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PART II
DISCIPLINARY ASSESSMENTS



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Page 61 PART II DISCIPLINARY ASSESSMENTS

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Page 63 1 Atmospheric Physics Research Entering the Twenty-First Century1 Summary Atmospheric physics seeks to explain atmospheric phenomena that occur on a variety of temporal and spatial scales in terms of physical principles. Areas included in atmospheric physics are atmospheric radiation, aerosol physics, the physics of clouds, atmospheric electricity, the physics of the atmospheric boundary layer, and small-scale atmospheric dynamics. Major Scientific Goals and Challenges In each of these areas, we generally have a useful understanding of the physical principles involved at the most fundamental level. However, understanding these physical principles alone does not ensure an adequate understanding of observed atmospheric phenomena because the various realizations of these phenomena are inherently complex and result from complicated interactions among physical processes. Further, these interactions occur across a great range of time and space 1 Report of the Ad Hoc Group on Atmospheric Physics: W.A. Cooper (Chair), National Center for Atmospheric Research; T. Ackerman, Pennsylvania State University; C. Bretherton, University of Washington; S. Cox, Colorado State University; J. Dye, National Center for Atmospheric Research; E. Gossard, Environmental Technology Laboratory, National Oceanic and Atmospheric Administration; D. Lenschow, National Center for Atmospheric Research, V. Ramaswamy, Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration; D. Raymond. New Mexico Institute of Mining and Technology; E. Williams, Massachusetts Institute of Technology.

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Page 64 scales, and many small-scale processes have significant influence on those occurring on larger scales. An understanding must be developed of the collective influence of individual physical processes on larger spatial-scale and longer time-scale phenomena, which are referred to here as organizing principles. Key Components of the Scientific Strategy The most critical components of a program to address the scientific issues and challenges are the following: • To develop and verify an ability to predict the influences of small-scale, atmospheric physical processes, such as turbulence, on large-scale atmospheric phenomena such as thunderstorms. In many cases, large-scale atmospheric phenomena arise from the collective effects of an ensemble of interactions that occur on much smaller spatial and much shorter temporal scales. Two central impediments to more accurate simulation and prediction of weather and climate, which are comprised of these large-scale events, are the physical understanding of the smaller-scale events and the inability to include them explicitly in models due to computational constraints. The solution lies in developing organizing principles to relate small-scale events to larger-scale phenomena. Progress is being made in this area due to improvements in modeling and computational power, expanded observations from the ground and in situ, and increased communication and collaboration among research scientists. Considerable attention is being directed toward the use of field observations to verify model results. In the atmospheric sciences, verification is essentially the process of establishing the accuracy of a theory to within some error estimate where the errors include those of both the computational version of the theory (the model) and the observing system. Error estimation is itself often an imprecise quantification in the atmospheric sciences because of a lack of understanding of the propagation of error in nonlinear systems and the inability to create repeatable atmospheric experiments. • To develop a quantitative description of the processes and interactions that determine the observed distributions of water substance in the atmosphere. The importance of water, whether vapor, liquid, or solid, in climate and weather processes is self-evident, but there are weaknesses in the current ability to specify the atmospheric water cycle. Among these are poor characterization of upper-troposphere water vapor, uncertainties in surface fluxes and precipitation efficiency, poor representation of ensemble effects of cumulus convection on the transport of water and the characterization of precipitation over oceans, and the

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Page 65 absence of a comprehensive understanding of the links between the atmospheric water cycle and other components of the hydrological cycle. Fortunately, recent improvements appear able to support a comprehensive new approach to these problems. These improvements include, or will include, in situ and remote sensing methods, characterization of precipitation over the oceans, new modeling capability, and comprehensive international programs to study the hydrological cycle at regional scales. • To improve the capability of making critical measurements in support of studies in atmospheric physics. Many areas of atmospheric physics instrumentation lag behind what is technically feasible. Platforms suited to atmospheric physics studies are among the most needed facilities. These include new observing satellites and high-altitude, long flight time aircraft. Measurements of water substance, radiation, and trace gases are of maximum importance. Initiatives to Support the Strategies Implementation of these research strategies requires the following disciplinary initiatives: • Atmospheric Radiation: to understand the interactions between radiation and components of the hydrologic cycle. • Cloud Physics: to understand water substance interactions and processes, for example, initial droplet formation, cloud chemistry, and the interaction between radiation and clouds, such as is needed in weather and climate research. • Atmospheric Electricity: to enable reduction of fatalities and economic losses due to lightning discharges, and to determine the usefulness of electrical activity as an observational tool for monitoring severe weather and more typical weather as well as climate. • Boundary Layer Meteorology: to understand and make use of the knowledge of boundary layer effects on weather, climate, and human activity. Expected Benefits and Contributions to the National Well-Being Many components of the research program recommended here address sources of uncertainty in climate prediction. In addition to expediting the reduction of this uncertainty, other benefits will accrue, such as the improved ability to predict regional and local weather, and will aid in the development of polices to mitigate anthropogenic impacts on the environment and the management of the nation's natural resources.

