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Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

Cover Image Copyright Year: 2009
Author(s): Heintzenberg, J.; Charlson, R.
Publisher: MIT Press
Content Type : Books & eBooks
Topics: Geoscience
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Abstract

More than half the globe is covered by visible clouds. Clouds control major parts of the Earth's energy balance, influencing both incoming shortwave solar radiation and outgoing longwave thermal radiation. Latent heating and cooling related to cloud processes modify atmospheric circulation, and, by modulating sea surface temperatures, clouds affect the oceanic circulation. Clouds are also an essential component of the global water cycle, on which all terrestrial life depends. Yet clouds constitute the most poorly quantified, least understood, and most puzzling aspect of atmospheric science, and thus the largest source of uncertainty in the prediction of climate change. Because clouds are influenced by climate change, and because complex, unidentified feedback systems are involved, science is faced with many unanswered questions. This volume begins by indentifying and describing the baffling nature of clouds. It explores the boundaries of current knowledge on the spatial/temporal variability of clouds and cloud-related aerosols as well as the factors that control clouds, and examines the extent and nature of anthropogenic perturbations. Particular emphasis is given to the connections of clouds to climate through radiation, dynamics, precipitation, and chemistry, and to the difficulties in understanding the obvious but elusive fact that clouds must be affected by climate change. Utilizing the insights of this unique gathering of experts, the book offers recommendations to improve the current state of knowledge and direct future research in fields ranging from chemistry and theoretical physics to climate modeling and remote satellite sensing.

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      Front Matter

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): i - xv
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Half Title, Strüngmann Forum Reports, Title, Copyright, Contents, The Ernst Strüngmann Forum, List of Contributors View full abstract»

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      Introduction

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 1 - 15
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Perturbations of Clouds and Related Aerosols, Cloud-controlling Factors, Extent and Nature of Anthropogenic Perturbations of Clouds, Current Understanding and Quantification of the Effects of Clouds in the Changing Climate System and Strategies to Reduce the Critical Uncertainties, Describing the Response of Clouds to Changing Climate: The Need for Multiple Indices of Climate Change, Context of this Forum: The Urgency of Current Demands by the Policy Community on the Scientific Community and the Need for High Scientific Standards, References View full abstract»

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      Trends in Observed Cloudiness and Earth's Radiation Budget: What Do We Not Know and What Do We Need to Know?

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 17 - 36
      Copyright Year: 2009

      MIT Press eBook Chapters

      The response of clouds and their radiative effects to global warming represents a longstanding and considerable area of uncertainty in our understanding of climate change. At present, it is not known whether changes in cloudiness will exacerbate, mitigate, or have little effect on the increasing global surface temperature caused by anthropogenic greenhouse radiative forcing. Another substantial uncertainty is the magnitude of radiative forcing resulting from the modification of cloud properties by anthropogenic aerosols. Global climate models provide scant reliable insight regarding these issues because of their inability to parameterize correctly or otherwise represent the small-scale convective, turbulent, and microphysical processes that control cloud properties. It is therefore crucial to document and assess global and regional low-frequency variations in clouds and radiation flux that have occurred over the past several decades, a period marked by rapidly rising temperature and changes in anthropogenic aerosol emissions. This will enable us to estimate from observations how clouds and their impacts on the radiation budget are responding to global warming and aerosol changes. Moreover, a trustworthy observational record will provide a good constraint on global climate model simulations. Previous investigators have documented multidecadal variations in various cloud and radiation parameters, but no conclusive results are yet available. Problems include the lack of global and quantitative surface measurements, the shortness of the available satellite record, the inability to determine correctly cloud and aerosol properties from satellite data, many different kinds of inhomogeneities in the data, and insufficient precision to measure the small changes in cloudiness and radiation that nevertheless can have large impacts on the Earth's climate. Many of these def iciencies emerge from the absence of an observing system designed to monitor variations in clouds and radiation on timescales relevant to climate; to compensate, observations must be assembled from a system originally designed for purposes of weather forecasting. Although we cannot go back in time to make more and better observations, we need to improve our processing of the available historical measurements to mitigate inhomogeneities, provide better retrievals of cloud and aerosol properties, and extend the record farther back in time. Furthermore, it is essential to construct an observation system with sufficient stability and longevity to measure long-term variations in cloudiness and the radiation budget with improved precision and accuracy. Our present observing system has unfortunately little prospect of enhancement at this time and, moreover, is in danger of future deterioration since there are no definite commitments to replace several critical instruments when current satellite missions end. View full abstract»

