2 decision aids, and recommendations for
action.
Somewhat different data formats and strategies may also be
required. Thus, the effort to optimize and improve national and
global observing systems should examine commonalities and
differences among the needs of weather prediction, climate
monitoring and projection, atmospheric chemistry and air quality
prediction, near-Earth space physics, and other environmental
disciplines.
New Observing Opportunities
New opportunities for acquiring atmospheric observations suggest
that the observing system of the future may be dramatically
different from that of today. Three examples illustrate this
point:
1. Commercial aircraft observations: Sensors
carried by commercial air transport aircraft are now producing
thousands of observations of winds and
1 A
four-dimensional data base is one in which data are stored
according to four independent variables, or coordinates: longitude,
latitude, height above mean sea level, and time.
2 Impact
variables are descriptions of atmospheric conditions that directly
affect a given operation, in contrast to the meteorological
variables used in theory and modeling, such as pressure and
temperature. For aviation, as an example, such impact variables
include cloud height, visibility, and intensity of icing or
turbulence.
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temperatures daily over the contiguous United States; the use of
humidity sensors is currently being evaluated. The ascent and
descent of these aircraft at major air terminals can provide a
dense set of atmospheric soundings in addition to enroute
observations (NRC, 1994a; Fleming, 1996).
2. Global positioning system: Signals from the
Global Positioning System (GPS) satellites intended for use in
navigation can also, through occultation techniques, produce
profiles of the atmospheric index of refraction (Ware et al., 1996)
which can be used to derive temperature and humidity profiles or
can be assimilated directly into models. With these GPS
measurements, global fields of important variables could be
measured relatively inexpensively and continuously for weather and
climate applications.
3. Adaptive strategies: High-resolution
measurements in specific locations may sometimes enhance prediction
of significant phenomena, suggesting that some observations can be
made adaptively in response to empirical evidence or modeling
techniques that identify sources of errors. For example,
operational models predicting severe weather or tropical storms
might be configured to specify locations where higher-resolution
initial conditions would enhance accuracy (Burpee et al., 1996).
Then, observations could be obtained in these regions from
satellites or remotely piloted scientific aircraft.
Requirements for Optimizing and
Integrating Observing Systems
The integrated portrayal of atmospheric evolution and
interactions in four-dimensional data bases containing both
existing and some critical new observations is essential for the
advances envisioned in this report. The following requirements must
be met to achieve this objective:
• Integration with modeling efforts: Efforts
to optimize observing systems must take into account the analysis
and prediction models that will assimilate the data, whether they
are weather, climate, upper-atmosphere, or chemical models. Thus,
rigorous end-to-end analyses—from observations through models
to accuracy of predictions—are required to assess the benefits
and costs of overall strategies and specific observation schemes.
For example, issues such as whether radiosonde observations can be
replaced with satellite data must take account of many
considerations, including forecast accuracy, operational costs, and
especially the integrity of the climate record.
• Increases in computing power: Advances in
computing power will be necessary to conduct fully coupled
simulations of the entire Earth system—including the
atmosphere, oceans, land surface, ice, and chemical and biological
subsystems—in order to study climate and its variability on
various scales. High-resolution weather forecasts, along with
ensemble approaches to assessing confidence in the prediction will
similarly require greater computer power. Both activities may
impose changing requirements on optimization criteria.
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• Assimilation of new forms of data:
Innovative assimilation schemes for incorporating new forms of data
into analysis and prediction models may enable significant
advances. For example, recent research has shown that using model
variables to calculate the model radiance fields and then iterating
the model to achieve agreement with radiance fields as observed by
satellites is superior to attempting to convert satellite
observations into traditional variables such as temperature and
humidity. Significant benefits from other new forms of data,
including those from commercial aircraft and air quality networks,
may derive from similar innovation and analysis. Earth Observing
System (EOS) satellites will provide global observations focused on
understanding climate variability and evolution. In addition, EOS
and other research satellite efforts will provide new streams of
data and new instruments that may be adapted for operational
use.
• Multiple uses of data bases: As data from
surface and upper-air observations, operational radar,
geostationary and polar-orbiting satellites, and other sources such
as air quality networks are incorporated into the data base, they
can be shared by the weather, climate, and applications
communities. Because resources are limited, it will be critical to
strive for the maximum benefit for all from the incremental
investments required to optimize and integrate the observational
and analysis systems. Indeed, the goal should be to integrate
atmospheric data bases with those developed by other disciplines to
represent ocean and land surface conditions.
