Aerosol-Cloud Interactions
CSL uses observations and models to constrain the single biggest unknown in radiative forcing
*Please note, references in BLUE indicate a CSL first or co-author publication.
Introduction
Aerosol and Clouds are the largest contributors to uncertainty in climate projections
The radiative forcing associated with aerosol-cloud interactions continues to contribute the largest uncertainty to total radiative forcing estimates (see figures at right from IPCC AR5 and Salawitch et al., 2017 ). In particular, aerosol effects on the amount and brightness of warm, shallow clouds that cover large areas of the oceans and provide a cooling effect that partially offsets the warming associated with greenhouse gases, are poorly constrained. This uncertainty in aerosol-cloud radiative forcing is further exacerbated by our uncertainty in how these warm clouds will respond to a warming climate (cloud feedbacks) and how they relate to climate sensitivity.
Here we present a small sample of the NOAA Chemical Sciences Laboratory (CSL) efforts over the past 5 years to (a) quantify the global aerosol budget, (b) improve the way that shallow clouds are represented in climate models, and (c) advance our process-level understanding of aerosol-cloud interactions in shallow cloud systems.
METHODS
Our team at CSL addresses Aerosol-Cloud Radiative Forcing and Cloud Feedbacks using a combination of airborne in-situ measurements, satellite and surface-based remote sensing, and numerical models that simulate the aerosol-cloud system at a range of scales.
Our modeling expertise rests on decades of development of detailed cloud microphysics methods and our hallmark coupling of aerosol-cloud-radiation interactions in large eddy simulations (LES) that resolve the cloud motions and aerosol-cloud processes important to aerosol-cloud radiative forcing. Over the last 5 years CSL activities have expanded to encompass modeling at mesoscale-to-regional scales, and even efforts to improve the representation of clouds in climate models.
We custom build instrumentation to measure aerosol size distribution, chemical composition and optical properties, alongside relevant gas phase species from a variety of ground-based and airborne platforms. Over the last 5 years we have measured intensively in the remote free troposphere on the Atmospheric Tomography Mission (ATom), and emissions from wildfires on the Fire Influence on Regional to Global Environments and Air Quality Mission (FIREX-AQ).
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Global Aerosol Budget
Using custom instrumentation, a team of CSL scientists ( Charles Brock , Christina Williamson , and Agnieszka Kupc) measured aerosol size distributions at high time resolution on the NASA Atmospheric Tomography Mission (ATom). Instrumentation is described in three CSL led papers Williamson et al., 2018 , Kupc et al., 2018 , and Brock et al., 2019 .
We repeatedly observed high concentrations of very small aerosol particles at high altitude over the tropics. Linking these observations with models over a range of scales - from box models run by CSL scientist Jan Kazil studying parcels of air evolving over a few minutes, to global-scale chemistry climate models run by CSL scientist Pengfei Yu , and collaborators from Colorado State University and SUNY Albany - we were able to determine that deep convective clouds were driving new particle formation in the tropical upper troposphere, as illustrated in the figure on the right from Williamson et al., 2019 .
By flying profiles from 12 m altitude to just a few 100 meters above the ocean surface, we were able to map these particles growing to sizes where they influence the properties of clouds. We have been able to calculate the local and global effect of these through linking our observations to global models.
This analysis is published in a CSL led paper in Nature, Williamson et al., 2019 , and further developed in a CSL co-author paper, Kupc et al., 2020 .
In the following short presentation we describe observations made by CSL scientists of newly formed particles in the tropical upper troposphere on ATom , and how, by linking with modeling studies, we showed this to be important on a global scale for cloud-radiation interactions. The results presented here were published in Williamson et al., 2019 .
New Particle Formation in the Remote Free Troposphere
In-situ observations conducted by CSL scientists ( Patrick Veres and Andy Neuman ) during the recent NASA ATom project led to the identification of a new, previously unobserved molecule, hydroperoxymethyl thioformate (HPMTF), shown on the right highlighted in the orange box. HPMTF was detected using CSL's state of the art, custom built iodide ion time-of-flight chemical ionization mass spectrometer . This discovery has significantly altered our understanding of the atmospheric marine sulfur budget and is described in detail in a CSL led publication, Veres et al. 2020 .
This work establishes HPMTF as one of the three primary DMS oxidation products where only methane sulfonic acid (MSA) and sulfur dioxide (SO2) were previously considered. Results from from a box model analysis imply a 30-40% reduction in direct SO2 yields from DMS oxidation relative to currently implemented DMS oxidation schemes. This difference is illustrated on the right where the bar graphs show a comparison of box model yields between classical DMS oxidation chemistry and an updated scheme that includes HPMTF formation.
