A policy statement of the American Meteorological Society as adopted by the Council (26 September 1991)
In the late 1980s, the American Meteorological Society, in conjunction with the University Corporation for Atmospheric Research (UCAR), prepared a position paper in which the top two research priorities for the next decade were identified: climate and mesoscale research. Mesoscale research, which deals with phenomena with spatial scales of 2–2000 km and time scales of minutes to hours, includes thunderstorms, hurricanes, tornadoes, flash floods, and major winter storms. These phenomena intrinsically involve weather processes at all scales and have fundamental influences on the hydrologic cycle and on climate itself. While the Society is pleased with increased emphasis on climate research in recent years, it is concerned that mesoscale research has not received adequate attention. This is unfortunate given the need for improved weather predictions and warnings, and the importance of mesoscale weather systems to climate. Major programs are under way to modernize the National Weather Service (NWS) observing network and to prepare for a satellite-based Earth Observing System (EOS) that will provide a long-term dataset of unprecedented quality and resolution.
For the moment, progress in understanding mesoscale processes remains limited by the insufficient temporal and spatial resolution of the current radiosonde network and the observing sites of the operational network. The new observing systems will, together, provide an observational base that will alleviate this problem. These systems include: 1) the Automatic Surface Observing System (ASOS), with its much improved and denser network of surface sensors; 2) the 137 Doppler radars for the contiguous United States, which will provide information on wind, turbulence, and precipitation fields; 3) the new improved geostationary (GOES I–M) and polar-orbiting (NOAA K–M) satellite systems; 4) the demonstration network of 30 clear-air radars for the central United States to measure winds continuously through the troposphere and lower stratosphere; 5) the quasi-operational ARINC Communication Addressing and Reporting System (ACARS) to provide over 30 000 automated daily reports from commercial aircraft of wind and temperature over the United States, covering the jet-stream levels and take-offs and landings; and 6) the anticipated availability of lightning data on a national basis. The AMS strongly endorses the operational implementation of the new observing systems. These systems will produce major improvements in the nation's capability to forecast short-term changes in the weather and issue timely severe weather and flood warnings. Yet, while the new and better data are our primary source of information, research is necessary to exploit the knowledge gained from the new datasets and to develop new numerical and conceptual models that will further improve forecasts and warnings.
For these reasons, the Society strongly believes that an intensive research effort on mesoscale processes is needed. The effort should be on the order of ten years in duration, national in scope, and should begin immediately. The Society endorses the scientific objectives outlined in the document "Predicting Our Weather: A Strategic Plan for U.S. Weather Research Program," approved by the Committee on Earth and Environmental Sciences (CEES), a committee of principal U.S. agencies involved in environmental research. This plan, which grew out of the National Stormscale Operational and Research Meteorology (STORM) Program, proposes a balanced program of research to be carried out over the next decade with the two goals of: 1) improving the 0–48-hour prediction of precipitation and severe-weather events; and 2) advancing the fundamental understanding of precipitation and other mesoscale processes and their role in the hydrologic cycle. The program provides for improved data access by researchers, basic process studies, research for numerical weather prediction and data assimilation, and accelerated transfer of scientific and technologic advances into operations. The plan is the result of years of work by the best scientists in the field, and in fact has already had substantial influence on the research and operational communities.
The implementation of this plan must be initiated now for the realization of the full potential of the new observing and processing systems under the modernization of the NWS programs. Resources must be provided for: 1) utilizing the new data as complex high-resolution numerical models are developed to improve the understanding of the physical processes leading to the development of precipitation and hazardous weather; and 2) meeting the most serious challenge of predicting events on such small temporal and spatial scales and limiting the yearly loss of life and property due to severe weather and flooding.
The Society also emphasizes that this must be a national effort involving as many scientists as possible from government laboratories, universities, and private industries. The new mesoscale data must be made available to as broad a spectrum of the scientific community as possible, and universities and the operational community must join in updating educational programs in mesoscale meteorology.
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Weather phenomena, as well as climate, are made up of diverse processes involving interactions between the atmosphere, the ocean, the biosphere, and the land surfaces, including snow and ice. These interactions occur over a range of spatial and temporal scales. Processes such as cloud formation, strong vertical motions, and rapid heat exchanges occur over spatial scales typically less than 100 km, and time scales of minutes to hours. These are the scales in which tornadoes, violent thunderstorms, flash floods, downslope wind storms, lake-effect snow squalls, downbursts, and wind shears take place. Cloud and precipitation processes can also become organized into the so-called mesoscale convective complexes and supercloud clusters with spatial scales of 100–2000 km and time scales of hours to days.
