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Quantifying the aerosol/cloud-mediated radiative effect at a global scale requires simultaneous satellite retrievals of cloud condensation nuclei (CCN) concentrations and cloud base updraft velocities (Wb ). Hitherto, the inability to do so has been a major cause of high uncertainty regarding anthropogenic aerosol/cloud-mediated radiative forcing. This can be addressed by the emerging capability of estimating CCN and Wb of boundary layer convective clouds from an operational polar orbiting weather satellite. Our methodology uses such clouds as an effective analog for CCN chambers. The cloud base supersaturation (S) is determined by Wb and the satellite-retrieved cloud base drop concentrations (Ndb ), which is the same as CCN(S). Validation against ground-based CCN instruments at Oklahoma, at Manaus, and onboard a ship in the northeast Pacific showed a retrieval accuracy of ±25% to ±30% for individual satellite overpasses. The methodology is presently limited to boundary layer not raining convective clouds of at least 1 km depth that are not obscured by upper layer clouds, including semitransparent cirrus. The limitation for small solar backscattering angles of <25° restricts the satellite coverage to ∼25% of the world area in a single day.

Collisional growth of cloud droplets is an essential yet uncertain process for drizzle and precipitation formation. To improve the quantitative understanding of this key component of cloud‐aerosol‐turbulence interactions, observational studies of collision‐coalescence in a controlled laboratory environment are needed. In an existing convection‐cloud chamber (the Pi Chamber), collisional growth is limited by low liquid water content and short droplet residence times. In this work, we use numerical simulations to explore various configurations of a convection‐cloud chamber that may intensify collision‐coalescence. We employ a large‐eddy simulation (LES) model with a size‐resolved (bin) cloud microphysics scheme to explore how cloud properties and the intensity of collision‐coalescence are affected by the chamber size and aspect ratio, surface roughness, side‐wall wetness, side‐wall temperature arrangement, and aerosol injection rate. Simulations without condensation and evaporation within the domain are first performed to explore the turbulence dynamics and wall fluxes. The LES wall fluxes are used to modify the Scalar Flux‐budget Model, which is then applied to demonstrate the need for non‐uniform side‐wall temperature (two side walls as warm as the bottom and the two others as cold as the top) to maintain high supersaturation in a tall chamber. The results of LES with full cloud microphysics reveal that collision‐coalescence is greatly enhanced by employing a taller chamber with saturated side walls, non‐uniform side‐wall temperature, and rough surfaces. For the conditions explored, although lowering the aerosol injection rate broadens the droplet size distribution, favoring collision‐coalescence, the reduced droplet number concentration decreases the frequency of collisions. Plain Language Summary A convection‐cloud chamber is useful in understanding how turbulence affects the interaction between aerosols and cloud droplets. The current convection‐cloud chamber (the Pi Chamber) is likely too small to explore how turbulence affects the collision‐coalescence among cloud droplets. To see whether collisional growth may be observable in a larger cloud chamber, we use numerical simulations to model the cloud droplet size distributions under several different configurations of the cloud chamber. The results suggest that the likelihood of detectable collisional growth increases significantly in a tall chamber with two warm and two cold saturated side walls and rough wall surfaces. Key Points Collision‐coalescence effects on a steady‐state droplet size distribution are stronger in a taller chamber Wet side walls are essential for maintaining cloud liquid water in a chamber with a low width‐to‐height aspect ratio Rougher surfaces increase surface heat and moisture fluxes, leading to larger liquid water content that promotes collision‐coalescence

Aerosol-cloud interactions (ACI) in warm clouds are the primary source of uncertainty in effective radiative forcing (ERF) during the historical period and, by extension, inferred climate sensitivity. The ERF due to ACI (ERFaci) is composed of the radiative forcing due to changes in cloud microphysics and cloud adjustments to microphysics. Here, we examine the processes that drive ERFaci using a perturbed parameter ensemble (PPE) hosted in CAM6. Observational constraints on the PPE result in substantial constraints in the response of cloud microphysics and macrophysics to anthropogenic aerosol, but only minimal constraint on ERFaci. Examination of cloud and radiation processes in the PPE reveal buffering of ERFaci by the interaction of precipitation efficiency and radiative susceptibility.