• Information organizing systems: A
requirement in forecasting is to identify the limited subset of
data that is critical for the accuracy of a prediction.
Machine-learning concepts, statistical models relating key
variables or decision parameters to computer forecasts, expert
systems, and the concepts of fuzzy logic all have potential for
organizing observed data and computer simulations in ways that
strengthen atmospheric information services.
• International collaboration:
Four-dimensional data bases will transcend national boundaries. It
is essential that the long tradition of sharing weather and climate
data globally be maintained. The proposed integration of satellite
observations between the United States and European space and
meteorological agencies is an important harbinger of progress; in
contrast, attempts to limit the availability of local data are
serious impediments to the cooperation required.
Many of the issues involved in improved observations and
improved weather services are being addressed in the federal
interagency effort to create a future North American Atmospheric
Observing System (NOAA, 1996) as a composite observing system for
the twenty-first century. Until it has been established rigorously
that new approaches are fully adequate alternatives, it is
essential to retain the full capabilities of the worldwide
radiosonde network because it is the foundation of the present
atmospheric observation and prediction system.
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Observing System Simulation
Experiments
Getting the highest return from investments in atmospheric
information and forecasts is the clear motivation for integrating
observing systems. Assessing alternatives is critical as automated
systems replace human observers in an attempt to reduce ongoing
operational costs by making capital investments in systems that
have lower overall life-cycle costs.
Observing system simulation experiments enabled by contemporary
computers and numerical models can help to determine the optimum
configurations of observing system components and numerical models
relative to forecast accuracy and overall costs, including capital
investments, operational costs, and the costs of disseminating and
archiving data. Such experiments allow rigorous and quantitative
examination of a wide range of strategies and can indicate whether
new resources will produce demonstrable benefits. Simulations of
observing and prediction systems can thus be a critical mechanism
for managing the advance of the discipline and its service to
society.
Atmospheric Science Imperative
2:
Develop New Observation Capabilities
The federal agencies involved in atmospheric
science should commit to a strategy, priorities, and a program for
developing new capabilities for observing critical variables,
including water in all its phases, wind, aerosols and chemical
constituents, and variables related to phenomena in near-Earth
space, all on spatial and temporal scales relevant to forecasts and
applications. The possibilities for obtaining such observations
should be considered in studying the optimum observing systems of
Imperative 1.
Contemporary numerical atmospheric computer models are
sufficiently varied and powerful that they can predict or simulate
a range of phenomena such as climate change or air pollution
episodes, as well as the course of the weather. However,
observations of critical variables on time and space scales
relevant to forecasts are essential to improving such numerical
simulations and predictions. Some of the required observations
present significant technological challenges.
In some cases, higher-resolution observations of traditional
meteorological variables such as water vapor are required to
portray important processes accurately. In others, both research
and applications require some new observations, for example, of
aerosols and trace chemicals and their vertical transports in both
clear and mixed-phase environments.
The required improvements will arise in part from new
instruments and new
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technology, as in new remote sensing techniques, and in part
from new observational strategies. For example, automation,
adaptive observations, and international cooperation will all
contribute to improved observations, as demonstrated by the
Tropical Ocean Global Atmosphere-Tropical Atmospheric Ocean array,
a network of buoys stretched across the equatorial Pacific.
Water in the Atmosphere
Water in all its phases and changes in these phases have a
pervasive role in weather, climate, and atmospheric chemistry;
thus, observing the phases and amounts of water in the atmosphere
is important to understanding atmospheric events on all scales. The
energy released by changes in the phase of water substance is of
major importance in driving global wind systems.
Water Vapor
On the storm scale, prediction of convective precipitation is
limited by uncertainty in the distribution of water vapor in the
atmosphere and the amount of water in the soil. On the longest time
scale, water vapor is the most important greenhouse gas, and
significant uncertainty in climate models can be traced to
inadequate understanding of the water budget and water-induced
radiation feedback. Water vapor in the atmosphere varies markedly
over small scales vertically and horizontally; thus, its
variability is significant at scales not resolvable by the
radiosonde network.