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Page 66 Recommended Atmospheric Physics Research Recommended Atmospheric Radiation Research • Develop and/or test the ability of theory and models of radiation transfer to (1) understand water vapor continuum absorption and in-cloud solar absorption; (2) develop the theory of scattering by nonspherical, including irregular, particles; and (3) understand radiative transfer in cloudy atmospheres. • Develop observational studies and analyses to (1) better utilize satellite and other remote sensor data. (2) represent the four-dimensional distribution of water vapor, and (3) quantify the direct radiative forcing of climate by trace gases and aerosols. Recommended Cloud Physics Research • Develop the ability to predict the extent, lifetimes, and microphysical and radiative properties of stratocumulus and cirrus clouds to (1) resolve stratocumulus-related issues as in atmosphere-ocean coupling, (2) resolve the aerosol-stratocumulus albedo feedback effect, and (3) resolve the role of cirrus clouds in global warming or cooling. • Improve models of atmospheric radiative transfer; test these models using observations to verify radiative transfer models under different atmospheric conditions and improve parameterizations of these effects in general circulation models (GCMs). • Increase the attention paid to clouds and their interaction with radiation to (1) permit estimation of the coverage and radiative properties of clouds and (2) improve the theoretical understanding of liquid and solid precipitation formation. • Conduct critical tests of precipitation mechanism theory with increased attention to dynamic consequences for (1) testing models of warm-rain and ice-phase precipitation processes, (2) evaluating the effects of precipitation production and evaporation on the dynamical evolution of storms, and (3) evaluating the importance of precipitation processes on advertent and inadvertent weather modification. • Develop the ability to predict size distribution of hydrometeors and aerosol populations to (1) determine their joint influence on the Earth's radiation balance, (2) understand their role in sustaining heterogeneous atmospheric chemical reactions and precipitation formation, and (3) evaluate the influences of microphysical processes on cloud models and the influence of clouds on climate models. • Investigate the interactions among aerosols, trace chemical species, and clouds; develop and improve the characterization of atmospheric aerosols to (1) characterize cloud condensation nuclei (CCN) activity in chemical global models and (2) develop representations of the radiative effects of aerosols.

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Page 67 Recommended Atmospheric Electricity Research • Determine mechanisms responsible for charge generation and separation in clouds to understand cloud formation mechanisms and elucidate the fundamental physics of lightning. • Determine the nature and sources of middle-atmospheric discharges to (1) increase knowledge of these recently discovered phenomena and their possible association with severe weather and (2) explore their effects on radio propagation and atmospheric chemistry. • Quantify the production of oxides of nitrogen (NOx) by lightning to better understand upper-troposphere ozone production or loss. • Investigate the possibility that the global electrical circuit and global and regional lightning frequency might be an indicator of climate change. Recommended Boundary Layer Meteorology Research • Understand the structure of cloudy boundary layers to enable characterization of the effects of boundary layer clouds on climate. • Improve understanding of turbulence and entrainment in the boundary layer to aid in the parameterizations of boundary layers in numerical models and to improve pollution modeling. • Improve measurements of exchange of water, heat, and trace constituents at the Earth's surface. This is fundamental information for use in most aspects of tropospheric functioning. • Understand model interactions of the planetary boundary layer, surface characteristics, and clouds for use in analytical and predictive models of daily temperature cycle, hydrologic studies, and pollution prediction. • Exploit new boundary layer, remote sensors for obtaining a more complete description of three-dimensional boundary layer flow to use in direct comparisons with boundary layer simulations. Recommended Small-Scale Dynamics Research • Develop better representations or parameterizations of physical processes occurring on scales smaller than the grid scale in climate models to improve GCM parameterizations. • Represent the effects of moist convection in large-scale models to improve models of potentially destructive mesoscale convective thunderstorm supercells. • Improve the dynamical representation of small-scale features in large-scale models to permit better understanding of local severe weather, large-amplitude gravity waves, clear air turbulence, and stratospheric-tropospheric exchange.

