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Satellite image of the northeast Pacific Ocean showing ship tracks, both in thin closed-cellular stratocumulus regions and in open-cellular regions.The southern Californian coastline is visible in the upper-right corner. Open-cellular regions are characterized by clouds in a lace-like network. Closed-cellular regions have a higher albedo and a finer granulation. Recent work has shown regions of open cells to be associated with precipitation and depleted aerosol, although the origins of such features are unclear. At times, closed cells can be embedded in broader regions of open cells, in which case they are referred to as pockets of open cells. Such features, although not clearly evident here, suggest that both states are possible for a given large-scale meteorological environment. Adapted with permission from NASA.
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It is thought that changes in the concentration of cloud-active aerosol can alter the precipitation efficiency of clouds, thereby changing cloud amount and, hence, the radiative forcing of the climate system. Despite decades of research, it has proved frustratingly difficult to establish climatically meaningful relationships among the aerosol, clou...
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Context 1
... in shafts of precipitation that readily form in aerosol-poor environments are common. Equally evocative is the apparent transition from low-albedo (open-cellular) shallow-cumulus cloud regimes to high-albedo (closed-cellular) stratocumulus cloud regimes as a result of aerosol infusions from passing ships 13 . An example of the latter is shown in Fig. 2. Such features differ from ship tracks (which are often taken as a signature of the cloud albedo effect) 14 because the open-cellular structure in the low-albedo region is associated with regions of precipitation, whereas the closed-cellular structure in the high-albedo region is usually observed to be non-precipitating 13,15,16 ...
Context 2
... acting to cool the surface, further stabilizing the layer and reducing cloudiness 85 . Here again a mechanism emerges whereby an increase in aerosol amount may reduce, rather than enhance, cloudiness, hence reinforcing the concept of a well-buffered system. This is not to say that lifetime effects are entirely without merit. Pockets of open cells (Fig. 2) provide dramatic support for the con- ventional wisdom that precipitation reduces cloud amount 13 . Rather, our argument is that the sensitivity of clouds and precipitation to changes in the aerosol is, on average, weaker than implied by simple arguments and is regime or state dependent. This capacity of the system to respond ...
Citations
... Wang et al., 2003;Wood, 2007). Given the large number of processes that may be affected, and the causal ambiguity inherent in empirical constraints (Gryspeerdt et al., 2016(Gryspeerdt et al., , 2019Stevens & Feingold, 2009), it is not surprising that the aggregate effect of aerosol-cloud adjustments on radiative forcing remains highly uncertain (Bellouin et al., 2020). The sum of radiative forcing from IRFaci and aerosol-cloud adjustment is the effective radiatve forcing due to aci (ERFaci). ...
Plain Language Summary
Uncertainty in predicting future global temperature inferred from the historical record of warming is dominated by how much the warming due to greenhouse gases has been offset by the cooling due to aerosols. Aerosols are small liquid and solid particles that play an important role in cloud formation. The majority of cooling from aerosols is through reflecting incoming solar radiation back to space by cloud. In this study, we constrain an ensemble of possible global model configurations with observations of cloud properties and radiation to reduce uncertainty in the response of clouds and ultimately radiation to anthropogenic aerosol. While observations substantially reduce the uncertainty in both changes in the number of droplets and amount of liquid cloud, the constraint on aerosol cooling is minimal. We argue that the relatively weak constraint is because large changes in cloudiness are accompanied by small change in reflected sunlight due to increased cloudiness.
... Tropical cloud feedbacks have been identified as a key uncertainty in climate models (Bony and Dufresne, 2005) and the relationship between cloud microstructures and macroscale properties remains poorly understood (Van Zanten, 2011). Any changes in the frequency of these relatively shallow clouds over the ocean has the potential to 55 significantly impact the net solar radiation budget due to their relatively high albedo verses the ocean surface (approximately an order of magnitude net increase in reflected short wave radiation) and minimal impact on outgoing longwave radiation (Stevens and Feingold, 2009). ...