Today, water vapor profiling technology and its integration with
other measurements and models to improve resolution are evolving
rapidly. Notable advances are being made with remote sensing
techniques. In situ water vapor measurements are essential to
resolve fine structure and to constrain remotely sensed data.
Radiosonde observations satisfy these needs only partially because
of large spatial and temporal gaps. Greater coverage along
commercial air routes and more soundings near airports will be made
possible through the Commercial Aviation Sensing Humidity (CASH)
program [sponsored by the Federal Aviation Administration (FAA) and
the National Oceanic and Atmospheric Administration (NOAA) Office
of Global Programs], which is installing humidity sensors on air
transport aircraft; these sensors will have sufficient resolution
to obtain profiles during aircraft ascent and descent. However,
improvements are needed because current devices cannot measure
humidity accurately on an aircraft flying through clouds.
Clouds
Clouds are the visible evidence of a host of complex processes
involved with the change of water vapor to both liquid and solid
forms. Many critical aspects of
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the cloud processes involved in the formation of precipitation
and in interactions between water drops and ice particles are still
not well understood and require detailed observational and modeling
studies.
These processes also have important impacts on chemical
reactions in aqueous and mixed-phase environments and must be
better understood in order to improve models of local and global
chemical cycles.
As described later, clouds are an important control on the
planetary energy budget, inducing significant modifications of the
incoming streams of solar radiation and the outgoing streams of
infrared radiation. Moreover, latent heat released during the
formation of liquid water is a significant source of energy for
both severe weather and large-scale synoptic systems.
Precipitation
The global distribution of precipitation is tied to the dynamics
and energetics of the atmosphere, the coupling of atmosphere to
ocean, and ocean currents themselves. Precipitation observations
thus reflect both regional and local conditions and are critically
needed for climate studies, verification of global models, and
forecasts of severe weather and flood conditions. Truly global
observations can be made effectively only by satellites; progress
with precipitation observations is being made with the Special
Sensor Microwave/Imager (SSM/I). Another satellite system, the
Tropical Rainfall Measurement Mission (TRMM), launched on November
27, 1997, promises to improve precipitation estimates and estimates
of energy released over the tropics, using a combination of
high-resolution radar, a passive microwave radiometer, and
measurements in the visible to infrared. Variability in the
location of tropical rainfall has been shown to be a critical link
in the changes leading to El Niño events.
Precipitation estimates over the United States have been greatly
enhanced by deployment of NOAA's WSR-88D radars, and these
improvements are being incorporated into river basin flood models.
Further development is necessary to (1) improve the accuracy of
rainfall estimates by upgrading WSR-88D radars to include
multiparameter capability, (2) to improve rain gauge reliability,
and (3) to strengthen the ability to estimate rainfall from
satellites in space. As in the case of water vapor, improved
precipitation-estimation algorithms must be based on combinations
of relevant data. An expert system might be developed to produce
precipitation estimates from input information including radar,
satellite, and rain gauge data, as well as season of the year and
type of cloud system.
Wind Observations
Owing to modem technology, the density of wind observations has
significantly increased over the United States and some other
continental areas in recent years. In the central United States,
radar wind profilers, sensitive to the motions
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of refractive index fluctuations in clear air, are now
demonstrating their usefulness as forecast model input and research
tools. WSR-88D Doppler radars, although used mainly to document
precipitating weather systems, can measure winds in clear air to
altitudes of several kilometers over land during the day. Some
civilian transport aircraft provide wind observations along air
routes and during ascent and descent.
Wind information over the open ocean is sparse. Radiosondes are
launched from islands and from some ships, but many of the remote
sites are being abandoned or becoming unreliable owing to fiscal
pressures in other nations. Winds inferred from satellite
observations of cloud motion can be useful but are sometimes
inaccurate.
More accurate and comprehensive wind observations are needed for
nearly all the endeavors of atmospheric science. Adaptive and
episodic observations, obtained on demand in data-sparse regions
where information is critical, could help. Presently, aircraft are
deployed to gather data in and around tropical cyclones through
penetrations and dropsondes; future strategies could also
incorporate remotely piloted scientific aircraft (Holland et al.,
1992) and drifting balloons that rise or fall in response to remote
control. Satellite-borne lidar systems for measuring wind could
become a reality in the next decade, with several promising
techniques now under development. Satellite scatterometer
measurements, such as those from the European Research Satellite
ERS-1, and microwave brightness from SSM/I, when constrained by
models or independent observations, should prove useful for
estimates of surface winds over the oceans. New airborne radars
developed and operated by the National Center for Atmospheric
Research and the NOAA Aircraft Operations Center are providing
unprecedented detail on the fine-scale structure of weather
systems, including severe storms and hurricanes.