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Page 68 Improvement of Capabilities • Studies in atmospheric physics require new measurement systems such as Doppler infrared radar, polarized lidar, millimeter radar, microwave radiometers, Doppler wind profilers, and polarimetric Doppler radar. • Develop new analysis techniques such as (1) multispectral algorithms to infer optical depth, cloud liquid water content, and trace gas concentrations from infrared spectrometers, and (2) techniques from other areas that include pattern recognition, intelligent systems, artificial intelligence, chaos theory, and computer visualization. Introduction Mission Atmospheric physics seeks to explain, in terms of basic physical principles, atmospheric phenomena that occur on a variety of temporal and spatial scales. Thus, atmospheric physics could be interpreted broadly as including all atmospheric phenomena. The field of atmospheric science, however, has traditionally separated large-scale dynamics (meso-, synoptic, and planetary scales) and atmospheric chemistry from atmospheric physics, and this tradition is followed here. Therefore, unavoidable overlap with material in other parts of this document is to be expected. Areas emphasized here include atmospheric radiation, aerosol physics, the physics of clouds, atmospheric electricity, processes in the planetary boundary layer, and small-scale atmospheric dynamics. In each of these areas, there is generally a useful understanding of the physical principles involved at the most fundamental level, including the laws of motion or electromagnetics. However, understanding these physical principles alone does not ensure an adequate understanding of observed atmospheric phenomena because the various realizations of these phenomena are inherently complex and result from complicated interactions among physical processes. Further, these interactions occur across a great range of time and space scales and many small-scale processes have significant collective influence on large-scale processes. For example, consider the influence that the collective interactions between photons and cloud drops have on cloud life cycle, the influence that cloud life cycle has on synoptic development, and the influence that synoptic development has on climate. Since no large-scale model can hope to include all these processes ab initio, there is a need to develop an understanding of the collective influence of individual physical processes on larger spatial-scale and longer time-scale phenomena, an intellectual process that is called here ''the development of organizing principles.'' Current research in the atmospheric sciences is primarily concerned with finding ways to represent, understand, organize, and predict the results of these complex interactive phenomena.

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Page 69 Major Research Themes and Past Accomplishments This section reviews some central research themes and recent results in atmospheric physics and identifies as challenges some key problems that impede progress. This review indicates that there have been some notable shifts in the field in the recent past. Most of them are related to the increased attention now being directed to climate studies. For example, studies of atmospheric radiation have received increased attention as their importance to atmospheric science has become more evident. Growing attention to climate modeling has increased the need to understand radiative transfer in the atmosphere, and recent results demonstrate the influences of radiation on mesoscale weather systems and weather forecasting. The scope of cloud physics has also broadened considerably, with increased attention directed to the roles of clouds in climate, mesoscale meteorology, and atmospheric chemistry. Because of the need to represent small-scale processes in weather and climate models, there has been increased attention to the problem of parameterization, and this is now an active area of research in cloud physics, atmospheric radiation, boundary layer research, and cloud dynamics. Many of the problems being addressed in small-scale dynamics relate to the interactions between this scale and larger scales, and may also involve significant parameterization efforts. Note that "parameterization" is defined here as including two conceptually different processes. It includes using both organizing principles and empirical relationships as a way of incorporating subgrid-scale processes in models. The former implies that the underlying physics of some process is reasonably well understood but cannot be included due to computational constraints so the physics is included via statistical methods or aggregate models. The latter implies that we have a less than adequate physical understanding of the process in question so the process is included by using some appropriate set of observations. Thus, in this context, it is understood that empirical processes include some amount of curve fitting and extrapolation. In practice, most parameterizations include a blend of both of these ideas, which is the view adopted in this document. Additionally, the simultaneous expansion of computational power and instrument performance in the past two decades has led to an increased focus on the relationship between model output and observational data sets, a process that is referred to as model validation or verification. Validation carries the connotation of establishing the legitimacy of some theory, usually by empirical or logical means. Verification has the connotation of testing the accuracy of some theory, usually by empirical means. Clearly, these concepts are closely allied. Unfortunately, atmospheric phenomena are generally so complex and span so many spatial and temporal scales that logically rigorous validation is impossible; we simply cannot compute or measure all of the relevant quantities. Verification, as the more limited concept of testing the accuracy of a theory—particularly if it includes testing only some aspects of the theory—is conceptually

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Page 70 more tractable. Despite this ambiguity in a precise definition of these terms, however, the concept of model verification by observation is deeply ingrained in current atmospheric physics and has been an important intellectual driver of research in recent years. It is likely to continue to be a strong driver. In this document, the term verification is used to denote the process of establishing the accuracy of a theory within some error estimate. It should be noted further that error estimation is itself often an imprecise quantification in atmospheric science because of a lack of understanding of the propagation of error in nonlinear systems and the inability to create repeatable experiments in the atmosphere. Studies in atmospheric physics are improving our understanding of radiative transfer, precipitation formation, transport, and other fundamental processes in the atmosphere. Beyond this, the principal benefit to society of studies in atmospheric physics is an improved ability to predict the effects of weather and climate. The present extent of human activity has demonstrable effects on our weather and climate, so it has become imperative to understand these effects and determine their consequences. This adds urgency to the current scientific thrust to understand these phenomena. Because the consequences of weather and climate are so important to us individually and collectively, more reliable predictions of weather and climate would be of great societal and economic value. Perspective for the Future Possible components of a program to address the challenges listed below, and to take advantage of the opportunities presented by recent advances, are discussed in the section that follows, which concludes by focusing on three particularly important aspects of the research program recommended—aspects that should be pursued with highest priority: 1. Develop and verify an ability to predict the influences of small-scale physical processes on large-scale atmospheric phenomena. 2. Develop a quantitative description of the processes and interactions that determine the observed distributions of water substance in the atmosphere. 3. Improve capabilities to make critical measurements in support of studies in atmospheric physics. These imperatives span the needs of all areas in atmospheric physics. Accomplishing the first two would be a significant scientific achievement that would also lead to immediate improvement in our ability to predict weather and climate.