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      Climatologies of Cloudrelated Aerosols: Part 1: Particle Number and Size

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 37 - 57
      Copyright Year: 2009

      MIT Press eBook Chapters

      A proper representation of aerosol properties is an essential first step in addressing potential impacts of aerosols on climate and clouds. In the context of temporal and spatial variability of concentration, size, and composition, maps of aerosol properties are usually based on insufficiently evaluated datasets of model simulations or satellite retrievals. Here, a new approach is offered. Quality data from ground-based remotesensing networks are merged into multi-model median background fields. Global monthly maps are created (at a 1° × 1° horizontal resolution) for aerosol column properties of aerosol optical depth (AOD), single-scattering albedo (ω0), and Ångström parameter (AnP, an easier to measure substitute for the asymmetry factor, g). Adopting the commonly observed bimodal size distribution shape for aerosol, AOD is partitioned into contributions from smaller ( accumulation mode, radii: 0.05–0.50 µm) and larger ( coarse mode, radii >0.50 µm) particle sizes. This simplifies the spectral extension of mid-visible optical properties and allows anthropogenic AOD estimates to be independent of interannual variations of (larger size) natural aerosol. The AOD is partitioned into its natural and anthropogenic components using global model estimates for the anthropogenic AOD fraction of all small particles, assuming that only small aerosol particles can be anthropogenic. Global modeling also provides the vertical distribution of AOD. Applying the arguments of Pöschl et al. (see Part 2 of this chapter), namely, that hygroscopic growth of atmospheric aerosol particles is relatively well constrained, all necessary ingredients are assembled to derive global monthly maps for concentrations of cloud condensation nuclei (CCN) and to assess regional CCN enhancements as a result of anthropogenic activi ty. View full abstract»

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      Part 2: Particle Hygroscopicity and Cloud Condensation Nucleus Activity

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 58 - 72
      Copyright Year: 2009

      MIT Press eBook Chapters

      Aerosol particles that act as cloud condensation nuclei (CCN) play a central role in the formation of clouds. Here, the basic concepts and key aspects that link the CCN activity of aerosol particles to their size, chemical composition, and hygroscopicity (i.e., their ability to absorb water vapor) are discussed. Literature data and recent field measurements suggest that the influence of chemical composition can be efficiently described by a single effective hygroscopicity parameter that relates the dry particle diameter to the so-called critical water-vapor supersaturation (i.e., the minimum supersaturation required to form a cloud droplet). This hygroscopicity parameter, κ, can be easily calculated from chemical composition data and is typically in the range of ∼0.1 for pyrogenic and secondary organic aerosols to ∼1 for sea spray aerosols. Continental and marine boundary layer aerosols tend to cluster into relatively narrow ranges of effective hygroscopicity (continental κ = 0.3±0.1; marine κ = 0.7±0.2). Thus the influence of aerosol chemical composition and hygroscopicity appears to be less variable and less uncertain than other factors that determine the effects of aerosols on warm cloud formation in the atmosphere (e.g., particle number concentration, size distribution, sources, sinks, and meteorological conditions). Nevertheless, more detailed investigations and representations of the hygroscopic properties of aerosol particles, as a function of chemical composition, are needed to elucidate fully aerosol-cloud interactions, especially for low water-vapor supersaturations, low aerosol concentrations, and organic components. Even for simple and well-defined inorganic reference substances, such as ammonium sulfate and sodium chloride, the critical supersaturations or critical dry particle diameters of CCN activation c alculated with different Köhler models can deviate as much as 20% from the most accurate available models. To ensure that measurement and model results can be compared properly, CCN studies should always report exactly which Köhler model equations and parameters have been applied. In addition, potential kinetic limitations of water uptake appear to be one of the most crucial open questions of CCN properties and activation. View full abstract»