Products of dimethyl sulfide (DMS) oxidation have long been linked to production and growth of marine aerosols and CCN, through the production of sulfuric acid (H2SO4), Quinn and Bates 2011 . This link is illustrated in the figure below where enhancements in HPMTF appear concurrently with high number concentrations of small particles at the top of the boundary layer.
Observations from the NASA ATom mission during two consecutive descents into and out of the marine boundary layer. Similar behavior is observed between HPMTF abundance and sub-micron particle number concentrations suggestive of the link between DMS oxidation and particle formation/growth. Figure taken from Veres et al. 2020 .
The role of HPMTF in particle formation and growth remains unclear; however, it is likely that the rate of sulfate production from DMS is impacted by this discovery. This work highlights large uncertainties in our understanding of DMS oxidation on marine aerosols and CCN abundance relative to other sources in the marine atmosphere, such as wave breaking and bubble bursting as illustrated on the right.
This initial work led by CSL scientists and involving collaborations between 15 international institutions has reinvigorated a decades long debate on the role of DMS emissions on climate. The results of this study highlight the inability of current models that incorporate DMS oxidation chemistry to accurately model the impact of marine sulfur oxidation on particle abundance and number concentrations.
CSL efforts to understand marine sulfur oxidation are ongoing utilizing both laboratory experimentation and field observations. The marine sulfur cycle will be a foci of the upcoming CSL led Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas ( AEROMMA ) field study planned for the spring of 2022.
Clouds in Climate Models
Global Climate Models (GCMs) are our only means of evaluating aerosol-cloud radiative forcing and feedbacks in future climate states. Unfortunately their relatively coarse resolution means that they do not resolve shallow cloud systems very well.
Improving the realism of clouds in GCMs is crucial to all attempts to evaluate both aerosol-cloud forcing and feedbacks. Higher horizontal spatial resolution (e.g., the 'Multi Model Framework') has been shown to improve the representation of shallow clouds, but is very expensive, and not a panacea.
Scientists at CSL, including Takanobu Yamaguchi , Ryuji Yoshida , Yaosheng Chen , and Graham Feingold , have developed a Framework for Improvement by Vertical Enhancement (FIVE) ( Yamaguchi et al. 2017 ) that computes selected processes on a locally high vertical resolution, one-dimensional grid, thereby capping computational cost. Results at right show that DOE's Energy Exascale Earth System Model (E3SM) coupled with FIVE greatly improves the representation of subtropical low clouds. Further improvements are attained through combining FIVE with a finer horizontal grid.
See the Model Development and Applications StoryMap for further details on the model development aspects of this project.
Aerosol-Cloud Interactions
1. Improving our Process Level Understanding of Aerosol-Cloud Interactions in Shallow Clouds
Cloud formation, evolution, and organization results from a complex set of aerosol and meteorological drivers that are hard to untangle. CSL research has shown that an improved understanding of the mutual role of these drivers, and their co-variability, is essential to improving our understanding of aerosol-cloud forcing and feedbacks.
At right a presentation by Takanobu Yamaguchi highlights some key examples of our research ( Goren et al. 2019 ; Yamaguchi et al. 2015 ; Yamaguchi et al. 2017 ), alongside illustrative satellite images. Model development efforts that form the foundations of this research are described here .
State-of-the-Art Modeling
Below we show our most detailed 'superdroplet Lagrangian particle model', coupled to our LES. This is the new generation of microphysics that is required to study the details of aerosol-cloud interactions when aerosol size and composition are of great importance.
Super-accurate microphysics: A Lagrangian Cloud Model embedded in a Large Eddy Simulation ( Hoffmann and Feingold 2019 ). Blue dots indicate growth via condensation, red dots, growth by collision-coalescence, and grey dots, the growth 'bottleneck' that has to be overcome before rain can form.
2. Trade-wind Cumulus: meteorological drivers of organization
As noted above, understanding shallow cumulus clouds over the subtropical oceans is key to constraining uncertainty in cloud feedbacks. CSL scientists participated in the NOAA Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign ( ATOMIC ) field program, in parallel to the European Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation (EUREC4A) campaign. Joint field assets included 4 aircraft, 4 ships, and multiple remotely controlled aircraft and ocean platforms.
At right we show GOES satellite animations and large eddy numerical model simulations by CSL scientist Pornampai Narenpitak of a cloud system evolving with time from a collection of small cumulus clouds to an organized system in which clouds resemble a bouquet of flowers. These organized structures result from a self-aggregation mechanism that concentrates water vapor in some regions at the expense of others. Studies such as these contribute to fundamental understanding of trade-wind cumulus clouds and their likely changes in a warmer world.
3. Cloud Field Organization: aerosol and meteorological drivers
Shallow clouds commonly change their patterns of organization in response to precipitation. In the case of stratocumulus, this can mean a transition from a high cloud fraction, closed cellular state to a low cloud fraction open cellular state. Closed cells reflect significantly more shortwave radiation to space than open cells. Again we find an important role for an aerosol-cloud radiative effect.