Anthropogenic aerosols (AA) can affect cloud and precipitation through aerosol–radiation interaction (ARI) and aerosol–cloud interaction (ACI). Over the past few decades, anthropogenic aerosol emissions have exhibited remarkable changes in the magnitude and in spatial pattern. The most significant changes are the increased emissions over both South Asia and East Asia. In this study, the atmospheric component of a state-of-the-art climate model that includes eight species of tropospheric aerosols, coupled to a multi-level mixed-layer ocean model, has been used to investigate the impacts of Asian anthropogenic aerosol precursor emission changes from 1970s to 2000s on large scale circulation and precipitation in boreal summer over East Asia. Results reveal significant changes in circulation and clouds over East Asia and over the tropical and western North Pacific (WNP). Increased Asian AA emissions lead to anomalous cyclonic circulation over the Maritime continent (MC) and anomalous anticyclonic circulation over the WNP, resulting in anomalous moisture transport convergence over the MC and therefore increased precipitation. They also lead to anomalous moisture flux divergence over both the WNP and large land areas of East Asia, especially over northern China, and therefore decreased precipitation there. These large scale circulation anomalies over the adjacent oceans are related to aerosol change induced ocean feedbacks, predominantly through ACI. It is the slow responses over the adjacent oceans (e.g., SST changes) through coupled atmosphere–ocean interaction in pre-monsoon seasons and summer that shape the changes of the East Asian summer monsoon and local precipitation. The results in this study suggest that increased Asian AA emissions from 1970s to 2000s may have played an important role for the observed southward shift of the Pacific intertropical convergence zone and precipitation belt, weakening of East Asian summer monsoon and reduced precipitation over northern China in East Asia during the latter half of the twentieth century.

Recent observations show that dust emitted within the Arctic (Arctic dust) has a remarkably high ice nucleating ability, especially between −20°C and −5°C, but its impacts on the number concentrations of ice nucleating particles (INPs) and radiative balance in the Arctic are not well understood. Here we incorporate an observation‐based ice‐nucleation parameterization indicating the high ice nucleating ability of Arctic dust into a global aerosol‐climate model. A simulation using this parameterization better reproduces INP observations in the Arctic and estimates >100 times higher dust INP number concentrations with ∼100% contribution from Arctic dust in the Arctic lower troposphere (>60°N and >700 hPa) during summer and fall (June–November) than a simulation applying a standard ice‐nucleation parameterization suitable for desert dust to Arctic dust. Our results demonstrate the importance of considering an ice‐nucleation parameterization suitable for Arctic dust when simulating INPs and their effects on aerosol‐cloud interactions in the Arctic. Plain Language Summary Dust is an important aerosol type acting as “ice nucleating particles,” which initiate the formation of ice crystals within mixed‐phase clouds (consisting of both supercooled water droplets and ice crystals) and influence the cloud lifetime and distribution. Recent observations show that dust is emitted from ice‐ and vegetation‐free areas in the Arctic region (hereafter Arctic dust), which has a remarkably high ice nucleating ability, compared with desert dust such as Asian dust and Saharan dust, because of the presence of certain organic matter. However, the impacts of Arctic dust with high ice nucleating ability on ice nucleating particles and mixed‐phase clouds in the Arctic are unknown. In this study, we investigate the importance of Arctic dust with high ice nucleating ability for ice nucleating particles in the Arctic using a global aerosol‐climate model. Our simulation results show that Arctic dust accounts for almost all dust ice nucleating particles in the Arctic lower troposphere (>60°N and about 0–3 km) during summer and fall (June–November). This study demonstrates the importance of considering the high ice nucleating ability of Arctic dust when simulating ice nucleating particles and their impacts on mixed‐phase clouds and radiative balance in the Arctic. Key Points Arctic dust, emitted within the Arctic, accounts for most of dust ice nucleating particles in the Arctic lower troposphere in summer to fall Importance of Arctic dust as ice nucleating particles in the Arctic strongly depends on its high ice nucleating ability at high temperatures Considering an ice‐nucleation parameterization suitable for Arctic dust is crucial for aerosol‐cloud‐climate simulations in the Arctic