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Page 71 Scientific Challenges and Questions Atmospheric Radiation The discipline of atmospheric radiative transfer and remote sensing has matured greatly in the past decade, and its importance to atmospheric sciences has become increasingly evident. A fundamental interaction that must be understood if we are to develop realistic climate projections is the interactions between radiation and the components of the hydrologic cycle, particularly clouds. A number of studies have shown that radiative processes are important in the development and maintenance of convective systems and therefore have important consequences for weather forecasting and mesoscale meteorology. Satellite remote sensing is providing the meteorological community with a wealth of data on the state of the global atmosphere, while ground-based remote sensing is beginning to supply detailed, temporally continuous data that can be used to understand physical processes in the atmosphere. Despite the maturation of the discipline, radiative transfer and remote sensing continue to present significant research challenges. Foremost is the continued dichotomy between the modeling of cloud-radiation interactions at the scale of climate models and the understanding of these same processes at the observational and cloud-scale level. Merging of these efforts will be needed before valid parameterizations of radiative transfer in a cloudy atmosphere can be developed. Secondly, the resources devoted to the development of high-quality radiation instrumentation, particularly for in situ and ground-based use, have been minuscule until very recently. At the same time, operational satellite instrumentation has been allowed to expire without adequate replacement. As a result, the quality and quantity of radiation data needed to address current problems are simply not available in many cases. Thirdly, support for basic theoretical and computational research on more realistic cloud-radiation problems has been very limited. This has hampered our ability to understand, for example, the nature of three-dimensional radiative transfer, the scattering of radiation by nonspherical hydrometeors, or the recent and past indications of anomalous absorption by clouds. At present and in the near future, the following actions are likely to be important aspects of the study of atmospheric radiation. The challenge will be to accomplish them, or to make significant progress in these areas. Radiation Transfer Models and Observations The accuracy of fluxes and heating rates given by current models of radiative transfer must be verified by rigorous comparisons of computed quantities with observations at the top and bottom of the atmosphere, and with radiative divergence observations within the atmosphere, especially under cloudy conditions. Recent comparisons of terrestrial infrared model calculations with observations

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Page 96 chemistry via NO production or alteration of the mixing ratios of other species requires investigation. Likewise, investigation of the spectral characteristics of these discharges is needed. A better understanding of cloud electrification could lead to a much better ability to predict when and where clouds are likely to become electrified. One example of the potential economic benefit of better understanding of cloud electrification is in the launch of space vehicles, which is often postponed because of lightning, at the expense of about a million dollars per day. Lightning threats to aircraft also result in significant course deviations and costs for commercial airlines. Low-voltage devices are sensitive to low-level transients, and there is concern about the use of composite materials in commercial airplanes because of their susceptibility to lightning damage. Thus, better understanding of electrical phenomena in the atmosphere would have widespread benefits, not only through reduced hazards to humans but also through direct economic payoffs. Determine the Rate of Generation of NOx by Lightning The concentration of NOx (NO + NO2) is one of the major factors in determining whether there is a net in situ loss or production of ozone in the troposphere. Ample field and laboratory evidence shows that lightning is an important source of NOx, but the impact on a global scale remains uncertain` Current estimates of the global production rate of NOx by lightning range between 2 and 200 Tg N/yr. However, models have not accepted the higher estimates and usually restrict the source strength to 3 to 5 Tg N/yr. Even these lower estimates are comparable to, or larger than, source strengths in the middle and upper troposphere from stratospheric exchange and subsonic aircraft, and the upper estimates are comparable to anthropogenic sources in the boundary layer. Likewise, lightning is reported to produce other chemical species in lesser concentrations, and further studies are needed here as well. The newly recognized phenomena of middle-atmosphere discharges, discussed above, may also have an impact on the chemistry of the middle atmosphere. Determining the importance of lightning as a source of NOx from observations is difficult, because it requires not only measurements of NOx within or near storm systems but also characterization of the number and type of lightning flashes within a given area and, for some regions, discharges. It also requires knowledge of the transport and evolving chemistry of the species from the cloud to the regional and global scales. Although this may be a difficult problem, it is one that is tractable and provides an opportunity for real advance. Most of the technical and modeling capabilities are in place, and relatively minor improvements to others would be necessary. Chemistry instruments to measure the proper species, airborne platforms from which measurements can be made, Doppler radars for cloud motion measurements, lightning interferometers to identify location and type of lightning in conjunction with surface networks of electric