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      Cloud Properties from In-situ and Remotesensing Measurements: Capability and Limitations

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 58 - 72
      Copyright Year: 2009

      MIT Press eBook Chapters

      The ability to measure cloud properties has advanced considerably over recent years. This chapter reviews the in-situ and remote-sensing instrumentation currently available and discusses some of the associated potential problems. The requirements for calibrations and use of well-documented software are outlined. Aerosol and cloud characteristics have vertical and geographical variations that are important, but poorly defined. The necessity to measure parameters on the scales of interest and to present those measurements in proper units is discussed. Recommendations are made regarding future actions that would be beneficial to the community. These include improvements required in the accuracy of the measurements, global datasets, and collaborations between those who make and use in-situ and remote-sensing measurements. View full abstract»

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      Clouds and Precipitation: Extreme Rainfall and Rain from Shallow Clouds

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 107 - 126
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter reviews present knowledge on extreme precipitation and moderate rainfall from low-level clouds. Primary focus is on the statistics of precipitation characteristics rather than on a detailed description of individual case studies. First, observed variability of precipitation from low-level clouds and the existing techniques to separate different microphysical stages from remote-sensing measurements are reviewed. Over the tropical areas of Pacific and Atlantic oceans, the global distribution of shallow rainfall exhibits a “butterfly” pattern. This feature encompasses heavily precipitating regions such as the intertropical, south Pacific, and south Atlantic convergence zones (ITCZ, SPCZ, and SACZ, respectively); the northern hemispheric counterpart of SPCZ and SACZ emerges only when shallow rain is isolated. The nature of extreme precipitation varies temporally. On a timescale of about a day, extreme precipitation is associated with synoptic-scale disturbances, including a notable example known as tropical plumes or moist conveyer belt, which could give rise to extreme daily precipitation in downstream arid regions. On an hourly timescale, extreme precipitation is caused by mesoscale moisture convergence, which is so intense that it maintains a continuous overturning of saturated air. Satellite observations imply that the global distribution of extreme precipitation shows a systematic difference from the total rainfall map in terms of, for example, the contrast between land and ocean. The distribution of low-level, precipitation-related latent heating associated with warm rain coincides with the butterfly pattern. Its cohabitation and separation with the deep heating suggests that warm rain plays a role in providing a thick layer of moist static energy source to the convection, and that it is also related to the tropical plumes which cause extr eme precipitation in the semiarid west coasts of continents. View full abstract»

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      Temporal and Spatial Variability of Clouds and Related Aerosols

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 127 - 147
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Hygroscopic Properties of Atmospheric Aerosols Appear to be Better Constrained than Other Key Parameters of Warm Cloud Formation, Ice Nuclei and Ice Microphysics Still Pose Many Open Questions, The Confounding Influence of Meteorology, Evidence for Large-scale Aerosol Impacts on Cloud Albedo, Cloud Amount, and Precipitation Remain Ambiguous, A Major Motivation to Study Aerosols and Climate Is to Understand 20th-century Temperature Change, Since Some Global Warming Signals Are Now Becoming Evident, We Should Search for Associated Cloud Changes, Existing Cloud, Aerosol, and Radiation Records Need to be Reprocessed, and the Continuity of the Current Observing System Must be Guaranteed, New Strategies Are Required to Observe Key Variables More Accurately for Understanding Perturbed Clouds in the Climate System, Acknowledgments, References View full abstract»

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      Laboratory Cloud Simulation: Capabilities and Future Directions

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 149 - 172
      Copyright Year: 2009