Observations in the Stratosphere
The stratosphere affects the world's weather through
interactions and exchanges with the troposphere and is an integral
part of the global climate and chemical system. Sampling
meteorological variables, trace gases, and aerosols in the
stratosphere will require a combination of ground-based balloon and
remote sensing, satellite measurements, and piloted and remotely
piloted aircraft—all blended with numerical models. Satellite
systems should overlap in time, to provide continuity of record as
well as to provide confidence in comparison of studies separated by
several years. In situ aircraft measurements will also be used to
evaluate new remote sensing devices. Remotely piloted aircraft may
become the platform of choice because of their lower cost and
high-altitude capability, provided that safety issues are resolved
and the technology proves to be operationally feasible. Both
space-borne and airborne measurements will benefit from
miniaturization of remote sensing and in situ instruments.
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Observations in Near-Earth Space
Phenomena created in the near-Earth environment by charged
particles and varying solar magnetic fields are referred to
collectively as ''space weather'' and have a variety of significant
effects on human activities, as discussed later. The prediction of
space weather phenomena requires observations that can be obtained
in part through present and planned satellite programs. For
example, the GPS radio occultation technique (Ware et al., 1996)
gives vertical profiles of electron density and total electron
content in the ionosphere.
Because space weather phenomena are forced by solar variability,
three sets of solar observations should be emphasized: (1) coronal
mass ejections, (2) magnetic fields including those in the corona,
and (3) near-Sun solar wind properties. Moreover, measurements of
the interaction of solar particles and the solar magnetic field
with the Earth's magnetosphere and ionosphere are important for
basic understanding and development of numerical models.
Atmospheric Research Recommendation 1:
Resolve Interactions at Atmospheric Boundaries and Among Different
Scales of Flow
The major weather, climate, and global
observation programs supported by the federal government and
international agencies should put high priority on improved
understanding of interactions of the atmosphere with other
components of the Earth system and of interactions between
atmospheric phenomena of different scales. These programs,
including the U.S. Weather Research Program, the U.S. Global Change
Research Program, and other mechanisms for supporting atmospheric
research, require observational, theoretical, and modeling studies
of such interactions.
Atmospheric studies are shaped today by the recognition that
interactions with neighboring systems are critical to improved
understanding and prediction. It is no longer sufficient to study
the individual components of the Earth system in
isolation—attention must now turn to understanding, modeling,
and predicting them as coupled systems. In many cases the relevant
coupled systems are the actual physical subsystems; in others, the
interacting subsystems are more abstract—for example, when
small-scale eddies interact with the largest scales of atmospheric
flow. Critical scientific questions focus on the exchanges of
energy, momentum, and chemical constituents among the atmosphere
and the surface below as well as the layers above leading to
near-Earth space.
In the initial study of a dynamical system, interactions with
the exterior environment can sometimes be ignored. This approach
served the numerical
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weather forecasting community well in its initial efforts.
However, as the length of the forecast period increased, the need
to account for the flows of dynamically significant quantities
across system interfaces became evident. In climate dynamics,
knowledge of this interaction between the components of the Earth
system is essential to progress. For example, it is now recognized
that the El Niño-Southern Oscillation (ENSO) is driven by
interactions within the combined tropical ocean-atmosphere
system.
Surface Processes
Processes occurring at the boundary between the atmosphere and
the land and sea are vital elements in weather, climate,
atmospheric chemistry, and global change. Atmospheric interfaces
with the ocean and the land are similar because both represent the
intersection of an atmospheric boundary layer with a boundary layer
on the surface below. The ocean interface is better understood
except under high winds; the land interface merits greater
emphasis.
Surface fluxes over the ocean are important in early
cyclogenesis (Kuo et al., 1991), and the churning up of cold ocean
water by hurricanes (Shay et al., 1992) helps determine their
evolution and motion (Bender et al., 1993). The weather and climate
are particularly sensitive to air-sea interaction over the tropical
South Pacific, where the warmest open-ocean surface temperatures
coincide with Earth's greatest annual precipitation (Webster and
Lukas, 1992).
