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PART II
DISCIPLINARY ASSESSMENTS
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radiative transfer
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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.
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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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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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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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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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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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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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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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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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
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
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-
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
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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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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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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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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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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• 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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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.
