Open Issues

Whether radiative flux perturbations exhibit the same additivity as practiced for traditional radiative forcings remains unknown. The relationships between flux perturbations and traditional forcings for greenhouse gases and aerosols need to be quantified across the ensemble of climate models used for IPCC. To calculate forcing (as precisely defined), GCM simulations using a double-call to the radiation scheme need to run typically for only one year in order to sample the full seasonal cycle....

Abstract

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...

B

0.2 0.4 0.6 0.8 1.0 1.2 Tunnel Reference LWC (g m-3) 0.2 0.4 0.6 0.8 1.0 1.2 Tunnel Reference LWC (g m-3) 67 ms-1 position 1 100 ms-1 position 1 67 ms-1 position 2 100 ms-1 position 2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 67 ms-1 position 1 100 ms-1 position 1 67 ms-1 position 2 100 ms-1 position 2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 o 100 ms-1 position 1 x 67 ms-1 position 2 Figure 4.6 PMS King hot-wire probe (a) comparisons to tunnel reference LWCs for low median volume diameter, MVD,...

Aerosol Impacts on Mixedphase Clouds

GCM studies suggest that if, in addition to mineral dust, hydrophilic black carbon particles are assumed to act as ice nuclei at temperatures between 0 and -35 C, then increases in aerosol number concentration from preindus-trial to present times may have resulted in greater glaciation of supercooled stratiform clouds and an increase in the amount of precipitation via the ice phase. This process could decrease the global mean cloud cover, leading to enhanced absorption of solar radiation....

Cloud Feedbacks in a Changing Climate

Cloud and radiative observations are only available for a short period (at best for about 25 years, more generally for just a few years), and no climate variation occurring at this timescale may be considered as an analog of long-term climate change. Until long time series (three decades or more) of cloud and radiation data become available, it is hopeless to assess directly the response of clouds to global climate changes using observations and to compare it with model simulations. This is...

Approaches for Better Use of Observations

Analysis of Time History in Satellite Retrievals Observations and analyses from a combination of various satellite instruments (e.g., from the A-Train constellation) provide a comprehensive dataset for studying global cloud and aerosol properties, their radiative effects, and their interrelationship. It is critical to determine whether the correlations imply causation (e.g., whether correlations between cloud albedo and aerosol optical depth from adjacent clear-sky regions constitute evidence...

Clouds in the Perturbed Climate System

Charlson Clouds in the Perturbed Climate System The Ernst Strungmann Forum is made possible through the generous support of the Ernst Strungmann Foundation, inaugurated by Dr. Andreas and Dr. Thomas Strungmann. Additional support for this Forum was received from the Deutsche Forschungsgemeinschaft (German Science Foundation).

Conclusions and Outlook

Clouds play a crucial role in determining Earth's energy balance. This must be accurately represented in climate models, if the models are to be used with confidence to project future climate change. Much of the variation in climate sensitivity observed in current climate models can be attributed to differences in the treatment of clouds, as evidenced by model-to-model variation in cloud feedback. Microphysical processes are now recognized to exert strong influences on cloud dynamics and that...

Are There Observational Proxies in the Presentday Climate for Future Cloud Perturbations

Climate sensitivity, defined as the equilibrium change in global mean temperature in response to a doubling in the atmospheric CO2 concentration, is still a very uncertain quantity (e.g., Randall et al. 2007). The primary reason for the spread in climate sensitivities, as simulated by different global climate models, is the difference in the representation of cloud processes and cloud-climate feedbacks (Soden and Held 2006 Dufresne and Bony 2008). Cloud processes determine the amount and...

Essential Observations

The Oklahoma Atmospheric Radiation Measurement (ARM) site, which has been operating for 15 years, provides an example of detailed ground-based cloud process observations over long time series. Although these measurements contribute greatly to the improvement of process understanding and the development of process representations in models, studies that provide reliable information about cloud-climate feedbacks derived directly from these data are still lacking. One reason might be that the...

Current Insitu Instrumentation and Its Limitations

The airborne platform used for in-situ measurements must be suitable. For example, in some applications a tethered balloon might be adequate and economical, whereas in others a long-range aircraft is essential. For campaigns requiring only a few instruments, a light aircraft or an unmanned airborne vehicle (UAV) might be the best option in other projects, a large capacity aircraft is necessary to carry all of the technical gear as well as the many operators and scientists involved. Some...