Although progress has been made in the scientific understanding of the structure and dynamics of mesoscale precipitation systems and of the scale-interactive processes that lead to their initiation and modulation, there are many areas where additional study is required. Vast gaps remain in our ability to simulate many of the mesoscale processes in numerical models. The new observing systems and the STORM field experiments incorporated in the CEES plan will provide the means to monitor the mesoscale processes and to improve our understanding of their evolution and structure. Yet, the ability to produce accurate forecasts of precipitation-system amounts and locations will depend on the improvement of forecasting models, their resolution, their data-assimilation schemes, and their representation of physical processes. It will also depend on their ability to represent properly the interactions between mesoscale systems and cloud microphysical processes involving latent heat release. The effect that precipitation processes have on diffusion, transport, and scavenging of chemical species important to public safety and air quality also demands intensive research. Similarly, the importance of surface characteristics to the development of mesoscale precipitation systems underscores the need for adequate information concerning the spatial and temporal patterns of evapotranspiration and solar radiation, important terms in the surface energy balance. These interactions promise to be an exciting area of research in the 1990s, because of the capability to observe the systems in considerable detail and study them with combined cloud–mesoscale models.
Many of the issues just discussed are equally important at the interannual and longer time scales. Convective systems are known to undergo shifts and reorganization over spatial scales of more than 5000 km and interannual time scales during major climate fluctuations such as the El Niño Southern Oscillation. Clouds are the most crucial factor causing uncertainties in model estimates of global warming. Changes in water vapor in the upper troposphere, although small in absolute amount compared with the lower troposphere, may potentially be far more important in determining warming near the earth's surface. According to the CEES report titled "Our Changing Planet: The FY 1990 Research Plan," "the representation of clouds and their overall effect on the earth radiation balance is one of the principal limitations of climate modeling today." Likewise, the report of the NASA Advisory Council titled "Earth System Science: A Closer View" (NASA 1988) states " . . . the effects of clouds . . . are a major concern. Significant research progress will follow from the development of models that furnish reliable predictions of cloudiness and its radiative impacts from a knowledge of the water vapor fields and atmospheric dynamics." In the representation of relevant physical processes, a typical climate model has spatial resolutions of 5° x 5° latitude and longitude and 10–15 vertical levels. While these resolutions are capable of representing the larger-scale atmospheric circulation, they are too crude to simulate explicitly the physical processes that generate clouds and redistribute and condense water vapor. Thus, climate models fail to determine uniquely how clouds and water vapor contribute to global-scale temperature changes, and are clearly incapable of depicting regional climate changes. The capability of these models to predict regional climates depends on increased model resolution and better parameterization of the physical processes inherent in mesoscale systems. Therefore, the improved understanding of these mesoscale processes is needed in global climate programs such as the Global Energy and Water Cycle Experiment (GEWEX). In particular, the mesoscale experiments planned by the United States under the U.S. Weather Research Program are essential to the success of the GEWEX Continental Scale International Project (GCIP) planned for this decade in the Mississippi basin.
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As has been recognized in the climate program, ready access to the voluminous data from the new instruments and models is a critical issue that confronts all atmospheric research scientists. In the mesoscale program, data-management systems will also be required to store and organize efficiently the vast amounts of data produced from operational, quasi-operational, and research systems, particularly radar and satellites. Universities will need access to these real-time mesoscale datasets for instruction and for weather research. Making these data routinely available at nominal cost to researchers and forecasters is necessary if the data are to be used effectively to improve operational forecasts and warnings.
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An important element of a total national mesoscale program is the education of researchers and operational meteorologists. Useful as the new scientific tools like mesoscale models will be, the final critical decisions on forecasts and warnings will continue to be made by weather forecasters for many years in the future. Researchers and forecasters must be well versed in the developing science of mesoscale meteorology and in the integration and interpretation of radar, satellite, and conventional observations. This kind of versatility will require close cooperation between the academic and operational communities as curricula for the professional education of forecasters are improved, and as modern meteorological laboratories are organized with appropriate access to observed and model data. Building of such cooperative relationships will be valuable to education and will ensure effective transfer of research findings into improved severe-weather and flood warnings, and weather forecasts.
Important steps toward meeting these needs are the newly formed "Cooperative Program for Operational Meteorology Education and Training" (COMET) and the proposed creation of the Experimental Forecast Facilities (EFFs) at selected NWS forecast offices. COMET is a program created by the National Oceanic and Atmospheric Administration (NOAA) and operated by UCAR for mesoscale training of NWS forecasters involving the new observing systems. EFFs are specifically designed to create an environment where forecasters and meteorologists from universities and research laboratories work together on applied research topics that have a direct application to forecast problems.
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The foregoing discussion has highlighted the essential elements of a comprehensive national program of mesoscale research that will be of great benefit to the nation. With a modest financial investment, this program will utilize the new operational systems for the mesoscale to produce: 1) theoretical and conceptual models that will aid in interpreting the new data; 2) the development of mesoscale data assimilation to combine optimally the many kinds of data that will be obtained from the new operational networks and special experiments; 3) the improvement of the representation of physical and chemical processes in numerical models, particularly those related to the hydrologic cycle, sensible, latent, and radiative heating throughout the troposphere, and energy exchange at the earth's surface; and 4) the improvement in both operational and climate models operating at global and regional scales.
The Society strongly recommends the prompt implementation of the strategic plan for mesoscale research approved by the Committee on Earth and Environmental Sciences.
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