      MIT Press eBook Chapters

      Since the atmosphere offers only observation possibilities and does not permit the setting of initial and boundary conditions, laboratory studies are important tools to examine cloud processes under well-defined, repeatable conditions and expand our understanding. An overview is provided of the capabilities and limitations of laboratory studies on physical cloud processes. Aerosol particle hygroscopic growth and activation, droplet dynamic growth, ice nucleation, and droplet-turbulence interactions are addressed and laboratory devices suitable for investigating and simulating these cloud physical processes are presented. These devices range from portable instruments for measuring selected particle and cloud droplet properties, to setups allowing process studies under simulated cloud conditions, to experiments allowing a nearly full-scale cloud simulation. The issue of laboratory-generated particles and their importance in laboratory cloud simulation is discussed. Finally, suggestions are presented for possible future research topics and devices in the field of laboratory cloud simulation. Specific suggestions include the initiation and/or continuation of investigations regarding particle hygroscopic growth and activation, the accommodation coefficients of water vapor on liquid water and ice, aerosol effects on primary ice formation in clouds, aerosol-based parameterizations of cloud ice formation, secondary ice formation/ice multiplication, the production and characterization of particles suitable for cloud simulation experiments, and experiments that combine turbulence and microphysics. The latter topic is important, as it offers the only possible way of simulating and quantifying the possible interactions and feedbacks between the microphysical (activation, growth, freezing) and turbulent transport processes within clouds View full abstract»

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      Cloud-controlling Factors: Low Clouds

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 173 - 196
      Copyright Year: 2009

      MIT Press eBook Chapters

      The way in which meteorological and aerosol factors conspire to determine the statistics and climatology of layers of shallow (boundary layer) clouds is reviewed, with an emphasis on factors that may be expected to change in a perturbed climate. The paramount role of theory is identified, both in service of advancing our understanding, but also in modeling and attributing specific causes and effects. In particular, it is argued that limits to current understanding of meteorological controls on cloudiness make it difficult, and in many situations perhaps impossible, to attribute changes in cloudiness to aerosol perturbations. Suggestions for advancing our understanding of low cloudcontrolling processes are offered; these include renewing our focus on theory, model craftsmanship, and increasing the scope and breadth of observational efforts View full abstract»

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      Deep Convective Clouds

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 197 - 215
      Copyright Year: 2009

      MIT Press eBook Chapters

      Deep convection plays a key role in the Earth's atmospheric general circulation and is often associated with severe weather. It spans a wide range of spatial scales, from subcentimeter for the cloud microscale to tens and hundreds of kilometers for convective towers and mesoscale convective systems. Deep convection extends, however, far beyond the scale of an individual convective system because its latent heating drives large-scale atmospheric tropical and subtropical circulations, such as the Hadley, Walker, and monsoon circulations. Thus to understand the role of atmospheric deep convection in the climate system, as well as in climate change, key processes across all of these scales must be taken into account. This chapter reviews the relevant aspects of the problem, points out the limitations of current modeling and observational approaches, and suggests areas for future research. It is argued that understanding the role of deep convection in the climate system, as well as in predictions of climate change, requires modeling efforts across all scales, from microscale to global, using a variety of models. Traditional atmospheric general circulation models are not sufficient because representation of deep convection, and how it may change in the perturbed climate, is highly uncertain View full abstract»

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      Large-scale Controls on Cloudiness

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 217 - 234
      Copyright Year: 2009

      MIT Press eBook Chapters

      The climatological distribution of clouds is tightly coupled to large-scale circulation. Net cloud radiative forcing is mainly the result of boundary layer clouds in large-scale subsidence. Deep convective cloud systems exert long- and shortwave cloud forcing that nearly cancel out each other. The extent of this cancellation depends strongly on the vertical motion profile, suggesting that if the cancellation is not coincidental, dynamic feedbacks probably play a role in its maintenance. Low cloud radiative forcing is tied to how cold the surface is compared to the free troposphere. It is an open question how this correlation should be represented in a way that generalizes to a perturbed climate. Simple empirical representations of deep and low cloud forcing are shown to provide strong feedbacks on an idealized model of a tropical overturning circulation. Global weather and climate models, however, still have profound difficulties in accurately representing the cloud response to large-scale forcings View full abstract»

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      Cloud-controlling Factors of Cirrus

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 235 - 268
      Copyright Year: 2009

      MIT Press eBook Chapters

      Factors controlling cirrus clouds comprise small- and large-scale atmospheric dynamics, ice nucleation behavior of natural and anthropogenic particles, and interaction with terrestrial and solar radiation. Current understanding of these factors is summarized. Key uncertainties in this active area of research are addressed, and future developments aimed at reducing these uncertainties are outlined View full abstract»