In this paper measurements are presented of the observed properties of aerosols and microphysics of clouds associated with the characteristics of precipitation in convective clouds that formed off the east coast of Barbados during EUREC4A. Most data were gathered by the instrumented British Antarctic Survey Twin Otter aircraft supported by detailed in-situ aerosol measurements at the Ragged Point observatory on Barbados as well as HALO and PoldiRad radars, dropsonde and satellite data. The development of precipitation was studied in the three aerosol regimes previously reported, i.e. one low aerosol regime and two containing desert dust that had been advected across the Atlantic Ocean. The later dust event also contained evidence of biomass burning aerosol. Results showed that the maximum intensity of rain was similar for all the aerosol regimes. Clouds that developed in an environment with high aerosol loading tended to be deeper than those that developed in the clean environment. It was also found that the greatest intensities occurred in clouds that had aggregated, in agreement with previous work.
... Single-layer, low marine warm clouds cover 25 % of the ocean surface (Charlson et al., 1987) and exert a strong cooling effect on climate due to their reflectivity (Hartmann et al., 1992;Hartmann and Short, 1980;Ramanathan et al., 1989). Aerosols modulate multiple radiative properties of low-level warm clouds, including droplet number concentration (N d ), liquid water path (LWP), geometric, cloud fraction, and lifetime, and their net impact on the cloud radiative forcing is the most uncertain component of the climate system (e.g., Stevens and Feingold, 2009;Christensen et al., 2020;Glass-meier et al., 2021). Though aerosols also exert a significant influence on ice and mixed-phase clouds, aerosol-cloud interactions (ACIs) make their largest contribution to global radiative forcing via liquid water clouds (Bellouin et al., 2020). ...
... When aerosols suppress precipitation (e.g., Suzuki et al., 2013), LWP and/or cloud fraction may be enhanced, resulting in brighter clouds and stronger ERFaci SW . The relationship between aerosols, LWP, and cloud fraction (Albrecht, 1989), however, is highly uncertain, varies regionally (Sato et al., 2018), and is influenced by processes that are buffered over multiple spatiotemporal scales (Stevens and Feingold, 2009). Additionally, E3SMv2's CFODD slope ("CNTL" simulation) agrees with MODIS-CloudSat within uncertainty, indicating that droplet collection efficiency is well-represented according to CFODD analysis. ...
Process-oriented observational constraints for the anthropogenic effective radiative forcing due to aerosol–cloud interactions (ERFaci) are highly desirable because the uncertainty associated with ERFaci poses a significant challenge to climate prediction. The contoured frequency by optical depth diagram (CFODD) analysis supports the evaluation of model representation of cloud liquid-to-rain conversion processes because the slope of a CFODD, generated from joint MODerate Resolution Imaging Spectroradiometer (MODIS)-CloudSat cloud retrievals, provides an estimate of cloud droplet collection efficiency in single-layer warm liquid clouds. Here, we present an updated CFODD analysis as an observational constraint on the ERFaci due to warm rain processes and apply it to the U.S. Department of Energy's Energy Exascale Earth System Model version 2 (E3SMv2). A series of sensitivity experiments shows that E3SMv2 droplet collection efficiencies and ERFaci are highly sensitive to autoconversion, i.e., the rate of mass transfer from cloud liquid to rain, yielding a strong correlation between the CFODD slope and the shortwave component of ERFaci (ERFaciSW; Pearson's R=-0.91). E3SMv2's CFODD slope (0.20 ± 0.04) is in agreement with observations (0.20 ± 0.03). The strong sensitivity of ERFaciSW to the CFODD slope provides a useful constraint on highly uncertain warm rain processes, whereby ERFaciSW, constrained by MODIS-CloudSat, is estimated by calculating the intercept of the linear association between the ERFaciSW and the CFODD slopes, using the MODIS-CloudSat CFODD slope as a reference.