Surface temperature, moisture, and fluxes are as strongly linked
to land surface properties as they are to atmospheric variables.
Satellites provide estimates of surface temperature, vegetation,
and moisture characteristics that can be converted into estimates
of fluxes but require further calibration against in situ
measurements of local characteristics (Kogan, 1995).
Methods of estimating fluxes over the ocean using satellites and
buoys are perhaps at a more advanced state than over land, with the
major uncertainties associated with light and strong winds, wave
spectra, and precipitating convection. The TOGA-TAO array is
providing a long time series of atmosphere and ocean data useful
for understanding sea-air interaction in the equatorial Pacific.
Continued developments in remote sensing of surface wind,
temperature, and radiative fluxes are needed, along with adaptive
sampling techniques for in situ and remotely sensed data in
cyclones.
Long-Term Interactions with the
Oceans
On decadal to centennial time scales, the interaction between
the upper and the deeper parts of the world ocean is believed to be
a primary control on the natural variability of surface temperature
on the planet. Moreover, the flow of heat between ocean layers sets
the time scale for significant response to forcing such as the
greenhouse effect. Better quantification and understanding of
these
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interactions will help in assessing the difference between
natural fluctuations of the large-scale surface temperature and the
response to external forcing.
Vertical transport of heat in the oceans is mediated by currents
that are often of very small horizontal scale compared to
basin-wide gyres. Flows connecting the ocean surface to deep water
are thought to occur in only a few locations on the planet. For
example, North Atlantic deep water is formed in very small regions
in the Norwegian Sea. Local phenomena such as the intermittent
formation of sea ice can have significant effects in modulating
flow from the cold briny surface waters to the very bottom of the
world ocean.
Surface interactions are also important. Accurate simulation of
the exchange of heat and fresh water at the ocean surface is a
demanding contemporary challenge for coupled ocean-atmosphere
climate models. Sea ice variation also is only poorly understood,
although it is clearly important in long-term climate change
because of its high reflectivity and its inhibiting effects on
thermal exchange between the atmosphere and ocean.
The coupling of the atmosphere and ocean in the tropics gives
rise to the ENSO cycle of alternating extremes of warm and cold sea
surface temperatures with periods of three to seven years that
affect the global climate system. The information between system
components is communicated through wind stress, sea surface
temperature, radiation, and precipitation. Although current models
exhibit solutions with ENSO-like oscillations that suggest useful
predictive capability, they are not always realistic. Improvement
in computation of the fluxes across the tropical atmosphere-ocean
boundary should lead to notable improvements in understanding and
practical application.
Clouds and Their Consequences
Clouds and cloud systems are relatively small but numerous and,
in sum, play a key role in shaping weather and climate. For
example, the reflection of sunlight by the planet is determined
largely by the cloud cover, and a significant fraction of the
infrared radiation flowing from the surface is absorbed by clouds
and reradiated to the surface as part of the greenhouse effect.
Hence, clouds are strong controls on the amount of solar energy
absorbed, the planetary energy budget, and the surface temperature.
Success in modeling a changing climate will require correct
specification of the effects of clouds and the way in which these
effects will themselves change as climate evolves.
The effects of clouds on radiation streams are determined by
their horizontal and vertical distribution and the type, size, and
concentration of their constituent particles. The properties of
cloud particles are a consequence of the initial concentrations of
aerosols and thermodynamic properties, subsequent vertical motion
and mixing, and encounters with particles having different
properties.
Deep, precipitating convection interacts with motions of larger
scales in other important ways as well. For decades, theoretical
studies have explored the
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effects of convective heating (the latent heating associated
with convective rain) on the growth of synoptic- to global-scale
waves in the tropics. More recently, it has been demonstrated that
the effects of ENSO on weather in other parts of the world are
determined largely by the atmospheric response to heating by deep
convection over the equatorial Pacific, which is in turn modulated
by the sea surface temperature distribution.
Today, numerical models that can resolve properties of
individual clouds and cloud ensembles are being used to study the
response of clouds to larger-scale forcing and to develop
convective parameterization schemes for climate models whose
resolution is too coarse to represent clouds explicitly. Toward
this end, the results of cloud-resolving models are presently being
compared to available observations, and cloud-resolving models are
being coupled with ocean models to increase the realism of the
simulation.