Cloud Feedbacks in Climate Models and Their Influence on Climate Sensitivity

The energy balance model of Earth's climate system is useful to assess the influences of particular processes on global mean surface temperature (T) and to compare these influences across different climate models. Within this energy balance framework, the time-dependent change in heat content of the climate system, AQ, is related to radiative forcing, AF, and the change in T, AT, as where A is the equilibrium climate sensitivity, as is readily seen by considering a system in a new equilibrium...

Detailed Planning

Most surface remote-sensing systems sample only two-dimensional slices of atmosphere, and instrumented aircraft samples are limited to long lines of data of a few mm2 cross section each. Their use, over the short duration of an intensive observation period, must therefore be highly optimized compromises must be made on where to locate surface systems and which flight tracks to allocate to each aircraft. Thus, the preparation of a field experiment involves preliminary studies of the expected...

Conclusion

With the arrival of new and powerful observational datasets, particularly the new space-based active radar and lidar sensors in the A-Train, we are entering a new era for the evaluation of clouds in large-scale models. Observations with active sensors have already demonstrated that NWP models have skill in representing clouds in the right place and the right time, and have also identified some shortcomings. It will soon be possible to assess key aspects of the simulation of clouds, such as the...

Data Sources

To develop a climatology for monthly global fields of mid-visible aerosol properties of AOD, ro0, and AnP, input from three different sources must be considered (a) remote sensing from space, (b) remote sensing from the ground, and (c) global modeling. Strengths and weaknesses of these data sources are briefly reviewed to justify the selected choice a synergetic approach combining the accuracy of ground-based remote-sensing statistics with the completeness from global modeling.

Introduction

Clouds are an extremely important element of Earth's climate system. They are highly reflective in the solar spectrum, yet strongly absorbing in the thermal infrared consequently, they produce a large impact on Earth's radiation budget. This impact, termed cloud radiative forcing (CRF), has been quantified through satellite observations globally, on average, clouds decrease the absorption of solar radiation by about 50 W m-2 (shortwave CRF) and decrease the upwelling thermal infrared radiation...

Lagrangian Experiments Linking Small and Large Scales

The temporal and spatial scales over which cloud fi elds evolve and interact with the general circulation in which they are embedded are typically days and thousands of kilometers. Scales involved in the general circulation exceed those accessible to single aircraft flights and limit seriously our ability to understand the two-way interactions between microscale processes and the general circulation. This need to observe the evolution of meteorological fi elds and the processes that influence...

How Are Clouds Responding to Anthropogenic Aerosols

A second key question is how clouds and CRF have been altered in response to changes in the concentration and composition of anthropogenic aerosol particles. Without a dynamic feedback on cloud macrophysics, an increase in the number of hygroscopic particles will produce a larger number of smaller cloud droplets and hence increase cloud albedo. Changes in cloud particle size, phase, and number concentration have furthermore been hypothesized to have a substantial influence on cloud...

Evaluating Cloud Properties Simulated by Largescale Models with Satellite Data

The evaluation of the clouds in GCMs has long been hampered by the lack of global observations of the vertical structure of clouds. The situation is now radically changing with the arrival of new observations from the A-Train constellation of satellites, including the spaceborne radar (CloudSat) and lidar (CALIOP CALIPSO) instruments. The observational definition and detection of clouds, however, depends strongly on the type of measurements and sensitivity of sensors, as well as the vertical...

References

E. Schwartz et al. 2003. Climate forcing by aerosols A hazy picture. Science 300 1103-1104. Andrews, T., and P. M. Forster. 2008. CO2 forcing induces semi-direct effects with consequences for climate feedback interpretations. Geophys. Res. Lett. 35 L04802. Bodas-Salcedo, A., M. J. Webb, M. E. Brooks et al. 2008. Evaluation of cloud systems in the Met Office global forecast model using CloudSat data. Geophy. Res. Abst. 10 EGU2008-A-10141. Bony, S., J.-L....

Limits to Existing Observationbased Studies

Despite many observational campaigns, no large body of observational evidence exists to provide a positive correlation between cloud albedo and aerosol perturbations on a large scale. Although it has been established that an increase in aerosol concentration results usually in an increase of CDNC, this does not necessarily lead to an increase in cloud albedo, since the LWP of the cloud also changes. Although the LWP may be responding to aerosol-induced changes, it may be determined to a first...

