as heat waves. Research is needed on the traceability of regional RM, for example, using detection and attribution methods. Incentives and implications of regional RM need to be examined, and new governance options have to be conceived. [ABSTRACT FROM AUTHOR]

Three different boundary layer cloud models are incorporated into the ECHAM5 general circulation model (GCM) and compared to CloudSat and CALIPSO satellite observations. The first boundary layer model builds upon the standard Tiedtke (1989) parameterization for shallow convection with an adapted convective trigger; the second is a bulk parameterization of the effects of transient shallow cumulus clouds; and lastly the Dual Mass Flux (DMF) scheme adjusted to better represent shallow convection. The three schemes improved (Sub)Tropical oceanic low-level cloud cover, however, the fraction of low-level cloud cover remains underestimated compared to CALIPSO observations. The representation of precipitation was improved by all schemes as they reduced the frequency of light intensity events <0.01 mm d−1, which were found to dominate the radar reflectivity histograms as well as be the greatest source of differences between ECHAM5 and CloudSat radar reflectivity histograms. For both lidar and radar diagnostics, the differences amongst the schemes are smaller than the differences compared to observations. While the DMF approach remains experimental, as its top-of-atmosphere radiative balance has not been retuned, it shows the most promise in producing nonprecipitating boundary layer clouds. With its internally consistent boundary layer scheme that uses the same bimodal joint distribution with a diffusive and an updraft component for clouds and turbulent transport, the ECHAM5_DMF produces the most realistic boundary layer depth as indicated by the cloud field. In addition, it reduced the frequency of large-scale precipitation intensities of <0.01 mm d−1 the greatest. [ABSTRACT FROM AUTHOR]

This study explores the viability of parameter estimation in the comprehensive general circulation model ECHAM6 using ensemble Kalman filter data assimilation techniques. Four closure parameters of the cumulus-convection scheme are estimated using increasingly less idealized scenarios ranging from perfect-model experiments to the assimilation of conventional observations. Updated parameter values from experiments with real observations are used to assess the error of the model state on short 6 h forecasts and on climatological timescales. All parameters converge to their default values in single parameter perfect-model experiments. Estimating parameters simultaneously has a neutral effect on the success of the parameter estimation, but applying an imperfect model deteriorates the assimilation performance. With real observations, single parameter estimation generates the default parameter value in one case, converges to different parameter values in two cases, and diverges in the fourth case. The implementation of the two converging parameters influences the model state: Although the estimated parameter values lead to an overall error reduction on short timescales, the error of the model state increases on climatological timescales. [ABSTRACT FROM AUTHOR]