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      Cloud-controlling Factors

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 269 - 268
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Introduction, Connecting Scales in Global Climate Models, Connecting Scales Using Alternative Modeling Techniques, Fundamental Problems in Cloud Processes, Observational Strategies and Proposals, Conclusions, References View full abstract»

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      Cloud Particle Precursors

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 291 - 317
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Introduction, Relevant Properties: Warm Cloud Particle Precursors, Sensitivity of Liquid Cloud Particle Formation to Particle Properties, Relevant Properties: Ice Cloud Particle Precursors, Sensitivity of Ice Cloud Particle Formation to Particle Properties, Future Changes in Cloud Particle Precursor Types, Properties, and Concentrations, Status of “Closure” and Recommended Research Directions, Acknowledgments, References View full abstract»

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      Cloud—Aerosol Interactions from the Micro to the Cloud Scale

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 319 - 338
      Copyright Year: 2009

      MIT Press eBook Chapters

      The effect of suspended particles (aerosol) on clouds and precipitation from the micro to the cloud scale has been well studied through laboratory, in-situ, and remote-sensing data but many uncertainties remain. In particular, there is scant observational evidence of aerosol effects on surface precipitation. Clouds and precipitation modify the amount of aerosol through both physical and chemical processes so that a three-way interactive feedback between aerosol, cloud microphysics, and cloud dynamics must be considered. The fundamental cloud microphysical properties are driven by dynamics; vertical motions and mixing processes between the cloud and its environment determine the concentration of cloud water, a key parameter for both climate and precipitation. However, aerosol particles can significantly affect the microphysics and dynamics of clouds by changing the size distribution of drops, their ability to grow to raindrops, their rates of evaporation, and their mixing with the environment. The physical system is strongly coupled and attempting to separate aerosol effects has only been done using some simple constructs, some of which will be shown to be of dubious utility. Both observations and modeling suggest that not only the magnitude, but perhaps also the sign of these effects, depends on the larger-scale meteorological context in which aerosol-cloud interactions are embedded. Some alternate approaches are considered, as we explore the possibility of self-regulation processes that may act to limit the range over which aerosol significantly affects clouds View full abstract»

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      Weather and Climate Engineering

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 339 - 367
      Copyright Year: 2009

      MIT Press eBook Chapters

      In this chapter I present an overview of the concepts and status of the science of weather and climate engineering. I begin by discussing the concepts of seeding clouds through glaciogenic and hygroscopic seeding. I review the status of research on these concepts for increasing rainfall, decreasing hail damage, and reducing hurricane intensity. Thereafter I present an overview of the concepts for climate engineering to counter greenhouse warming. These include seeding in the stratosphere with sulfate-producing gases and aerosols, and carbonaceous aerosols. I also consider hygroscopic seeding of marine stratocumulus boundary layer clouds to enhance their albedo and cause a cooling effect. Also considered is seeding mid-level stratus clouds to enhance their albedo during the day and increasing outgoing longwave radiation during the night time. Cirrus clouds present a major obstacle to climate modification owing to their widespread global coverage and their tendency to warm the surface, thus reinforcing greenhouse warming. Speculations on the seeding of carbonaceous aerosols to clear cirrus through a semi-direct effect are presented. Most of the proposed concepts require a great deal of research to quantify their impacts and potential adverse consequences. I include a long list of the reasons as to why we should not apply climate engineering. Despite these, I anticipate that if we find ourselves in a true climate crisis, politicians will call for climate engineering measures in an attempt to alter adverse climate trends. If this should ever be the case, let us be sure that we do so with the most advanced level of knowledge of the climate system and the full consequences of our actions. View full abstract»

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      Air Pollution and Precipitation

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 369 - 399
      Copyright Year: 2009

      MIT Press eBook Chapters

      The physical hypothesis that air pollution in the form of small particles should lead to less efficient formation of precipitation has been established for several decades and is widely considered to be scientifically sound. As yet, however, no convincing proof exists that such a microphysical control of precipitation efficiency has been the prime cause of rainfall reduction in any area of the globe. There is a need for new experimental designs to test this hypothesis in a holistic way, taking into account all possible confounding influences on rainfall trends in a climate that is clearly not stationary in the face of global warming and natural decadal variability. View full abstract»

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      What Do We Know about Large-scale Changes of Aerosols, Clouds, and the Radiation Budget?