General Conclusions

In terms of making cloud in-situ measurements, there have been many advances in recent years. However, it is difficult to provide suitable measurements for use in climate models and some of the reasons are summarized below Cloud microphysical measurements are collected today without commonly accepted standards for calibration and data analysis. Users are often unaware of the limitations of the calibrations, if any were done, and the problems associated with the data analysis. Table 4.5...

Name Index

S. 3, 127, 134, 182-185, 239, 326, 355, 453, 458, 461, 501 Ackerman, B. 375 Ackerman, S. A. 482 Adams, P. J. 259 Aitken, J. 320 Akiyama, T. 117, 119, 122 Albrecht, B. 3, 108, 112, 134, 181-184, 190, 278, 321, 420, 453, 500, 545 Allan, J. D. 66 Allan, R. P. 32 Alpert, P. 25, 377, 378, 395 Anderson, J. G. 144, 475 Anderson, T. L. 2, 5, 7, 41, 68, 127, 287, 288, 440, 547, 564 Andreae, M. O. 3, 59, 61-64, 127, 128, 296, 302, 312, 371, 409 Andrews, T. 570 Ansmann, A. 94, 249, 518...

Presentday Status

Over the last decade, atmospheric research began to concentrate on aerosols, because they were identified as a key regulator of atmospheric change, including clouds. From a modeling perspective, increased computing capacities permitted better characterization of aerosols in global models. Elaborate aerosol submodules now process emissions from different aerosol components (e.g., sulfate, organic matter, black carbon, dust, and sea salt). Particle size is resolved into several size classes at...

Relevance between the Changes in Moisture Transport and Cirrus

Centralized deep convection supplies more moisture to the tropopause under the higher SST condition, and high thin clouds, which had invisible optical thickness, become thicker and visible (Figure 20.1). Moisture transport increases in the tropics, despite the decreasing vertical velocity over almost all latitudes. The rise in moisture transport through deep convection increases the detainment of moisture at the top of the convection until anvils form. This contributes to the increase in the...

Summary

Various studies of observational data have established statistical relationships between cloud microphysical properties and aerosol concentrations consistent with an assumed aerosol indirect effect. However, covariance of aerosol concentrations and meteorological cloud-controlling factors leads to a variety of responses beyond the enhancement of CDNC at increased aerosol concentrations. Time history in 3-D data is needed to overcome some of the issues. To interpret correlations found in...

Representation of Aerosols in Globalscale Chemical Transport Models and Global Climate Models

Although there are many similarities between treatment of aerosol processes in chemical transport models (CTMs) and global climate models, it is useful to distinguish the two modeling approaches. Global climate models simulate their own meteorology and couple aerosol cycles with clouds, precipitation, and radiation transfer, thereby allowing the projection of future climate under different emissions scenarios. Because climate modeling emphasizes long-term simulation of climate, treatment of...

Surface Radiation Measurements

The Global Energy Balance Archive (GEBA) contains measurements of downwelling solar radiation at the surface that were made at various locations around the world during the past several decades, primarily in North America and Eurasia and only over land (Gilgen and Ohmura 1999). Not all stations are trustworthy many report unrealistically large interannual and decadal variations in solar radiation flux, particularly in the United States and in developing countries. Furthermore, sensor...

Shortcomings of IPCCs Radiative Forcing Concept

The forcing summary graph put forth by the IPCC (2007) includes only the direct forcing and Twomey aerosol indirect effect. It does not reflect the full magnitude or uncertainty of cloud-mediated aerosol forcing. However, some effects (e.g., the aerosol cloud lifetime effect,) for which confidence in understanding is limited do not necessarily have the smallest magnitude, and therefore they cannot be omitted. The inclusion of such uncertain effects emphasizes the imperative need to narrow the...

Ways to Reduce Critical Uncertainties in the Prediction of Clouds in a Changing Climate

Comparison of climate simulations with observations reveals a large number of systematic biases in current models. Faced with the long-standing biases of climate models and uncertainties in climate change projections, the optimal way to improve models is still open to question. The increase of the (horizontal and vertical) resolution of large-scale models is often cited as a way to improve model simulations. The experience of many modeling centers indicates that increasing the resolution does...

Satellite Cloud Measurements

International Satellite Cloud Climatology Project The most commonly used cloud dataset in climate studies is the ISCCP (Rossow and Schiffer 1999). Originally intended to provide a cloud climatology and data for studying cloud processes at synoptic scales, ISCCP uses narrowband radiance data obtained from weather satellites to retrieve cloud fraction and other cloud properties in 280 km grid boxes around the globe every three hours. A radiative transfer model has been applied to these cloud data...