... Before satellites, there was very little observational coverage of the SH; meteorologists attempting to calculate properties of Earth's climate even previously simply mirrored the albedo between the NH and SH for a first estimate despite expecting them to be different (Thomas Vonder Haar, personal communication). With better instruments and longer satellite records, the Earth's hemispheric albedo symmetry has been found to persist into the present day with no sign of trends, and the degree of symmetry is now known to be as strong as <0.1% of incoming solar radiation (Datseris & Stevens, 2021;Jönsson & Bender, 2022;Stephens et al., 2015;Stevens & Feingold, 2009;Voigt et al., 2013), making it the smallest energetic hemispheric difference in Earth's climate (Stephens et al., 2016). The reason this feature exists -already noted in Vonder Haar & Suomi (1971) -is that clouds compensate for the clear-sky differences in (Loeb et al., 2018). ...
... The longer a cloud stays in the atmosphere, the stronger its effect on the amount of solar radiation ultimately absorbed by Earth and thus on the effective planetary albedo (Siebesma et al., 2020;Stevens & Feingold, 2009). Precipitation formation, which removes cloud water from the atmosphere, can have a significant impact on how cloud contributions to albedo by changing their lifetime, and different processes can change the efficiency of precipitation formation. ...
Earth's Northern and Southern Hemispheres (NH and SH, respectively) have significantly different properties: the NH has a higher concentration of bright land surface area and aerosol emissions than the SH, making the Earth's clear-sky albedo hemispherically asymmetric. However, satellite observations have shown that higher cloud amount and reflectivity in the SH exactly compensate for this, making Earth's planetary albedo hemispherically symmetric. A physical explanation for this symmetry has not yet been found, but because it would give constraints for global cloud cover and its features, discovery of one may be a powerful tool in predicting the behavior of clouds in a changing climate.
The first chapter of this thesis investigates the hemispheric albedo symmetry in observations, and finds that its variability primarily stems from the tropics. General circulation models (GCMs) exhibit a large spread in albedo asymmetry biases; comparing these with observations reveals that the extratropics control mean-state modeled albedo asymmetry.
The second chapter compares the evolution of albedo asymmetries in GCMs when forced with increased CO2 concentrations. Models agree on an initial asymmetry response due to Arctic warming and albedo reductions, but diverge thereafter, with some models recovering their pre-industrial asymmetry. Those that recover their asymmetry do so via SH extratropical cloud loss and thus have stronger positive cloud feedbacks, illustrating that an albedo symmetry-maintaining mechanism could have implications for climate sensitivity.
Sources of modeled albedo asymmetry biases are investigated in a single atmospheric GCM using a perturbed parameter ensemble in the third chapter. The most significant parameters to simulated albedo asymmetry are those controlling warm rain formation, turbulent dissipation, and sea salt aerosol emissions. Parameters controlling warm rain formation and turbulent dissipation primarily affect extratropical low cloud cover, and those affecting ice particle formation disproportionately affects SH midlatitude albedo. Parameter settings that reproduce the observed albedo symmetry tend towards more strongly positive shortwave cloud feedbacks.
The link between hemispheric asymmetries in clouds and large-scale circulation is investigated with idealized atmospheric GCM experiments in the fourth chapter. Introducing hemispheric asymmetry in ocean heat fluxes that emulate heat divergence (convergence) in the SH (NH) drives an atmospheric response that qualitatively reproduces the observed cloud distribution. We conclude that the hemispheric albedo symmetry is not possible without implicating surface forcing from ocean circulation and heat transport.
... We observed that the cases with more CCN distributed in two layers resulted in longer-lasting events and higher cloud tops. In [39], it was proposed that an increase in CCN may act to increase cloud top height (i.e., cloud depth). The basis for this hypothesis was that an increase in CCN should act both to increase cloud droplet number concentration and to reduce cloud droplet effective radius in warm clouds, hence delaying the onset of precipitation. ...