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 401 - 432
      Copyright Year: 2009

      MIT Press eBook Chapters

      In this chapter we examine how aerosol and cloud fields undergo perturbations by anthropogenic activities. Recent surface observations and satellite remote sensing have detected signatures of large-scale changes in the atmospheric aerosol amounts, associated changes in the cloud fraction, and microphysical structures on a global scale. Models can simulate these signatures fairly well, but problems still remain. Fields of anthropogenic aerosol optical depth (AOD) from several atmospheric models have been found to be consistent with the spatial pattern obtained from satellite data. Further studies are needed to differentiate between natural and anthropogenic aerosols and to interpret observed temporal and regional trends in aerosol parameters. The strength of the cloud-aerosol interaction can be characterized by the regression of AOD or aerosol index (AI) on cloud droplet number (Nc). From recent studies, the corresponding slopes dlog(Nc)/ dlog(A) vary between 0.19 and 0.7. Further work is needed to see whether such variability is the result of methodological problems or differences in cloud environments for which the studies have established the cloud-aerosol relation. View full abstract»

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      The Extent and Nature of Anthropogenic Perturbations of Clouds

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 433 - 449
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Introduction, General Discussion on Perturbed Clouds, Microphysics, Radiation, Precipitation, Dynamics, Chemistry, References View full abstract»

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      Global Indirect Radiative Forcing Caused by Aerosols: IPCC (2007) and Beyond

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 451 - 467
      Copyright Year: 2009

      MIT Press eBook Chapters

      Anthropogenic aerosols are thought to exert a significant indirect radiative forcing because they act as cloud condensation nuclei in warm cloud-forming processes and ice nuclei in cold cloud-forming processes. Although many of the processes associated with the perturbation of cloud microphysics by anthropogenic aerosols were discussed, IPCC (2007) provided only an estimate of full quantification of the radiative forcing attributable to the first indirect effect (which they referred to as the cloud albedo effect). Here we explain that this approach is necessary if one is to compare the radiative forcing from the indirect effect of aerosols with those from other radiative forcing components such as that from changes in well-mixed greenhouse gases. We also highlight the problems in assessing the effect of anthropogenic aerosols upon clouds under the strict definitions of radiative forcing provided by the IPCC (2007). Although results from global climate models, at their current state of development, suggest that an analysis of indirect aerosol effects in terms of forcing and feedback is possible, a key rationale for the IPCC's definition of radiative forcing, a straightforward scaling between an agent's forcing and the temperature change it induces, is significantly compromised. Feedbacks from other radiative forcings are responses to radiative perturbations, whereas feedbacks from indirect aerosol effects are responses to both radiative and cloud microphysical perturbations. This inherent difference in forcing mechanism breaks down the consistency between forcing and temperature response. It is likely that additional characterization, such as climate efficacy, will be required when comparing indirect aerosol effects with other radiative forcings. We suggest using the radiative flux perturbation associated with a change from preindustrial to present-day co mposition, calculated in a global climate model using fixed sea surface temperature and sea ice, as a supplement to IPCC forcing View full abstract»

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      Simulating Global Clouds: Past, Present, and Future

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 469 - 486
      Copyright Year: 2009

      MIT Press eBook Chapters

      This chapter contains sections titled: Introduction, Projected Changes in Clouds and Cloud Effects, Open Issues in Global Simulation of Perturbed Clouds, Global Process-oriented Simulation, Challenges in Observations, Global Cloud-resolving Simulations, Summary, Acknowledgments, References View full abstract»

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      Observational Strategies from the Micro- to Mesoscale

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 487 - 510
      Copyright Year: 2009