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Page 97 field sensors, and chemistry and dynamical models on different scales to examine the transport, transformation, and redistribution of the species are now available. One area in which measurement capability is lacking involves the global frequency and location of lightning, which has been discussed above. The main challenge of this problem is to combine and focus the necessary expertise in several interdisciplinary areas. Improve Observational Capabilities in Support of Studies in Atmospheric Physics Although some studies of the atmosphere use advanced, modern, state-of-the-art instrumentation to make reliable measurements, there are many glaring weaknesses in our overall ability to measure atmospheric characteristics in support of the studies outlined above. We should learn from the great amount of time wasted in trying to interpret inadequate measurements that it would be more efficient to devote a larger fraction of the community's resources to instrument development, characterization, and improvement. There are numerous examples where current measurement capabilities are inadequate but technical solutions are possible. Yet the atmospheric science community devotes only a very small fraction of its effort to instrument development. Universities are hampered by lack of the long-term funding commitments needed to conduct such research. Private companies do not fill the gap because of the small market. The National Center for Atmospheric Research and other national facilities are always pressed to support a high level of deployment, but have limited personnel and resources to devote to instrument development. The result is seriously inadequate attention to this problem by the atmospheric physics community, perhaps contributing to a community shift toward computer simulations in which technological progress is impressive and new frontiers open every few years. This problem strikes at the infrastructure of the science—the ability to test theoretical understanding with observations. A substantial increase in the overall level of effort devoted to instrument development is needed, especially in the National Science Foundation-supported community. This is a clear "imperative" and is discussed further below. Develop New Analysis Techniques for Meteorological Data There is an opportunity to make an initial assault on many of the research challenges identified in this report by using the large data sets previously collected by satellite sensors and in-process experiments. We must seek new ways to express and visualize these data. Examples include multispectral algorithms to infer optical depth, cloud liquid water content, and trace gas concentration from infrared spectrometers. A second opportunity stems from advances made in research applications of remote sensing from the ground. These research tools

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Page 98 could be configured to provide routine data on variables important to the parameterization and validation of various physical processes simulated in regional and climate models. Candidate instrumentation includes Doppler infrared lidar, polarized lidar, millimeter radar, microwave radiometers, infrared and solar interferometers, Doppler wind profilers, and 3 cm and 10 cm polarimetric Doppler radars. At the same time that we seek to optimize the utility of existing data and instrumentation, we must continue an ambitious program of development of new instrumentation if we are to meet the challenges of the next century. The advent of the Department of Energy's (DOE's) Atmospheric Radiation Measurement (ARM) program, the planned launching of the National Aeronautics and Space Administration's (NASA's) Earth Observing System (EOS) platform in 1998, and the new interest in unmanned aerospace vehicles (UAVs) hold the promise of a wealth of new data on radiation and the hydrologic cycle. Some of these data streams will be similar to those being acquired by current systems, but many will be substantially different. There is and will be a tremendous opportunity to bring new and different analysis techniques to bear on these data and associated atmospheric problems. This field can benefit by importing and adapting techniques from areas such as signal processing, pattern recognition, intelligent systems and artificial intelligence, chaos theory, and computer visualization. Such techniques may also have considerable applicability to analyses of model results. As the complexity and resolution of atmospheric models continue to grow, we have to improve our ability to extract meaningful information from them. A number of atmospheric scientists are working in these areas, but more support, particularly for innovative and perhaps risky research, is needed. Unfortunately, in times of static or diminishing support, it is precisely this type of research that is most likely not to be funded. Exploit Rapidly Increasing Computational Power As the price of computer cycles continues to fall, new modeling approaches can be used both to circumvent the need for parameterizations and to tackle new issues. In the next decade we may expect numerical modeling of time-dependent problems spanning three orders of magnitude in spatial scale to become feasible. For example, one fundamental problem for which this may provide important new understanding is the organization of deep tropical convection into clusters, superclusters, and synoptic-scale waves. It should be feasible to simulate the motions explicitly in individual cells within a synoptic-scale region of the tropical ocean or over the midwestern United States, avoiding the need for cumulus parameterization. Increasingly sophisticated techniques for placing areas of increased grid resolution adaptively within a flow, so as to cover only the convective regions and not the large expanses of stably stratified nonturbulent flow in between, are a promising way to improve the efficiency of such large computa-