The panel at right shows an example of CSL led simulations by Yamaguchi and Feingold (2015 ) of stratocumulus with different aerosol conditions, demonstrating that higher aerosol concentrations yield more reflective clouds.
However, conditions unrelated to aerosol such as time of day, or meteorological conditions also affect organization and cloud reflectance. At far right, work by Kazil et al. (2017) shows four different cloud scenes over the course of the day that exhibit different cloud morphologies and reflectance.
Together these two examples amplify the need to understand the co-varying aerosol and meteorological drivers that determine the radiative effect of a cloud scene (Feingold et al. 2016 ; Muelmenstaedt and Feingold 2018 )
4. Aerosol-Cloud Interactions in a 'Buffered System'
'Buffering' is a concept introduced by Stevens and Feingold (2009) to convey the idea, along with examples, that strong aerosol perturbations to a cloud system are often significantly muted by adjustments internal to the system. A decade later we continue to find evidence of buffering in shallow cloud systems.
Our simulations of tropical cumulus in the Philippines by Yamaguchi et al. (2019) show that large aerosol perturbations (factor 6.5) result in very small changes in precipitation and cloud fraction. Under low aerosol loadings, clouds rain efficiently and produce a cloud field characterized by small cells. The more strongly the clouds are perturbed by aerosol, the more they have to overcome microphysical suppression of rain to produce similar amounts of rain. They do so by growing deeper, and by organizing small clouds into large clusters. Thus while rainfall amounts are very similar ('a buffered system'), the efficiency with which rain is produced decreases with increasing aerosol.
5. Marine Cloud Brightening
Lessons learned from our many years of aerosol-cloud interaction studies point to a complex system with internal feedbacks and buffering. Based on our experience we are well positioned to evaluate various proposals for marine cloud brightening (MCB) in the event of uncontrolled greenhouse gas warming.
In the video at right, CSL scientist Graham Feingold discusses current CSL research on the topic. Our work consists of two components that both identify the pitfalls of commonly held assumptions that aerosol injections always brighten clouds.
(1) The frequently occurring reductions in cloud water in response to aerosol injections. Our team led by Franziska Glassmeier ( Science , 2021) highlight the fact that cloud liquid water adjustments to aerosol perturbations are frequently negative, and become more negative with time. The success of MCB is premised on near zero adjustments. Our work contends that short-lived ship-tracks do not evolve for long enough to adequately represent climate-relevant cloud brightening and therefore overestimate the negative shortwave radiative forcing.
(2) The importance of the size distribution of seeded material for optimal brightening. To address this work we use a new Lagrangian microphysics model embedded in our large eddy simulation (LES). Our results indicate the common occurrence of negative liquid water adjustments, particularly in thin, non-precipitating clouds, that can offset the degree of brightening (Hoffmann and Feingold 2021).
6. Aerosol, Clouds, and Radiation: Research in support of Renewable Energy
Shallow cumulus clouds are hard to predict and have a strong influence on the amount of shortwave radiation reaching the surface. Supporting Numerical Weather Prediction (NWP) efforts at NOAA , and leveraging the US Department of Energy 's measurement suite in Oklahoma, we are studying the influence of clouds and aerosol on the surface radiation that can be collected by solar photovoltaic panels. Our research contributes directly to improved solar renewable energy predictions.
Our work combines large eddy simulation of shallow cumulus with 3D radiative transfer modeling. At right we show that we are able to simulate the high irradiance at the edges of brightly illuminated clouds that 1D models fail to capture ( Gristey et al. 2020a and 2020b ). Finally our work demonstrates the relationship between cloud and aerosol properties, and the downwelling irradiance, opening up exciting possibilities for NWP parameterizations.
7. Thinking Out-of-the-Box: new approaches to understanding cloud systems
In the field of cloud modeling, there is a strong tendency to build ever more complex models, representing more and more detailed, coupled processes. While this approach has its place, it is often difficult to assess the importance of a given process with respect to a certain model realization. In addition to our hallmark large eddy simulation work, we have over the years also explored more simple Dynamical Systems approaches to complex systems. These may take the form of coupled delay differential equations as in the Koren and Feingold (2011) representation of cloud systems as a Predator-Prey problem, or they may be analogs from the field of cellular biology.
At right we show two examples of CSL research: (1) how a simple Predator-Prey model can replicate some essential features of a very complex large eddy simulation ( Koren et al. 2017 ); and (2) a description of mesoscale cellular convection as a cellular network, and how one can replicate the cloud system with a set of simple growth rules applied from cellular biology (Glassmeier and Feingold, 2017) .
These two studies exemplify how CSL scientists develop and apply lower dimensional models alongside high dimensional large eddy simulation to advance understanding of the aerosol-cloud system.