Aerosols and Atmospheric
Chemistry
Suspensions in the atmosphere of tiny liquid or solid particles
are known as aerosols and can cause macroscopic changes in air
quality, atmospheric heating, and air chemistry. Aerosols have both
short-term and long-term effects in the atmosphere: cloud particles
form on them; they absorb and scatter radiation passing through the
atmosphere, thus altering the local heat balance; and they are
sites of chemical reactions with atmospheric trace gases. Aerosols
should be the focus of much research in the coming decades,
including gathering more data on aerosol chemistry and its
evolution in time and space, and improved understanding of the
light-scattering proportions of different types of aerosols (NRC,
1996a).
Trace gas concentrations in the atmosphere depend on sources and
sinks are usually at the surface of the Earth and on processes
within the atmosphere. Atmospheric transport and modification of
chemical constituents in mesoscale flow include reaction, advection
and diffusion, and interactions with water in its various phases.
Improved understanding is clearly important for air quality
modeling and prediction.
The Fundamental Problem of
Nonlinearity
Many of the key challenges in atmospheric sciences embody the
fundamental problem of all geophysical fluid flow—the
nonlinear interaction between phenomena with various length and
time scales in the flow and phenomena of different length and time
scales in the boundary conditions, external forcing, and within the
flow itself.
Most geophysical flow problems involve interactions with a
boundary and thus develop turbulent boundary layers. Energy,
momentum, and other properties are passed across these boundary
layers from the fixed surfaces to the larger-scale flows.
Geophysical problems rarely allow simple closures; some
interac-
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tions must usually be retained, even if in somewhat simplified
form. In fact, it is typical in atmospheric boundary layers for
scales varying as much as eight orders of magnitude in both space
and time to be involved in fluxes from the surface. Crude treatment
of these interactions limits the accuracy of long-range
forecasts.
Yet even without significant boundary effects or internal
interactions with water processes, nonlinear interactions between
flow components of different scales has been a continuing challenge
for more than a century. Significant progress has been made, as
reflected by contemporary understanding of turbulence,
predictability, and the phenomena of chaos. Nevertheless, we have
yet to develop ways to describe chaos mathematically by finding
analytic methods for specifying the nature of attractors and the
statistical characteristics of resulting flows (Dutton, 1992). Of
particular interest is the extent to which long-term climate
simulations can determine global or local statistics accurately,
even though the numerical solutions are surely not
deterministic.
Until the difficulties created by nonlinearity are dramatically
reduced, they will remain pre-eminent challenges in the attempt to
understand geophysical flows and predict them quantitatively.
Atmospheric Research Recommendation 2:
Extend a Disciplined Forecast Process to New Areas
A strategy and implementation plan for
initiating experimental forecasts and taking advantage of a
disciplined forecasting process should be developed by appropriate
agencies and the scientific community for climate variations, key
chemical constituents and air quality, and space weather
events.
Much of the effort in the atmospheric sciences is aimed, either
explicitly or implicitly, at extending the range and improving the
accuracy of forecasts of atmospheric phenomena—weather and air
quality on the short term, climate variation on the longer term. As
practiced for more than a century, weather prediction involves the
traditional steps of the scientific method:
• Collect and analyze observations of present
conditions.
• Develop and use subjective or quantitative methods and
models to infer future conditions from these observations.
• Assess the accuracy of the forecast with observations of
actual conditions.
• Analyze forecast results to determine how methods and
models can be improved.
Through this process, weather prediction imposes a demanding
discipline on both individual forecasters and all of the
atmospheric sciences. The accuracy of the forecast every day, in a
wide variety of locations, is a continuing measure of the
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success and progress in this field—and of the integration
of theory, technology, and practical methods.
The impact of this discipline has intensified with the advent of
operational numerical weather prediction because precise
quantitative comparisons between forecasts and actual observations
can be made easily. The inclusion of computers in the forecast
cycle also facilitates improvement because proposed modifications
in the computer model can be applied to difficult cases
retrospectively.
As shown in Part II, several of the atmospheric sciences are now
developing capabilities for making quantitative forecasts and will
benefit substantially by establishing experimental operational
forecast procedures and thus engaging the discipline of
forecasting.