The influence of entrainment, a key process characterized by the entrainment rate in cumulus parameterization, on aerosol‐cloud interactions has been widely recognized. However, despite qualitative links established between entrainment and aerosol loading, a quantitative relationship based on observational evidence remains elusive. This study utilizes aircraft observations of cumulus clouds during two field campaigns to determine the quantitative relationship between entrainment rate and aerosol loading. In both campaigns, the entrainment rate is negatively correlated with aerosol loading. It is speculated that increased aerosol loading enhances cloud edge droplet evaporation, which leads to increased buoyancy and vertical velocity within the cloud, thereby reducing the entrainment rate. Further analysis shows that the response of entrainment rate to aerosol perturbations is more significant in smaller cumulus clouds with weak buoyancy and less pronounced under opposite conditions. These findings shed new light on improving the description of aerosol‐cloud interactions in cumulus parameterizations. Plain Language Summary Clouds play a crucial role in regulating Earth's climate, and understanding how they form and evolve is important for accurate weather and climate predictions. One key process affecting cloud development is entrainment, where drier air from outside the cloud mixes into the cloud, influencing its growth. Scientists know that aerosols can impact entrainment, but the exact relationship hasn't been clear. This study uses observations from research aircraft flown through cumulus clouds to quantify the relationship between entrainment and aerosol. The results show that aerosol concentration is negatively correlated with entrainment rate, meaning less dry air mixes into the cloud. We speculate that this happens because the increased aerosols enhance the evaporation of cloud droplets at the cloud edges, making the cloud more buoyant and less likely to mix with the surrounding air. Interestingly, this decline in entrainment rate to aerosol is stronger in smaller and weaker clouds. These findings provide valuable insights into how aerosols influence cloud development and can help improve the representation of aerosol‐cloud interactions in climate models, leading to more accurate climate projections. Key Points The relationship between cumulus entrainment rate and aerosol loading is quantified based on observation for the first time The plausible physical mechanism linking entrainment rate to aerosol loading has been elucidated A steeper decline in entrainment rate with increasing aerosol loading is observed in small and weakly buoyant cumulus clouds

Relative dispersion (ε), as a parameter characterizing droplet spectral shape, exerts a considerable impact on cloud radiation and precipitation processes, and its accurate parameterization is urgently needed in models. Current ε parameterizations, which are based on droplet number concentration or simply set as constants, are inadequate to satisfy the demand. This study shows, utilizing in‐situ cloud and fog observations from five underlying surface regions (urban, suburban, mountainous, coastal and rainforest) of China, that ε uniformly and stably manifests as initially increasing then decreasing as volume‐mean diameter increases across these regions. Based on this relationship, a ε parameterization is established, which exhibits improved predictive capabilities in evaluating both cloud albedo effect and cloud lifetime effect. The parameterization is expected to enhance cloud simulation accuracy and minimize discrepancy between observed and simulated cloud radiation and precipitation, particularly for weather and climate models that commonly use the double‐moment cloud microphysical schemes. Plain Language Summary Clouds play a crucial role in the Earth's weather and climate. One key factor in understanding cloud behavior is the width of cloud droplet size distribution, quantified by relative dispersion. Accurately representing relative dispersion in weather and climate models is essential, yet current methods are often overly simplistic. Many models either rely on fixed values or use empirical monotonic equations based solely on droplet number concentration. However, the relationship between relative dispersion and droplet number concentration varies significantly across regions and can even be contradictory. In this study, we analyzed cloud and fog observations from five different regions, encompassing urban, suburban, mountainous, coastal, and rainforest environments. Our analysis revealed a consistent pattern: relative dispersion first increases and then decreases as the volume‐mean droplet diameter grows. Based on this insight, we developed a new method to predict relative dispersion. This approach has the potential to improve the accuracy of estimating cloud albedo and lifetime effects, enhancing the representation of aerosol‐cloud interactions in weather and climate models. Key Points Correlation between droplet spectral dispersion and volume‐mean diameter remains consistent across different regions Compared to previous dispersion parameterizations, the parameterization with volume‐mean diameter provides better predictions for dispersion The dispersion parameterization with volume‐mean diameter could reduce the uncertainty in simulating aerosol indirect effects