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Page 99 tions. These calculations can contribute to fundamental insights into self-organizing mechanisms for moist convection and help in the development of convective parameterizations. As a second example, higher-resolution large eddy simulations of the inversion-capped cloudy atmospheric boundary layer should be able to properly resolve turbulent eddies near the entrainment interface and the surface layer, thereby providing a valuable tool for better understanding and parameterizing entrainment and surface-layer structure. Increased computer power also allows new and often more fundamental modeling approaches to be used. In radiation, the exploration of three-dimensional radiative transfer in realistic cloud fields and nonspherical scattering problems are becoming feasible and should be fostered. Large eddy simulation models, including explicit drop and aerosol size distributions and even simple chemistry, are a promising tool for exploring climatically important feedbacks such as those between clouds, aerosols, and radiation, as well as for understanding drizzle processes in boundary layer clouds. The effects of orography in climate and forecast models can be represented much more faithfully with increased spatial resolution, while parameterizations of the boundary layer benefit from increased vertical resolution. These are only a few examples of problems in which increased computer power can make a fundamental contribution. However, experience has taught us that increased computer power and model resolution are not substitutes for improvements in the underlying basic physics of the model. All of the preceding topics are opportunities that should be pursued for their scientific and societal value. However, some stand out because of their particular scientific importance or their likely societal impact. We suggest that the following three topics are of particular importance and should be special foci for research in the coming decade. Focus 1: Develop and Verify an Ability to Predict the Influences of Small-Scale Physical Processes on Large-Scale Atmospheric Phenomena The atmosphere is an interactive system with mutually dependent processes that span scales from microscopic to global. In most cases, the important influences among processes arise from the collective effects of an ensemble of interactions. A central impediment to our ability to predict weather and climate is the fact that even when the fundamental physical laws governing individual processes are understood, their collective influences on other phenomena cannot be predicted because of the complexity and number of interactions involved. This problem is particularly evident in climate, weather, and cloud models, where many small-scale processes have collective influences on the largest-scale phenomena. Examples are the interaction of solar radiation with cloud droplets, the effects of cumulus convection and turbulence on the transport of mass and momentum, chemical interactions that influence cloud microstructure and hence

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Page 100 radiative properties on a global scale, and the generation of lightning and the global electric field from many small transfers of electrical charge during collisions of hydrometeors. To model all of the processes involved in the atmosphere would require representing processes that occur with scale sizes from at least 0.0001 cm to 10,000 km, a range of 1013. Most modern computer models can only handle scales that cover a range of about 103, so we are far from circumventing this problem even with the most optimistic future increases in computing power. The solution lies in developing organizing principles that make it possible to understand the collective influence of these small-scale processes on larger-scale phenomena in the atmosphere. We need to develop a better understanding not only of the interactions between phenomena that occur with different scales but also of the interactions among processes historically studied by different groups of scientists. In many cases, the linkage between these processes is the essence of the outstanding problem. For example, cloud physicists understand how soluble particles in the atmosphere influence the sizes and number of cloud droplets, and chemists understand the fundamental chemical reactions involved in producing soluble particles. Nevertheless, this understanding has not yet led to a predictive capability for the number of soluble particles in the atmosphere and hence for cloud droplet concentrations and sizes, because these processes interact with each other and with radiation, chemical cycles in the atmosphere, global circulation patterns, and the hydrological cycle in ways that are beyond our current understanding. We suggest that increased effort directed to this class of problems will provide substantial dividends in the next decade. Our optimism is based on the following considerations: 1. In many cases the treatment of physical processes in models is inconsistent with current understanding of the phenomena. This has resulted partly from a failure to bridge the communications gap between those involved in fundamental studies and those involved in modeling the effects of these studies. An initial basis for improvement in the representation of processes such as turbulent transport, entrainment, boundary layer influences, and precipitation formation already exists. 2. Modeling and computing capabilities are improving rapidly. Model simulations of phenomena on small scales can be used to develop representations of these processes on larger scales, and the techniques and capabilities for this approach are now available but are only beginning to be exploited. 3. New observational capabilities are appearing or expected that will aid in these studies. New satellites for global monitoring will provide extensive new data sources; new weather observing networks are now available; improved atmospheric soundings may become available from new uses of commercial aircraft soundings and remote sensors; new long-range and high-altitude aircraft are

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Page 101 available to support this research; and new fixed sites for global monitoring that are being established or are in operation will provide crucial data. 4. There have been recent satisfying advances suggesting that we are on the threshold of significant progress toward improved understanding of these interactions. Success in predicting radiative transfer for clear skies, new understanding of the structure and lifetime of stratocumulus and cirrus clouds, new appreciation of the roles of aerosols in the atmosphere, the success of cloud-resolving models in representing ensemble influences of clouds, increasingly realistic models of the atmospheric boundary layer, and the development of techniques for inferring hydrometeor and cloud characteristics from satellite observations are all important steps that can support continued and enhanced focus on these and related problems. 5. There has been a significant shift in perspective among scientists capable of addressing these problems—for example, cloud physicists, who traditionally were oriented more toward microscale studies, have developed increasingly global perspectives. This shift has to continue, for example, through field programs that seek to make observations with characteristic scales similar to the grid scale in climate models. Planned programs are formulating better ways of studying the interactive and large-scale consequences of the phenomena being studied. A key consequence of an improved ability to predict the consequences of small-scale processes will be the development of improved representations or parameterizations for physical processes that occur on scales smaller than the grid scale in models. This will permit the effects of subgrid-scale processes to be represented in simulations of climate, weather, and clouds. Two complementary approaches to parameterization are needed. First, the fundamental interactive processes (e.g., linking radiative properties to hydrometeor spectra and hence to aerosols and cloud dynamics) require clarification. Second, existing knowledge and existing or new data can be used to develop representations that are consistent with current understanding. Such parameterizations, although sometimes unsatisfying because they fall short of representing true physical relationships, are still useful and necessary to support studies of the complex interactions in the atmosphere. Currently, a wide gulf exists between the parameterization approaches used in climate models and data, and process model studies associated with field programs. The parameterization issue cannot be resolved without bringing to bear the accumulated knowledge from process studies. At the same time, narrowly focused process studies that exhaustively study single cases cannot supply the information needed to improve model parameterizations. Until a much better representation of reality is introduced into parameterizations of processes such as the effects of clouds on radiation, there is little hope of improving confidence in climate and climatic change simulations. Emphasis should be placed on the acquisition of data sets in which cloud and hydrometeor properties, fluxes of