Context of this Forum The Urgency of Current Demands by the Policy Community on the Scientific Community and the Need

Given the various forecasts of impending climatic catastrophes, there can be little doubt that the issue of global warming has captured world attention. Such forecasts range from modest increases of global mean temperature to severe climatic shifts, flooding coastlines, crop failures, and beyond. Yet the term global warming is also a source of some ambiguity insofar as the verb to warm has both transitive and intransitive meanings. There is no scientific doubt that the increase in manmade...

Status Ten Years

Ten years ago, aerosols were treated quite simplistically in global models. Tro-pospheric aerosols were characterized primarily by coarse size (r > 0.5 pm) dust, as a natural component, and by accumulation mode size (0.05 > r > 0.5 pm) (non-absorbing) sulfate, as the anthropogenic component (where r is the particle radius). Even by ignoring organic aerosol sources, it was not easy to convert industrial sulfur emissions via chemical and transport processes to somewhat realistic sulfate...

Limitations of Observational Proxies

Since cloud and radiation observations are available on a global scale for only a climatically short time period (at best, ca. 25 years), our ability to observe cloud-climate feedbacks directly is limited. Thus the question arises as to whether there might be some observational proxies in the present-day climate for future cloud perturbations. The dynamic and thermodynamic forcings of clouds that occur on shorter timescales do not correspond directly to the dynamic and thermodynamic changes...

Spectral Extension

The three particle properties of interest in radiative transfer simulations are AOD, ro0, and g, which is estimated from AnP. All three properties are spectrally dependent. Thus, for solar and infrared broadband simulations, which are required in climate applications, the merged monthly global fields of the new climatology for the mid-visible spectral region must be extended to other wavelengths of the solar and infrared spectrum. This spectral extension involves the following strategy First,...

Precipitation Formation in Warm Clouds

The influences of precipitation and drizzle processes on cloud lifetime, cloud water content, and cloud radiative properties discussed above cannot be simulated well in current GCM cloud parameterization schemes. For example, the autoconversion rate, which is the rate at which cloud droplets collide and coalesce with each other to form precipitation size drops, is a nonlinear function of the total water condensate. Thus, the mean LWC from a GCM model grid box is essentially meaningless for the...

The Radiative Flux Perturbation as an Alternative

Standard radiative forcing is computed as the change in radiation fluxes at the tropopause attibutable to an external perturbation, with tropospheric temperature and humidity profiles held fixed, but allowing for fast (order of months) temperature adjustment in the stratosphere. In contrast, radiative flux perturbation allows for rapidly responding tropospheric meteorological fields to adjust as well. The radiative flux perturbation concept does amount to fixed-SST forcing (Shine et al. 2003...

Weather and Climate Engineering

Colorado State University, Department of Atmospheric Science, Fort Collins, CO, U.S.A 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...

Cloud Droplet Formation and Khler Theory

Cobalt Sulfate Van Hoff

In the Earth's atmosphere, cloud droplets do not generally form by homogeneous nucleation of supersaturated water vapor (i.e., condensation of water molecules from the gas phase in the absence of a preexisting condensation nucleus). This would require the initial formation of droplet embryos (clusters of water molecules) with a very small radius of curvature. Because of surface tension, however, the equilibrium vapor pressure over such a strongly curved surface is much greater than over a flat...

Closure Experiments

Closure experiments aim to measure the consistency of the atmospheric state parameters with respect to models of the underlying physical processes (Ogren 1995 Quinn et al. 1996). The methodology consists of measuring input parameters to initialize a model and derive output parameters concomitantly, the control parameters are measured for comparison with model predictions. To illustrate this, we will describe the observational strategy of the ACE-2 Cloudy-Column experiment (Brenguier, Chuang et...

Abbreviations

ACE Aerosol Characterization Experiment 1, southern hemisphere 2, North Atlantic Regional Asia, Asian Pacific Regional ACES Artificial Cloud Experimental System ACTOS Airborne Cloud Turbulence Observation System AeroCom Aerosol Comparisons between observations and models AERONET AErosol RObotic NETwork AGCM Atmospheric General Circulation Model AIDA Aerosol Interactions and Dynamics in the Atmosphere Air NEPM National Environmental Protection Measure for ambient Air quality AIRS Atmospheric...