Over the years, there have been discussions about the possibility of air pollution affecting the process of rain formation. In this study, we have developed a simplified model that represents the atmospheric dynamics and cloud microphysics to explore the impact of pollution on rain formation. We used an existing three-dimensional minimal model consisting of five equations, for which we added a simple bulk parametrization that represents the role of cloud condensation nuclei (CCN) in cloud formation processes. We conducted numerical tests using two CCN profiles, with either one or two accumulation layers and modified their abundance to explore the effects of different CCN concentrations and distributions. We conducted four numerical tests corresponding to the two afore-mentioned profiles with polluted and low-polluted scenarios. The numerical simulations suggested that a layer with high CCN concentration close to the surface tends to suppress precipitation, while the same concentration distributed over two layers tends to enhance the efficiency of rain formation. The simulations also showed that CCN particles far from the surface produced higher cloud tops and longer events, consistent with previous research. Although the model includes a stable representation of precipitating turbulent convection with bulk cloud microphysics, we expect its simplicity and conservation properties to allow for deeper theoretical analyses that can help us better understand the physical processes involved in the studied phenomenon. We hope this model will serve as a tool to explore different aerosol-related scenarios within the context of minimal models.
... In winter, the relationship between the AI and PE was not significant in the two study regions. The negative correlation between the AI and PE indicates the inhibition effect of aerosol on precipitation efficiency, which may be due to the increase of aerosol concentration leading to the formation of more and smaller cloud droplets in the cloud, and the lower collision-coalescence efficiency is not conducive to the growth of raindrops [57][58][59]. The correlation between aerosol and precipitation efficiency is the most significant in spring, while in summer and autumn, it is less significant, mainly because the clouds and precipitation in the SCB and YGP in these two seasons are controlled by a ...
... In winter, the relationship between the AI and PE was not significant in the two study regions. The negative correlation between the AI and PE indicates the inhibition effect of aerosol on precipitation efficiency, which may be due to the increase of aerosol concentration leading to the formation of more and smaller cloud droplets in the cloud, and the lower collisioncoalescence efficiency is not conducive to the growth of raindrops [57][58][59]. The correlation between aerosol and precipitation efficiency is the most significant in spring, while in summer and autumn, it is less significant, mainly because the clouds and precipitation in the SCB and YGP in these two seasons are controlled by a variety of synoptic scale systems, such as the East Asian monsoon, the South Asian monsoon, the autumn rain system in West China and the quasi-stationary front [60][61][62], so the impact of aerosol on cloud and precipitation is overwhelmed. ...
... As can be seen from Figure 8a, the AI is significantly negatively correlated with precipitation in the two study regions, with determination coefficients of 0.46 and 0.43, respectively, indicating the inhibition effect of increasing aerosol on precipitation. Previous studies [57][58][59] have suggested that aerosol inhibits the warm precipitation process by reducing the radius of cloud droplets. Aerosols have a prominent excitation effect on convective precipitation, as they inhibit the warm rain process. ...
The aerosol–cloud–precipitation correlation has been a significant scientific topic, primarily due to its remarkable uncertainty. However, the possible modulation of aerosol on the precipitation capacity of clouds has received limited attention. In this study, we utilized multi-source data on aerosol, cloud properties, precipitation, and meteorological factors to investigate the impact of aerosols on precipitation efficiency (PE) in the Sichuan Basin (SCB) and Yun-nan-Guizhou Plateau (YGP), where the differences between terrain and meteorological environment conditions were prominent. In the two study regions, there were significant negative correlations between the aerosol index (AI) and PE in spring, especially in the YGP, while the correlations between the AI and PE in other seasons were not as prominent as in spring. In spring, aerosol significantly inhibited both the liquid water path (LWP) and the ice water path (IWP) in the YGP, but negatively correlated with the IWP and had no significant relationship with the LWP in the SCB. Aerosol inhibited precipitation in the two regions mainly by reducing cloud droplet effective radius, indicating that warm clouds contributed more to precipitation in spring. The suppressive impact of aerosols on precipitation serving as the numerator of PE is greater than that of the cloud water path as the denominator of PE, resulting in a negative correlation between aerosol and PE. The AI–PE relationship is significantly dependent on meteorological conditions in the YGP, but not in the SCB, which may be related to the perennial cloud cover and stable atmosphere in the SCB. In the future, as air quality continues to improve, precipitation efficiency may increase due to the decrease in aerosol concentration, and of course, the spatio-temporal heterogeneity of the aerosol–cloud–precipitation relationship may become more significant.