      MIT Press eBook Chapters

      In a changing climate, clouds are perturbed through large-scale variations in the general circulation induced by both greenhouse gases and aerosols. Some aerosols perturb cloud microphysical properties as well. The challenge for micro- and mesoscale observational studies of perturbed clouds is thus to establish the links between these two contrasting forcings, in an effort to understand how clouds respond to changes in the general circulation and to quantify how this response might be modulated by changes in their microphysical properties. The two generic classes of micro- to mesoscale observational strategies, the Eulerian column closure and the Lagrangian cloud system evolution approaches, are described using examples of low-level cloud studies, and recommendations are made on how they should be combined with large-scale information to address this issue View full abstract»

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      Observational Strategies at Meso- and Large Scales to Reduce Critical Uncertainties in Future Cloud Changes

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 511 - 530
      Copyright Year: 2009

      MIT Press eBook Chapters

      The response of clouds to climate change remains very uncertain. This is attributable to both an incomplete knowledge of cloud physics and to the difficulties that large-scale models have in simulating the different properties of clouds. An observational strategy is proposed to improve the representation of clouds in large-scale models and to reduce uncertainties in the future change of cloud properties. This consists of determining first what key aspects of the simulation of clouds are the most critical, with respect to future climate changes, and then of using specific methodologies and new datasets to improve the simulation of these aspects in large-scale models View full abstract»

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      Aerosols and Clouds in Chemical Transport Models and Climate Models

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 531 - 556
      Copyright Year: 2009

      MIT Press eBook Chapters

      Clouds exert major influences on both short- and longwave radiation as well as on the hydrological cycle. Accurate representation of clouds in climate models poses a major problem because of the high sensitivity of radiative transfer and water cycle to cloud properties and processes, an incomplete understanding of these processes, and the wide range of scales over which these processes occur. Small changes in the amount, altitude, physical thickness, and/or microphysical properties of clouds that occur as a result of human influence can exert changes in Earth's radiation budget comparable to the radiative forcing by anthropogenic greenhouse gases, thus either partly offsetting or enhancing the warming due to these gases. Because clouds form on aerosol particles, changes in the amount and/or composition of aerosols affect clouds in various ways. The forcing of the radiation balance due to aerosol-cloud interactions (indirect aerosol effect) has large uncertainties because a variety of important processes are not well understood, precluding their accurate representation in models View full abstract»

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      Current Understanding and Quantification of Clouds in the Changing Climate System and Strategies for Reducing Critical Uncertainties

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 557 - 573
      Copyright Year: 2009

      MIT Press eBook Chapters

      To date, no observation-based proxy for climate change has been successful in quantifying the feedbacks between clouds and climate. The most promising, yet demanding, avenue to gain confidence in cloud-climate feedback estimates is to utilize observations and large-eddy simulations (LES) or cloud-resolving modeling (CRM) to improve cloud process parameterizations in large-scale models. Sustained and improved satellite observations are essential to evaluate large-scale models. A reanalysis of numerical prediction models with assimilation of cloud, aerosol, and precipitation observations would provide a valuable dataset for examining cloud interactions. The link between climate modeling and numerical weather prediction (NWP) may be exploited by evaluating how accurate cloud characteristics are represented by the parameterization schemes in NWP models. A systematic simplification of large-scale models is an important avenue to isolate key processes linked to cloud-climate feedbacks and would guide the formulation of testable hypotheses for field studies. Analyses of observation-derived correlations between cloud and aerosol properties in combination with modeling studies may allow aerosol-cloud interactions to be detected and quantified. Reliable representations of cloud dynamic and physical processes in large-scale models are a prerequisite to assess aerosol indirect effects on a large scale with confidence. To include aerosol indirect effects in a consistent manner, we recommend that a “radiative flux perturbation” approach be considered as a complement to radiative forcing. View full abstract»

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      Abbreviations

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 575 - 580
      Copyright Year: 2009