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Page 102 radiation, aerosol size distributions, and concentrations of significant trace gases are measured simultaneously. Such data sets will also have to characterize different cloud regimes and geographical areas. This is an extremely difficult problem, justifying a multifaceted approach. This approach should include increased attention to the acquisition of long-term, temporally and spatially consistent data sets characterizing the properties of clouds, radiation, water vapor, and trace gases. It must also include continued efforts to develop models that explicitly link these quantities. In addition, a concerted effort must be made to link the observational, data analysis, and modeling communities to focus on this problem. Promising approaches include the following: • using cloud-resolving models and nested models to determine interactions with large-scale variables; • using cloud models with explicit microphysics to develop parameterizations for microphysical processes suitable for simulations of weather and climate; • developing parameterizations exclusively from process study observations, then using satellite observations to generalize and extrapolate to the global scale; • isolating a limited number of empirical parameters that have a physical basis, then fitting to a set of observations to determine the best values of these parameters; or • using operational models as sources of finer-scale ''data'' from which to develop parameterizations. The inability to represent interactions among phenomena, especially those that occur on small scales, is the primary weakness in current models of the climate and weather, so we regard the need for these results as "imperative" and offer this as a key challenge for the next decade. Although a central justification for such studies is the need to improve representations of these processes in models, we nevertheless argue that the challenge—and opportunity—is to understand the linkages among various physical and chemical processes. A short-range approach is inappropriate because we are still at an early stage in understanding these interactions. Significant effort must be directed toward improving the foundations that will lead to adequate representations of these processes. Focus 2: Develop a Quantitative Description of the Processes and Interactions That Determine the Observed Distributions of Water Substance in the Atmosphere Precipitation is the source of essentially all fresh water on Earth, so the hydrological cycle is truly "vital" to humans and to most plant and animal life on

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Page 103 land. Although precipitation and cloudiness are the most evident results of the cycling of water through the atmosphere, water also has many other effects on weather and climate. Water vapor is the most important greenhouse gas, and variations in cloudiness and ice cover are the primary sources of variability in the albedo of the Earth. Clouds scavenge particles and trace gases from the atmosphere, and thunderstorms maintain the Earth's electric field. The latent heat released or absorbed as water changes phase is the source of energy that drives hurricanes and other severe weather systems. Thus, we cannot understand weather and climate without a good understanding of the distribution of water substance in the atmosphere. Weaknesses in current specifications of the atmospheric water cycle include poor characterization of upper-tropospheric water vapor, uncertainties in surface fluxes, poor understanding of the factors controlling precipitation efficiency, inability to represent the ensemble effects of cumulus convection on the transport of water, poor characterization of rainfall over the oceans, and the absence of a comprehensive understanding of the links between the atmospheric cycle and other components of the hydrological cycle. Emerging technologies and recent developments now can support a comprehensive new approach to these problems. Improved characterization of water vapor in the atmosphere may become possible from sensors on commercial aircraft, remote sensors employing radiometric or GPS technology, and improved research instruments capable of accurate measurements at low humidity. Satellite characterization of precipitation over the oceans will be possible in the coming decade. Existing and new modeling capabilities and data sets can be used to characterize the effects of cumulus convection, and improved models and understanding of boundary layer fluxes are emerging. Comprehensive international programs to study the hydrological cycle over regional scales appear feasible and are planned. These expected new results should provide an opportunity to characterize the distribution of water in the atmosphere with new confidence and to relate this distribution to the underlying interactions with the global hydrological cycle. Such an improved characterization is needed for accurate determination of radiative transfer in the atmosphere, for climate predictions of precipitation amounts and global temperature, and for improved weather forecasts. These studies thus provide a good match between the opportunities presented by new research capabilities and the needs of current research. However, a comprehensive and systematic approach is required if the many factors and processes entering the atmospheric hydrological cycle are to be understood. Focus 3: Improve Capabilities to Make Critical Measurements in Support of Studies in Atmospheric Physics Although impressive advances in instrumentation have been made in some government laboratories, especially in the development of high-technology re-