... Making the quantification of LWP adjustment to aerosol perturbations even more challenging is the presence of feedbacks among system-wide microphysical, dynamical and thermodynamical processes at different spatiotemporal scales, acting to 35 buffer the system's response to perturbations (Stevens and Feingold, 2009). Quantifying aerosol effects on LWP in such a buffered system requires understanding not only of individual causal mechanisms but also their timescales (Glassmeier et al., 2021;Fons et al., 2023;Gryspeerdt et al., 2022). ...
Marine low-level clouds are key to the Earth’s energy budget due to their expansive coverage over global oceans and their high reflectance of incoming solar radiation. Their responses to anthropogenic aerosol perturbations remain the largest source of uncertainty in estimating the anthropogenic radiative forcing of climate. A major challenge is the quantification of the cloud water response to aerosol perturbations. In particular, the presence of feedbacks through microphysical, dynamical and thermodynamical pathways at various spatial and temporal scales could augment or weaken the response. Central to this problem is the temporal evolution in cloud adjustment, governed by entangled feedback mechanisms. We apply an innovative conditional Monte Carlo subsampling approach to a large ensemble of diurnal large-eddy simulation of non-precipitating marine stratocumulus to study the role of solar heating in governing the evolution in the relationship between droplet number and cloud water. We find a persistent negative trend in this relationship at night, confirming the role of microphysically enhanced cloud-top entrainment. After sunrise, the evolution in this relationship appears buffered and converges to ∼ -0.2 in the late afternoon. This buffering effect is attributed to a strong dependence of cloud-layer shortwave absorption on cloud liquid water path. These diurnal cycle characteristics further demonstrate a tight connection between cloud brightening potential and the relationship between cloud water and droplet number at sunrise, which has implications for the impact of the timing of advertent aerosol perturbations.
... Importantly, many of the processes driving cloud LWP and CF changes are not fully or precisely accounted for in climate models. In particular, climate models tend to more systematically produce increases in cloud LWP and CF than is estimated from observations and from higher-resolution modeling studies that resolve and account for more complexities of aerosol-cloud interactions (Quaas et al., 2008;Stevens and Feingold, 2009;Seinfeld et al., 2016;Malavelle et al., 2017). 70 In addition to accounting for these aspects specific to MCB, it is important to keep in mind that MCB, as with all SRM interventions, does not address some impacts of high CO 2 concentrations; most notably, SRM does not mitigate increases in ocean acidity, and the hydrological sensitivity to SRM is higher than that of GHG. ...
... Models used for evaluating climate change and MCB intervention have been found to be very sensitive to the manner in which aerosols, clouds, and their interactions are treated. The physical and chemical processes involving aerosols and clouds are 200 among the most challenging, complex, and difficult to represent in atmospheric models (Stevens and Feingold, 2009;Carslaw, 2022). Approximations and simplifications required for treatment of these processes in climate models are responsible for many variations in model behavior when those tools are exposed to present day aerosol perturbations (Malavelle et al., 2017), and produce a lot of uncertainty when used to study historical changes in climate and to project future changes (Masson-Delmotte and et al, 2021). ...