Climate forecasting presents one such opportunity, on both the
seasonal and the decadal scales. The TOGA program has stimulated
considerable progress in monitoring and predicting seasonal to
interannual climate fluctuations such as El Niño. In the
early 1980s, observations in the tropical Pacific Ocean were so
limited that the 1982-1983 El Niño was in progress for
several months before its magnitude and implications were known.
The Tropical Atmosphere Ocean observing system is now operational
and includes 70 moored ocean buoys to observe surface
meteorological properties and the upper-ocean thermal structure,
surface drifting buoys that measure sea surface temperature,
expendable bathy-thermographs from ships of opportunity that probe
the ocean temperature field to 700 m, and island tide gauges that
monitor sea level variability. Research and operational satellites
complement the in situ measurements. This system allowed the
evolution of the extreme El Niño event of 1997-1998 to be
monitored on a day-to-day basis. Moreover, these observations have
supported the development of coupled ocean-atmosphere models of the
tropical Pacific Ocean that can predict tropical Pacific sea
surface temperatures with confidence months to a year in advance.
These developments in the observing system, together with related
advances in coupled forecast models, have led to the implementation
at NOAA's National Centers for Environmental Prediction of a
routine, model-based, short-term climate forecast system for the
United States. Similarly, the newly formed International Research
Institute for Climate Prediction, created by NOAA, has taken on the
complementary responsibility for the rest of the world.
Successful forecasts of climate variations over a decade or more
would be of great value to a variety of industrial and government
activities. These forecasts would be statistical in nature and thus
focused perhaps on seasonal departures from means; as statistics,
they might include an estimate of expected accuracy. Verifying such
climate forecasts poses special problems because of the length of
time necessary to accumulate a meaningful ensemble of cases.
Another opportunity is created by the notable advances in
atmospheric chemistry over the past 20 years. The rapidity with
which the scientific community established the cause of the
Antarctic ozone hole only a few years after its
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discovery and impressive advances in both global and mesoscale
chemical modeling demonstrate that the necessary links between
dynamics and chemical processes are increasingly well understood
and that operational models are feasible. The discipline of
forecasting applied to air quality modeling will lead to improved
understanding of the factors that produce fluctuations in chemical
constituent concentrations. As the forecasting process matures, it
will provide continuing benefits in managing the environment of
urban areas and assessing the consequences of chemical
perturbations induced by pollutants, fires, volcanoes, and other
environmentally significant events.
A third opportunity arises with space weather—the
collection of phenomena associated with emissions of energy and
mass from the Sun and instabilities in the Earth's magnetosphere.
Space weather events can produce considerable disturbances in the
ionosphere, which in turn induce communications disruptions and
produce transient induced currents that lead to failures of
electrical power networks. The streams of high-energy radiation can
also result in satellite malfunction and may be lethal for
astronauts in space without sufficient protection. Today, the
increasing societal dependence on more than 250 satellites in
geosyn-chronous and low-Earth orbits for communication, navigation,
and Earth observations creates a new urgency for understanding and
predicting space weather. This urgency will increase as hundreds of
new satellites are placed in orbit for global cellular phone and
other information transfer services.
Space weather phenomena have been predicted for many years using
statistical methods, primarily by the military. Today, advances in
observations of solar phenomena and the solar wind can be coupled
with increasingly quantitative research models of the
solar-terrestrial system to develop an operational space weather
forecasting system.
In these three examples, as with prediction of the weather a few
days in advance, the relentless discipline of forecasting can be
expected to stimulate the interplay between improvements in
observation, theory, and practice required to develop predictive
capabilities of broad value to society.
Atmospheric Research Recommendation
3:
Initiate Studies of Emerging Issues
The research community and appropriate
federal agencies should institute interdisciplinary studies of
emerging issues related to (1) climate, weather, and health; (2)
management of water resources in a changing climate; and (3)
rapidly increasing emissions to the atmosphere.
A number of interdisciplinary scientific issues are emerging as
important candidates for attention by atmospheric scientists.
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Climate, Weather, and Health
Both threats to human health or other biological systems and
infectious diseases have been linked to climate variability (Shope,
1991; IPCC, 1996; Patz et al., 1996). Temperature, rain, sunshine,
wind, humidity, and soil moisture may affect the emergence and
spread of infectious diseases (Landsberg, 1969; Colwell and Huq,
1994; Epstein, 1995; Morse, 1995; Patz et al., 1996). Climatic
factors provide limiting conditions for the distribution of
vector-borne diseases; weather events can determine the timing,
outbreak, and spread of disease. For example, air temperature
controls the latitude and altitude distribution of mosquitoes that
are vectors for encephalitis, meningitis, dengue, and yellow fever.
