Explore the vast range of titles available.

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

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.

Aerosol measurements during the DOE ARM Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign were used to quantify the differences between clean and smoky cloud condensation nuclei (CCN) budgets. Accumulation‐mode particles accounted for ∼70% of CCN at supersaturations <0.3% in clean and smoky conditions. Aitken‐mode particles contributed <20% and sea‐spray‐mode particles <10% at supersaturations <0.3%, but at supersaturations >0.3% Aitken particles contributions increased to 30%–40% of clean CCN. For clean conditions, the Hoppel minimum diameter was correlated to the accumulation‐mode number concentration, indicating aerosol‐correlated cloud activation was controlling the lower diameter cutoff for which particles serve as CCN. For smoky conditions, the contributions of Aitken particles increase and the correlation of cloud activation to accumulation‐mode particles is masked by the lower‐hygroscopicity smoke. These results provide the first multi‐month in situ quantitative constraints on the role of aerosol number size distributions in controlling cloud activation in the tropical Atlantic boundary layer. Plain Language Summary Tiny airborne particles provide the “seeds” on which cloud droplets form, and clouds are in turn important for regulating climate around the world. The small number of measurements characterizing these particles in conditions that are not affected by man‐made emissions make it difficult to represent these cloud processes in computer models that compare current climate to pre‐industrial conditions. Aerosol measurements collected for 17 months on an isolated island in the tropical Atlantic Ocean show how the size and number of particles affect cloud characteristics. The long timescale and wide range from very clean to very smoky aerosol conditions revealed not only differences in the particles that activate in clouds but also in the mechanisms that control that droplet formation process. In clean air, the size required to form a cloud droplet is influenced by the number of particles, as well as how quickly particles take up water during growth in cloud. However, in smoky air, the larger number and size of particles mean that cloud activation processes are less affected by the number of particles that take up water. Key Points Clean cloud condensation nuclei (CCN) at <0.3% supersaturation were ∼70% accumulation, <10% sea spray, and <20% Aitken mode particles Hoppel minimum diameters correlated to accumulation‐mode particles showing aerosol‐correlated activation for clean conditions (<400 cm−3) Smoky accumulation‐mode particles were 30 nm larger and had 15%–30% more CCN, which dampened correlations to cloud activation

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

Aerosol–cloud interactions (ACI) are a major source of uncertainty in anthropogenic climate forcing. Anthropogenic aerosols modulate the cloud albedo by impacting the relative dispersion (ε) of cloud droplet size distributions (CDSDs), known as the dispersion effect. This effect can be either suppressive or enhancing, introducing considerable uncertainties in polluted continental clouds. Here, we developed in situ measurements in polluted East China and simultaneously measured the CDSDs and aerosol properties. A wide range of ε from 0.20 to 0.78 with a mean value of 0.43 was observed. Machine learning analysis shows that aerosol number concentration and hygroscopicity dominate the changes of ε in polluted continental stratiform clouds under weak vertical velocities (<0.5 m s−1) and low liquid water content (∼0.1 g m−3) conditions. Overall, the dispersion effect enhances the aerosol indirect effect by ∼11%. These results highlight the importance of aerosol properties in evaluating ACI in polluted environments.

Weekly cycles (WCs) in cloud properties have been reported and linked to aerosol effects. Yet the extent to which human activities contribute to their occurrence remains unclear. Here, we revisit aerosol–cloud interactions from the WCs over central Europe using long‐term satellite and reanalysis data. Significant WCs in aerosol and cloud droplet number concentration (Nd) are detected with minima/maxima on Monday/Friday, indicating a clear signal of the Twomey effect. Notably, Nd–to–aerosol sensitivity from WCs is found to decrease at larger aerosol concentrations, confirming the nonlinear behavior of the aerosol–Nd relation (in log–log space) reported previously, but from a distinct perspective. Nevertheless, no discernible WCs in liquid water path are found. The pronounced WCs in cloud cover are demonstrated to be driven by natural variability. Our results indicate that the WCs offer a useful pathway for investigating the Twomey effect, but are not as effective for detecting cloud adjustments. Plain Language Summary Aerosol–cloud interactions are the largest uncertainty in the anthropogenic forcing of climate. Specifically, an increase in aerosols increases cloud droplet number concentration (the Twomey effect), which further changes liquid water path and cloud cover (cloud adjustments), ultimately alters radiations. Weekly cycles would be a useful tool for such study, assuming no 7‐day periodicity in meteorological dynamics. In this study, we revisit aerosol–cloud interactions from weekly cycles using long‐term satellite observations and reanalysis data. The analysis is restricted to central Europe—a region with strong weekly cycles in anthropogenic emissions. We find significant weekly cycles in aerosol and cloud droplet number concentration with minima/maxima on Monday/Friday. Importantly, the sensitivity of cloud droplet number concentration to aerosol is found to decrease in polluted conditions, confirming the behavior of the nonlinear cloud response to aerosol as reported previously. It is demonstrated that the weekly cycle in liquid water path is negligible; though a pronounced cycle in cloud cover detected, it is predominately caused by natural variability. Hence, caution is warranted when attributing observed weekly cycle in cloud cover to aerosol effects. Conclusively, weekly cycles are useful for detecting the Twomey effect but less effective for cloud adjustments. Key Points The analysis of weekly cycles reveals a reduced sensitivity of cloud droplet number concentration to aerosol at larger aerosol loading The strong weekly cycle in cloud cover is mainly a result of natural variability rather than being attributed to aerosol effects Weekly cycles offer a useful pathway to investigating the Twomey effect, but are not as effective for detecting cloud adjustments

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

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.

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.