Uncertainty in estimations of the net contribution of anthropogenic aerosol particles, particularly of aerosol-cloud interactions (ACIs) to the Earth’s radiation budget, limits our ability to understand past and project future climate change. Earth System Models (ESMs) are among the key tools for assessing the magnitude and impacts of changes in various forcing agents on the global climate system. Hence, improving aerosol and cloud descriptions in ESMs is an important way forward to increase the confidence in estimates of climate impacts of aerosol perturbations in the past, present and future. In the framework of the FORCeS project, experimental and theoretical approaches were combined to bridge the current key gaps in the fundamental understanding of essential aerosol and cloud processes and their descriptions in selected European ESMs. Regarding aerosol types and processes, we focused on organic aerosol, particulate nitrate, absorbing aerosols, and ultrafine aerosol sources including new particle formation and growth. In terms of cloud processes, we targeted cloud droplet activation, hydrometeor growth and evaporation, ice formation and multiplication as well as aerosol processing and scavenging by clouds. The selection was made based on the identified knowledge gaps in the scientific understanding of these processes and/or their current representation in ESMs, as well as a novel perturbed parameter ensemble approach to detecting potential structural deficiencies in an ESM. Here, we review the state-of-the-art, outline our approach for arriving at recommendations for improving the representation of key aerosol and cloud processes within ESMs, and then provide such recommendations applicable in models operating at the Earth system scale. The limitations of the recommendations, applicability, as well as alternative approaches and future research directions are discussed. Overall, the findings highlight the need for continuous efforts towards smart ways for representing the aerosol number size distribution as well as consistent representations of key parameters (e.g., liquid water content and cloud droplet number concentration). Furthermore, we provide guidance for future ESM evaluation emphasising, in particular, the need for exploring the consistency of key parameters, process-based (as opposed to parameter-based), and the complementarity of in-situ and remote-sensed measurements for model evaluation.

Atmospheric aerosols often contain surface‐active organics, which reduce surface tension and enhance cloud droplets activation. This effect is often neglected in the application of Köhler theory where a constant surface tension equivalent to pure water is assumed. Using a cloud parcel model, we evaluated the impact of four representative surface‐active organics, humic‐like substances (HULIS), sodium dodecyl sulfate (SDS), cis‐pinonic acid, and dicarboxylic acids, on cloud condensation nuclei (CCN) activation under varied atmospheric conditions. Our results indicate that HULIS significantly enhance CCN activation, particularly at high aerosol concentrations, low updraft velocities, and small particle sizes. SDS, cis‐pinonic acid, and dicarboxylic acids also increase activation but to a lesser degree. The surface activity of HULIS has a stronger influence on CCN activation than its hygroscopicity, with particle size being the most sensitive parameter. This study emphasizes the need to incorporate surface‐active organics into climate models to improve the prediction of aerosol‐cloud interactions.

We use an independent observational estimate of aerosol‐cloud interactions (ACI) during the 2014 Holuhraun volcanic eruption in Iceland to evaluate four ACI parameterizations in a regional model. All parameterizations reproduce the observed pattern of liquid cloud droplet size reduction during the eruption, but strongly differ on its magnitude and on the resulting effective radiative forcing (ERF). Our results contradict earlier findings that this eruption could be used to constrain liquid water path (LWP) adjustments in models, except to exclude extremely high LWP adjustments of more than 20 gm−2$g\\hspace*{.5em}{m}^{-2}$ . The modeled ERF is very sensitive to the non‐volcanic background aerosol concentration: doubling the non‐volcanic aerosol background weakens the ACI ERF by ∼30%${\\sim} 30\\%$ . Since aerosol biases in climate models can be an order of magnitude or more, these results suggest that aerosol background concentrations could be a major and under‐examined source of uncertainty for modeling ACI. Plain Language Summary Particles suspended in the atmosphere (aerosols) play a key role in cloud formation. These aerosol‐cloud interactions have a major but uncertain influence on climate. We compare four different ways to calculate aerosol‐cloud interactions in a numerical atmospheric model. We compare model results to observed changes in clouds measured from satellites during the Holuhraun eruption in Iceland in 2014, which released large amounts of volcanic gases forming atmospheric aerosols. We find that all four approaches reproduce the observed reduction in cloud droplet sizes during the eruption, but that they disagree on its intensity and its impacts on the Earth's energy budget. An earlier study found that aerosol‐cloud interactions did not significantly increase the amount of liquid water in the clouds; using a more recent version of the satellite observations we find that large increases are possible. We also show that the eruption's impacts on the Earth's energy budget strongly depend on non‐volcanic aerosols already present in the atmosphere: doubling non‐volcanic aerosols reduces the impacts by ∼30%${\\sim} 30\\%$ . Aerosol biases in climate models can be far greater, indicating that this could be a major source of uncertainty for aerosol‐cloud interactions and for understanding past, present and future climates. Key Points Four aerosol‐cloud parameterizations tested in a regional model are consistent with observed cloud changes during the 2014 Holuhraun eruption Liquid water path (LWP) observations during the eruption are not enough to exclude large LWP adjustments in models Aerosol radiative impacts are as sensitive to background aerosols as to aerosol‐cloud interactions parameterization choice