Contributing CSL scientists
Charles A Brock , Yaosheng Chen , Graham Feingold , Karl D Froyd , Franziska Glassmeier (now at TU Delft), Fabian Hoffmann (now at LMU, Germany), Tom Goren (now at Univ. of Leipzig), Jake Gristey , Jan Kazil , Agnieszka Kupc (now at University of Vienna), Daniel M Murphy , Pornampai Narenpitak , Andy Neuman , Gregory P Schill , Patrick Veres , C hristina J Williamson , Takanobu Yamaguchi , Ryuji Yoshida
References
*Please note, BOLD indicates the author was affiliated with CSL at the time of publication.
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Goren, T., J. Kazil, F. Hoffmann, T. Yamaguchi, and G. Feingold, Anthropogenic air pollution delays marine stratocumulus break-up to open-cells , Geophysical Research Letters, doi:10.1029/2019GL085412, 2019.
Gristey, J.J., G. Feingold, I.B. Glenn, K.S. Schmidt, and H. Chen, Surface solar irradiance in continental shallow cumulus clouds fields: Observations and large eddy simulation , Journal of Atmospheric Sciences, doi:10.1175/JAS-D-19-0261.1, 2020.
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Murphy, D., Froyd, K., Bian, H., Brock, C., Dibb, J., DiGangi, J., Diskin, G., Dollner, M., Scheuer, E., Schill, G., Weinzierl, B., Williamson, C., Yu, P., 2019: The distribution of sea-salt aerosol in the global troposphere , Atmos. Chem. Phys., 19 (6), 4093-4104, doi: 10.5194/acp-19-4093-2019
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Veres, P. R.; Neuman, J. A.; Bertram, T. H.; Assaf, E.; Wolfe, G. M.; Williamson, C. J.; Weinzierl, B.; Tilmes, S.; Thompson, C. R.; Thames, A. B.; Schroder, J. C.; Saiz-Lopez, A.; Rollins, A. W.; Roberts, J. M.; Price, D.; Peischl, J.; Nault, B. A.; Møller, K. H.; Miller, D. O.; Meinardi, S.; Li, Q.; Lamarque, J.-F.; Kupc, A.; Kjaergaard, H. G.; Kinnison, D.; Jimenez, J. L.; Jernigan, C. M.; Hornbrook, R. S.; Hills, A.; Dollner, M.; Day, D. A.; Cuevas, C. A.; Campuzano-Jost, P.; Burkholder, J.; Bui, T. P.; Brune, W. H.; Brown, S. S.; Brock, C. A.; Bourgeois, I.; Blake, D. R.; Apel, E. C.; Ryerson, T. B., 2020, Global airborne sampling reveals a previously unobserved dimethyl sulfide oxidation mechanism in the marine atmosphere . PNAS, 2020, 117, (9), 4505-4510, doi:10.1073/pnas.1919344117.
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Yamaguchi, T., G. Feingold, and J. Kazil, Aerosol-cloud interactions in trade wind cumulus clouds and the role of vertical wind shear , Journal of Geophysical Research, 124(22), 12244-12261, doi:10.1029/2019JD031073, 2019.
Yamaguchi, T., G. Feingold, and Kazil, J., Stratocumulus to cumulus transition by drizzle , Journal of Advances in Modeling Earth Systems, 9(6), 2333-2349, doi:10.1002/2017MS001104, 2017.
Yamaguchi, T., G. Feingold, and V.E. Larson, Framework for improvement by vertical enhancement: A simple approach to improve low and high level clouds in large scale models , Journal of Advances in Modeling Earth Systems, doi:10.1002/2016MS000815, 2017.
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Yamaguchi, T., and G. Feingold, On the relationship between open cellular convective cloud patterns and the spatial distribution of precipitation , Atmospheric Chemistry and Physics, 15, 1237-1251, doi:10.5194/acp-15-1237-2015, 2015.
Yu, P., Froyd, K., Portmann, R., Toon, O.B., Fretas, S., Bardeen, C., Brock, C., Fan, T.Y., Gao, R.S., Katich, J., Kupc, A., Liu, S., Maloney, C., Murphy, D., Rosenlof, K., Schill, G., Schwarz, J., Williamson, C., 2018: Efficient in-cloud removal of aerosols by deep convection , Geophys. Res. Lett, 46 (2), 1061-1069, doi: 10.1029/2018GL08054
Zeng, L., Zhang, A., Wang, Y., Wagner, N. L., Katich, J. M., Schwarz, J. P., Schill, G. P., Brock, C., Froyd, K. D., Murphy, D. M., Williamson, C. J., Kupc, A., Scheuer, E., Dibb, J., & Weber, R. J., 2020: Global Measurements of Brown Carbon and Estimated Direct Radiative Effects , Geophysical Research Letters, 47(13), e2020GL088747. doi:10.1029/2020gl088747