In the tropics, rainfall controls the emergence of the anopheles
mosquito, the vector for malaria.
The increasing frequency of water-borne diseases, such as
typhoid, hepatitis, and bacillary dysentery, is associated with
flooding. A direct threat to human well-being arises because the
Earth's protective stratospheric ozone shield has been decreasing
(WMO, 1995), leading to detectable increases of ultraviolet (UV-B)
radiation at the Earth's surface (Herman et al., 1996). Thus,
health officials expect a consequent increase in skin cancer,
weakened immune systems, and other health-related concerns (Taylor
et al., 1988; Cooper et al., 1992; International Agency for
Research on Cancer, 1992; Johnson and Tinning, 1995).
Understanding the links between weather, climate, and various
diseases and their vectors will require collaboration among a
number of disciplines, particularly epidemiology and entomology. As
this understanding increases, multi-disciplinary observation
technologies, sophisticated numerical modeling techniques, timely
weather and climate analyses and predictions, and international
cooperation in environmental research will combine to offer new
tools in combating disease.
Water Resources
The design and operation of water resource management systems
have long taken account of expected extremes in both weather and
climate conditions. Now, however, population expansion coupled with
the possibility of climate change mandates that systems for water
supply or flood control and mitigation be designed for evolving,
rather than stationary, weather and climate conditions. The effects
of possible climate change on the spatial and temporal distribution
of floods and droughts, as well as on patterns of precipitation,
temperature, and wind, must be better understood to improve water
management designs and strategies. Thus, the stability of climate
and possible changes in the statistical structure of extreme
weather events are important new issues for water resource
management.
Moreover, the operation of water management systems is evidently
linked to both weather and climate forecasts; thus, issues of
predictability become impor-
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tant. Forecast lead times and operational flexibility in
managing water systems interact to determine the range of possible
actions and the possible benefits.
Atmospheric involvement in water resources extends to certain
aspects of the nation's ground water supply as well. Emerging
research problems involve aspects of multiphase ground water flow
and coupled mass-biochemical contaminant transport that come into
play as rainwater is absorbed by the ground and transported
laterally through underground rock strata. Robust description and
understanding of these complex phenomena involve laboratory and
field experiments and the development of mathematical and numerical
models capable of simulating unsteady processes with time constants
varying over many orders of magnitude.
With the atmosphere providing the rapid transport portion of the
hydrological cycle, it is evident that atmospheric science, in
collaboration with hydrology and soil science, must focus more
sharply on the complex issues associated with understanding,
predicting, and managing the fluxes of water on which society
depends.
Part of the World Climate Research Program, the Global Energy
and Water Cycle Experiment (GEWEX), was initiated in 1988 to
observe and model the hydrological cycle and energy fluxes in the
atmosphere, at the land surfaces and in the upper oceans. GEWEX
will significantly increases our understanding of the water-energy
cycle and thus provide the basis for a more sophisticated water
management system.
Rapidly Increasing Emissions to the
Atmosphere
Beginning with the Industrial Revolution, environmental quality
has been increasingly threatened. With the expansion of worldwide
industry, new threats to environmental quality pose new research
questions. One example is the economic development along the
western rim of the Pacific Ocean and the concomitant increase in
the emission of gases produced by combustion and industrial
processes. The rapid expansion of emissions in this region is now,
or will soon be, mirrored elsewhere around the world during the
twenty-first century, a trend that seems likely to result in
chemical and climate impacts on both regional and global
scales.
These increasing emissions pose new questions concerning the
regional, hemispheric, and global consequences of rapidly
intensifying sources of pollution. What will the fate of these
materials be as they move across the globe? Will they reach the
Arctic? Will they wash out and fertilize specific regions of the
ocean? How and at what rate will gaseous sulfur convert to
particulates? Will dispersion and conversion rates differ in El
Niño years?
The potential impacts of these emissions, both globally and
locally, merit considerable study. To be effective, this study
should be carried out in a highly collaborative mode, with social
scientists, economists, and others.
Representative terms from entire chapter:
space weather
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