A modeling protocol is introduced (defined by a series of model simulations with specified model output). The protocol is designed to improve understanding of climate impacts from Marine Cloud Brightening (MCB) Climate Intervention. The model simulations are not intended to assess consequences from a realistic MCB deployment intended to achieve specific climate targets but instead to expose responses produced by MCB interventions in 6 regions with pervasive cloud systems that are often considered as candidate regions for such a deployment. A calibration step involving simulations with fixed sea surface temperatures is first used to identify a common forcing, and then coupled simulations with forcing in individual regions and combinations of regions are used to examine climate impacts. Synthetic estimates constructed by superposing responses from simulations with forcing in individual regions are considered as a means to approximate the climate impacts produced when MCB interventions are introduced in multiple regions. A few results comparing simulations from 3 modern climate models (CESM2, E3SMv2, UKESM1) are used to illustrate similarities and differences between model behavior and the utility of estimates of MCB climate responses that have been synthesized by summing responses introduced in individual regions. There are substantial differences in the cloud responses to aerosol injections between models, but the models often show strong similarities in precipitation and surface temperature response signatures when forcing is imposed with similar amplitudes in common regions.
... The decrease in LWP that we diagnose could be due to an increase of rainfall ( Fig. 2), as clouds are still precipitating despite the reduction in r eff and/or because of entrainment of dry air inducing cloud evaporation, as indicated by the dry southern part of the domain in July 2008 (Fig. 3a). The reduction of in-cloud LWP counteracts the brightening from the Twomey effect, a process known as 'buffering' 38 . However, we find a strong increase in cloud cover (also known as cloud fraction, CF), due to the volcanic aerosol injection. ...
With global warming currently standing at approximately +1.2 °C since pre-industrial times, climate change is a pressing global issue. Marine cloud brightening is one proposed method to tackle warming through injecting aerosols into marine clouds to enhance their reflectivity and thereby planetary albedo. However, because it is unclear how aerosols influence clouds, especially cloud cover, both climate projections and the effectiveness of marine cloud brightening remain uncertain. Here we use satellite observations of volcanic eruptions in Hawaii to quantify the aerosol fingerprint on tropical marine clouds. We observe a large enhancement in reflected sunlight, mainly due to an aerosol-induced increase in cloud cover. This observed strong negative aerosol forcing suggests that the current level of global warming is driven by a weaker net radiative forcing than previously thought, arising from the competing effects of greenhouse gases and aerosols. This implies a greater sensitivity of Earth’s climate to radiative forcing and therefore a larger warming response to both rising greenhouse gas concentrations and reductions in atmospheric aerosols due to air quality measures. However, our findings also indicate that mitigation of global warming via marine cloud brightening is plausible and is most effective in humid and stable conditions in the tropics where solar radiation is strong.
... These threshold processes have been dubbed the cloud lifetime effect (Stevens and Feingold, 2009) since aerosols shape not only the intensity of convection and lightning production, but also its duration. A corollary of these dynamics is that a given quantity of atmospheric instability can have differing amounts of lightning production depending on aerosol concentrations. ...
A multi-variable investigation of thunderstorm environments in two distinct geographic regions is conducted to assess the aerosol and thermodynamic environments surrounding thunderstorm initiation. 12-years of cloud-to-ground (CG) lightning flash data are used to reconstruct thunderstorms occurring in a 225 km radius centered on the Washington, D·C. and Kansas City Metropolitan Regions. A total of 196,836 and 310,209 thunderstorms were identified for Washington, D.C. and Kansas City, MO, respectively. Hourly meteorological and aerosol data were then merged with the thunderstorm event database.
Evidence suggests, warm season thunderstorm environments in benign synoptic conditions are considerably different in thermodynamics, aerosol properties, and aerosol concentrations within the Washington, D.C. and Kansas City regions. However, thunderstorm intensity, as measured by flash counts, appears regulated by similar thermodynamic-aerosol relationships despite the differences in their ambient environments. When examining thunderstorm initiation environments, there exists statistically significant, positive relationships between convective available potential energy (CAPE) and flash counts. Aerosol concentration also appears to be a more important quantity than particle size for lightning augmentation.