Precipitation from Lowlevel Clouds

Low-level clouds are typically non-precipitating, or just lightly precipitating, and are thus unlikely to cause hazardous weather events. Nevertheless, precipitation from low-level clouds is important meteorologically and climatologi-cally for the following reasons Precipitation from stratocumulus has a direct impact on the dynamic maintenance of its parent cloud (Nicholls 1984). In terms of the so-called second indirect effect of aerosols, an increase in aerosol concentration is hypothesized...

The Emst Strngmann Forum

Julia Lupp

The Ernst Strungmann Forum is founded on the tenets of scientific independence and the inquisitive nature of the human mind. Through its innovative communication process, the Ernst Strungmann Forum facilitates the continual expansion of knowledge by providing a creative environment within which experts can scrutinize high-priority issues from multiple vantage points. The themes that are selected transcend classic disciplinary boundaries they address topics that are problem-oriented and of...

Major Ground Networks

AERONET (AErosol RObotic NETwork http aeronet.gsfc.nasa.gov ) is a federation of ground-based remote-sensing aerosol networks that was established in the mid-1990s by NASA and CNRS it was expanded thereafter through the efforts of national agencies, universities, and individuals (Holben et al. 1998). Using standards for instruments, calibration techniques, processing, and data distribution, individual sites are connected into a network. The basic instrument is a CIMEL sun sky photometer. Data...

CCN Activity of Aerosol Particles

The ability of an aerosol particle to act as a CCN (CCN activity) that is, its ability to induce water-vapor condensation and cloud droplet formation under a given set of conditions depends primarily on (a) water-vapor supersaturation, (b) dry particle size, and (c) hygroscopicity (soluble particles) or wettability (insoluble particles) (Seinfeld and Pandis 1998 Pruppacher and Klett 2000 McFiggans et al. 2006 Andreae and Rosenfeld 2008). In science and engineering, hygroscopicity is generally...

Laboratory Cloud Simulation

Frank Stratmann1, Ottmar M hler2, Raymond Shaw1'3, and Heike Wex1 1Leibniz Institute for Tropospheric Research, Leipzig, Germany 2Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe, Karlsruhe, Germany 3Michigan Technological University, Houghton, MI, U.S.A. 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,...

Diffusion Turbulence

Air-sea gas exchange Entrainment Shallow cumulus Teleconnections Stratocumulus Convective systems weeks Figure 12.1 Depiction of the continuum of relevant cloud-related processes across the full range of spatiotemporal scales, showing the underlying categorical behavior (gray text) from which processes emerge. scales (i.e., the microscale), or through global warming attributable to additional greenhouse gases. These perturbed clouds include possibly all tropospheric clouds, including contrails...

Introduction Clouds and Earths Radiation Budget

Clouds greatly impact the Earth's radiation budget (ERB) because they reflect solar or shortwave (SW) radiation back to space and restrict the emission of thermal or longwave (LW) radiation to space. One conventional measure of this radiative effect is cloud radiative forcing (CRF) that is, the difference between actual radiation flux and what it would be if clouds were absent (Ramanathan et al. 1989). A more proper term might be cloud radiative effect, since forcing is usually reserved for...

Current Understanding and Quantification of the Effects of Clouds in the Changing Climate System and Strategies to

Anthropogenic aerosols are thought to exert a significant indirect radiative forcing because they act as CCN in warm cloud formation and as ice nuclei in cold cloud-forming processes. Haywood et al. (Chapter 19) address this issue by comparing 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. They highlight problems in assessing the effect of anthropogenic aerosols upon clouds...

AOD 550nm

Figure 3.3 Mid-visible aerosol optical depth (AOD) at 550 nm wavelength. Comparison between annual maps from global modeling (M), the merged product (X), and sun photometer gridded samples (A). Deviations to model simulations suggest AOD underestimates in global modeling. Figure 3.3 Mid-visible aerosol optical depth (AOD) at 550 nm wavelength. Comparison between annual maps from global modeling (M), the merged product (X), and sun photometer gridded samples (A). Deviations to model simulations...

Importance of Small Cloud and ERB Changes and the Difficulties in Measuring Them

Global climate is very sensitive to even slight changes in clouds and radiation, hence requiring very precise measurements of variations in these parameters. For example, an instantaneous doubling of CO2 (100 increase) would produce about a 4 W m-2 reduction in outgoing radiation flux, which is less than 2 of the 235 W m-2 in global mean outgoing radiation. Since the trend in CO2 concentration over the span of a few decades is much smaller than a doubling, the trend in ERB is also...