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Page 104 mote sensing instruments, other areas have fallen far behind what is technologically feasible. For many measurements, research aircraft still employ inadequate, decades-old instruments for lack of better alternatives. Critical measurements of humidity in the upper atmosphere cannot be made because of deficiencies in the humidity sensors used for routine soundings. Our ability to make critical measurements is also lagging far behind current needs in studies of hydrometeor size distributions or radiation. Numerous examples exist in all areas of atmospheric physics.2 In most cases there are good candidate techniques for making the needed measurements, but they await implementation. The instrumentation in current use is seriously out of date and is not taking advantage of modern knowledge and modern technology. There is an opportunity for a concerted effort in instrumentation to have a large impact. New experimental instruments to measure temperature and wind from aircraft are beginning to answer crucial questions about cumulus convection that have been posed for 50 years or more. Similar advances can be expected from other improvements in the basic instruments used in atmospheric science research. There are special needs for new equipment and instrumentation to study the issues raised in the first two imperatives. Foremost is the need for platforms suited to the studies that must be conducted, including new observing satellites, high-altitude research aircraft, and aircraft with long range and flight duration. Remotely piloted vehicles will provide new opportunities to collect measurements at high altitude and over long periods. Examples of missing observations that would make critical contributions include ice mass (especially in anvils), ice crystal radiative characterization (especially near the tops of cirrus and other clouds), wind fields with improved coverage and resolution, upper-troposphere humidity, and cloud fractional coverage as a function of cloud area and altitude. Instrumentation presently available is particularly weak in the following areas: 2 For example, measuring the temperature inside a cloud from aircraft is a problem that has apparently been solved only recently by new radiometric thermometers, and these are not yet thoroughly evaluated or in widespread use. Measurements of equivalent potential temperature in rain are not reliable, so important dynamical influences of precipitation cannot be evaluated. Measurements of cloud-condensed water still do not provide the accuracy required for most studies. Measurement of low concentrations of large precipitation particles. and the sizes and shapes of small ice particles, have not been possible until quite recently, and the new instruments that can make these measurements are still experimental. Unless corrected by GPS measurements, the accuracy of measured horizontal winds from research aircraft is only a few meters per second due to the character of inertial navigation systems, and the degradation of GPS accuracy in nonmilitary applications still prevents removal of these errors in many cases. Broadband radiometers used on research aircraft are not sufficiently accurate and do not respond fast enough to provide the needed measurements of flux divergence in clouds.

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Page 105 • the characterization of hydrometeor size distributions and shapes; • measurements of total water content and ice water content; • water vapor mixing ratio, especially in the upper troposphere; • cloud condensation nuclei, where instruments have to be more widely available and suited to use at high altitude; • ice nucleus measurements; • nephelometers; • other radiation instrumentation for the characterization of irradiance, radiance, and net flux as a function of wavelength, optical depth, and volume absorption coefficient, and for determining the optical properties of ensembles of ice crystals; •chemistry instrumentation, including trace gas detectors; • equipment for the detection of organic constituents, especially aerosol instrumentation; • airborne remote sensing equipment such as lidars, short-wavelength radars, and near-field profilers for temperature and humidity that can provide some extension away from an airborne platform and thus increase the representative-ness of its sample; and • measurement of mean vertical motions to better than 10 cm/s, averaged over areas ranging from cloud-scale to mesoscale. There are some particular needs for instrumentation to support studies of atmospheric radiation: • Produce an inexpensive observing system for radiative fluxes that is capable of making continuous, accurate, and reliable observations. Such a system is crucially needed to increase the data base of surface radiative fluxes. • Make a concerted effort to upgrade all radiometers, particularly broadband flux radiometers used on aircraft. The typical thermopile devices in use are not sufficiently accurate, and their response times are too slow to supply the required in situ data. • Develop simultaneous multiwavelength, active and passive systems to probe the atmosphere from both space and the surface and to retrieve hydrologic parameters. These systems must be robust and well calibrated. Examples are combinations of passive microwave radiometers and radar, or Doppler lidar and radar. • Develop observational techniques to sample routinely the regional-scale hydrological cycle. Required observations beyond our current capabilities include water vapor profiles in the upper troposphere and lower stratosphere, ice water path and content, and precipitation. Although major additional resources are needed for improvements in instrumentation, a modest program, perhaps combining the efforts of university scien-

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Page 106 tists, government laboratories, and research support facilities, could lead to significant improvements in our ability to make critical measurements in the atmosphere. Contributions to National Goals Human activity is having documented effects on the weather and climate. For example, carbon dioxide and sulfate concentrations in the atmosphere have been changed dramatically by human activity, and the effects of these changes are beginning to appear in the climate of the Earth. In some major cities, pollution advisories warning of hazards to health and leading to restrictions on activities are becoming increasingly common. Biomass burning by humans is the predominant source of particulates and of some chemicals in some regions, and perhaps worldwide. As the global population continues to increase and to industrialize, these problems will become more urgent. Many of the components of the research program recommended here address sources of uncertainty in climate prediction. Other benefits will result from improved abilities to predict regional climate and weather. When policies to mitigate anthropogenic effects on climate and weather or to manage water and other natural resources are debated, the results of the research proposed here will provide the critical basis for making sound decisions. The difficult choices are likely to involve accepting damage to the economy or to health, penalizing developing or industrialized countries, and burdening current or future generations. In this context, the importance to society of the scientific information that accrues from the research recommended here is indisputable.