Aerosols enhance cloud lifetime and brightness along the stratus-to-cumulus transition

Christensen, Matthew; Jones, W. Kinzy; Stier, Philip

doi:10.1073/pnas.1921231117

Cited by 115 publications

(132 citation statements)

References 52 publications

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“…Such trajectory approaches are particularly useful when they exploit the high temporal resolution that is available from geostationary satellites. Aerosol retrievals from geostationary satellites may be combined using trajectory modelling to link these to clouds that form in these air masses (Kikuchi et al, 2018), or also the aerosol retrieval from a polar orbiter could be related to clouds retrieved from geostationary satellites that form in the same air masses (Christensen et al, 2020).…”

Section: Horizontal Co-locationmentioning

confidence: 99%

“…Aerosols can induce transitions between cloud regimes, for instance by changing drizzle behaviour (Rosenfeld et al, 2006;Feingold et al, 2010;Wood et al, 2011). The direction and magnitude of these changes depends on the cloud state and regime, because responses to aerosol changes occur due to processes spanning a range from microphysics to the mesoscale (Christensen and Stephens, 2012;Kazil et al, 2011;Wang et al, 2011). These processes include precipitation suppression (Albrecht, 1989), rapid feedbacks involving cloud-top entrainment (Ackerman et al, 2004;Bretherton et al, 2007;Hill et al, 2009;Bulatovic et al, 2019) and rapid feedbacks involving cloud lateral entrainment (Xue and Feingold, 2006;Small et al, 2009) as well as responses in dynamics (Xue et al, 2008;Stevens and Feingold, 2009;Wang and Feingold, 2009).…”

Section: Introductionmentioning

confidence: 99%

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Constraining the Twomey effect from satellite observations: issues and perspectives

et al. 2020

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Abstract. The Twomey effect describes the radiative forcing associated with a change in cloud albedo due to an increase in anthropogenic aerosol emissions. It is driven by the perturbation in cloud droplet number concentration (ΔNd, ant) in liquid-water clouds and is currently understood to exert a cooling effect on climate. The Twomey effect is the key driver in the effective radiative forcing due to aerosol–cloud interactions, but rapid adjustments also contribute. These adjustments are essentially the responses of cloud fraction and liquid water path to ΔNd, ant and thus scale approximately with it. While the fundamental physics of the influence of added aerosol particles on the droplet concentration (Nd) is well described by established theory at the particle scale (micrometres), how this relationship is expressed at the large-scale (hundreds of kilometres) perturbation, ΔNd, ant, remains uncertain. The discrepancy between process understanding at particle scale and insufficient quantification at the climate-relevant large scale is caused by co-variability of aerosol particles and updraught velocity and by droplet sink processes. These operate at scales on the order of tens of metres at which only localised observations are available and at which no approach yet exists to quantify the anthropogenic perturbation. Different atmospheric models suggest diverse magnitudes of the Twomey effect even when applying the same anthropogenic aerosol emission perturbation. Thus, observational data are needed to quantify and constrain the Twomey effect. At the global scale, this means satellite data. There are four key uncertainties in determining ΔNd, ant, namely the quantification of (i) the cloud-active aerosol – the cloud condensation nuclei (CCN) concentrations at or above cloud base, (ii) Nd, (iii) the statistical approach for inferring the sensitivity of Nd to aerosol particles from the satellite data and (iv) uncertainty in the anthropogenic perturbation to CCN concentrations, which is not easily accessible from observational data. This review discusses deficiencies of current approaches for the different aspects of the problem and proposes several ways forward: in terms of CCN, retrievals of optical quantities such as aerosol optical depth suffer from a lack of vertical resolution, size and hygroscopicity information, non-direct relation to the concentration of aerosols, difficulty to quantify it within or below clouds, and the problem of insufficient sensitivity at low concentrations, in addition to retrieval errors. A future path forward can include utilising co-located polarimeter and lidar instruments, ideally including high-spectral-resolution lidar capability at two wavelengths to maximise vertically resolved size distribution information content. In terms of Nd, a key problem is the lack of operational retrievals of this quantity and the inaccuracy of the retrieval especially in broken-cloud regimes. As for the Nd-to-CCN sensitivity, key issues are the updraught distributions and the role of Nd sink processes, for which empirical assessments for specific cloud regimes are currently the best solutions. These considerations point to the conclusion that past studies using existing approaches have likely underestimated the true sensitivity and, thus, the radiative forcing due to the Twomey effect.