      MIT Press eBook Chapters

      More than half the globe is covered by visible clouds. Clouds control major parts of the Earth's energy balance, influencing both incoming shortwave solar radiation and outgoing longwave thermal radiation. Latent heating and cooling related to cloud processes modify atmospheric circulation, and, by modulating sea surface temperatures, clouds affect the oceanic circulation. Clouds are also an essential component of the global water cycle, on which all terrestrial life depends. Yet clouds constitute the most poorly quantified, least understood, and most puzzling aspect of atmospheric science, and thus the largest source of uncertainty in the prediction of climate change. Because clouds are influenced by climate change, and because complex, unidentified feedback systems are involved, science is faced with many unanswered questions. This volume begins by indentifying and describing the baffling nature of clouds. It explores the boundaries of current knowledge on the spatial/temporal variability of clouds and cloud-related aerosols as well as the factors that control clouds, and examines the extent and nature of anthropogenic perturbations. Particular emphasis is given to the connections of clouds to climate through radiation, dynamics, precipitation, and chemistry, and to the difficulties in understanding the obvious but elusive fact that clouds must be affected by climate change. Utilizing the insights of this unique gathering of experts, the book offers recommendations to improve the current state of knowledge and direct future research in fields ranging from chemistry and theoretical physics to climate modeling and remote satellite sensing. View full abstract»

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      Name Index

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 581 - 589
      Copyright Year: 2009

      MIT Press eBook Chapters

      More than half the globe is covered by visible clouds. Clouds control major parts of the Earth's energy balance, influencing both incoming shortwave solar radiation and outgoing longwave thermal radiation. Latent heating and cooling related to cloud processes modify atmospheric circulation, and, by modulating sea surface temperatures, clouds affect the oceanic circulation. Clouds are also an essential component of the global water cycle, on which all terrestrial life depends. Yet clouds constitute the most poorly quantified, least understood, and most puzzling aspect of atmospheric science, and thus the largest source of uncertainty in the prediction of climate change. Because clouds are influenced by climate change, and because complex, unidentified feedback systems are involved, science is faced with many unanswered questions. This volume begins by indentifying and describing the baffling nature of clouds. It explores the boundaries of current knowledge on the spatial/temporal variability of clouds and cloud-related aerosols as well as the factors that control clouds, and examines the extent and nature of anthropogenic perturbations. Particular emphasis is given to the connections of clouds to climate through radiation, dynamics, precipitation, and chemistry, and to the difficulties in understanding the obvious but elusive fact that clouds must be affected by climate change. Utilizing the insights of this unique gathering of experts, the book offers recommendations to improve the current state of knowledge and direct future research in fields ranging from chemistry and theoretical physics to climate modeling and remote satellite sensing. View full abstract»

    • Full text access may be available. Click article title to sign in or learn about subscription options.

      Subject Index

      Heintzenberg, J. ; Charlson, R.
      Clouds in the Perturbed Climate System:Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation

      Page(s): 592 - 597
      Copyright Year: 2009

      MIT Press eBook Chapters

      More than half the globe is covered by visible clouds. Clouds control major parts of the Earth's energy balance, influencing both incoming shortwave solar radiation and outgoing longwave thermal radiation. Latent heating and cooling related to cloud processes modify atmospheric circulation, and, by modulating sea surface temperatures, clouds affect the oceanic circulation. Clouds are also an essential component of the global water cycle, on which all terrestrial life depends. Yet clouds constitute the most poorly quantified, least understood, and most puzzling aspect of atmospheric science, and thus the largest source of uncertainty in the prediction of climate change. Because clouds are influenced by climate change, and because complex, unidentified feedback systems are involved, science is faced with many unanswered questions. This volume begins by indentifying and describing the baffling nature of clouds. It explores the boundaries of current knowledge on the spatial/temporal variability of clouds and cloud-related aerosols as well as the factors that control clouds, and examines the extent and nature of anthropogenic perturbations. Particular emphasis is given to the connections of clouds to climate through radiation, dynamics, precipitation, and chemistry, and to the difficulties in understanding the obvious but elusive fact that clouds must be affected by climate change. Utilizing the insights of this unique gathering of experts, the book offers recommendations to improve the current state of knowledge and direct future research in fields ranging from chemistry and theoretical physics to climate modeling and remote satellite sensing. View full abstract»