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Section: Horizontal Co-locationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Constraining the Twomey effect from satellite observations: issues and perspectives

et al. 2020

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“…Thus, the aerosol perturbations can impart evaporation-entrainment feedback (particularly for non-precipitating cumulus clouds) to shorten the cloud lifespan [52]. However, it was also reported that anthropogenic aerosols can suppress the precipitation to yield clouds with longer lifespans, higher liquid water content, and larger fractional cloudiness [52,53]. Hence, further studies are needed to clarify the contrasting observations to pinpoint the exact influence of anthropogenic aerosols on cloud lifespan in the Arctic region.…”

Section: Air Pollution and Emissionsmentioning

confidence: 99%

Air Pollution and Its Association with the Greenland Ice Sheet Melt

et al. 2020

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The Greenland Ice Sheet (GrIS) has been a topic of extensive scientific research over the past several decades due to the exponential increase in its melting. The relationship between air pollution and GrIS melting was reviewed based on local emission of air pollutants, atmospheric circulation, natural and anthropogenic forcing, and ground/satellite-based measurements. Among multiple factors responsible for accelerated ice melting, greenhouse gases have long been thought to be the main reason. However, it is suggested that air pollution is another piece of the puzzle for this phenomenon. In particular, black carbon (BC) and other aerosols emitted anthropogenically interact with clouds and ice in the Arctic hemisphere to shorten the cloud lifespan and to change the surface albedo through alteration of the radiative balance. The presence of pollution plumes lowers the extent of super cooling required for cloud freezing by about 4 °C, while shortening the lifespan of clouds (e.g., by altering their free-energy barrier to prompt precipitation). Since the low-level clouds in the Arctic are 2–8 times more sensitive to air pollution (in terms of the radiative/microphysical properties) than other regions in the world, the melting of the GrIS can be stimulated by the reduction in cloud stability induced by air pollution. In this study, we reviewed the possible impact of air pollution on the melting of the GrIS in relation to meteorological processes and emission of light-absorbing impurities. Long-term variation of ground-based AERONET aerosol optical depth in Greenland supports the potential significance of local emission and long-range transport of air pollutants from Arctic circle and continents in the northern hemisphere in rapid GrIS melting trend.

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“…Meskhidze et al (2009) and Gryspeerdt et al (2014) showed that short term development is also correlated to the aerosol environment, but that accounting for the initial cloud state is vital. This technique has recently been extended to longer timescales (Christensen et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

Observing the timescales of aerosol-cloud interactions in snapshot satellite images

Gryspeerdt¹,

Goren²,

Smith³

2020

Preprint

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Abstract. The response of cloud processes to an aerosol perturbation is one of the largest uncertainties in the anthropogenic forcing of the climate. It occurs at a variety of timescales, from the near-instantaneous Twomey effect, to the longer timescales required for cloud adjustments. Understanding the temporal evolution of cloud properties following an aerosol perturbation is necessary to interpret the results of so-called "natural experiments" from a known aerosol source, such as a ship or industrial site. This work uses reanalysis windfields and ship emission information matched to observations of shiptracks to measure the timescales of cloud responses to aerosol in instantaneous (or "snapshot") images taken by polar-orbiting satellites. As found in previous studies, the local meteorological environment is shown to have a strong impact on the occurrence and properties of shiptracks, but there is a strong time dependence in their properties. The largest droplet number concentration (Nd) responses are found within three hours of emission, while cloud adjustments continue to evolve over periods of ten hours or more. Cloud fraction is increased within the early life of shiptracks, with the formation of shiptracks in otherwise clear skies indicating that around 5–10 % of clear-sky cases in this region may be aerosol-limited. The liquid water path (LWP) enhancement and the Nd-LWP sensitivity are also time dependent and strong functions of the background cloud and meteorological state. The near-instant response of the LWP within shiptracks may be evidence of a retrieval bias in previous estimates of the LWP response to aerosol derived from natural experiments. These results highlight the importance of temporal development and the background cloud field for quantifying the aerosol impact on clouds, even in situations where the aerosol perturbation is clear.

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Aerosols enhance cloud lifetime and brightness along the stratus-to-cumulus transition

Cited by 115 publications

References 52 publications

Constraining the Twomey effect from satellite observations: issues and perspectives

Constraining the Twomey effect from satellite observations: issues and perspectives

Air Pollution and Its Association with the Greenland Ice Sheet Melt

Observing the timescales of aerosol-cloud interactions in snapshot satellite images

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