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1 Introduction THE PROBLEM Each season reminds us of our vulnerability to destructive or incapac- itating weather events such as tornado outbreaks, windstorms, hailstorms, heavy snowstorms, and flash floods. Our society relies increasingly on tech- nology that is vulnerable to the weather, thereby increasing the need for improved weather predictions and warnings. There is, for example, height- ened awareness of the crucial role played by weather in major air and space transportation accidents, such as the Dallas-Ft. Worth airliner crash, the Centaur Atlas launch lightning accident, and the Challenger space shuttle disaster. The costs of inadequate warnings of tornadoes, floods, and other severe weather can be very large. Events of 1989 alone dramatically reminded many Americans of their vulnerability to severe weather events. In addition to Hurricane Hugo's costly (21 U.S. mainland deaths and about $7 billion in damage) strike into South Carolina, the nation reported a near-record 845 tornadoes that caused 48 deaths and much property damage. The outbreaks that caused the most deaths occurred on November 15 near Huntsville, Alabama, (21 deaths) and on November 16 in New York State (9 deaths), the latter tornado activity being very atypical in terms of location and time of year. Heavy rains brought flash floods to parts of Kentucky in February, Pennsylvania in May, Louisiana in both June and November, and northern Florida in October, when up to 40 cm (16 in.) of rain fell in one day. Fargo, North Dakota, recorded a record snowfall of 62 cm (24.4 in.) from

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a local snowburst embedded within a severe blizzard-producing storm in January. Similar severe weather events, many of them mesoscale, occur every year, have similar impacts, and typically cause some billions of dollars in economic losses as well as significant loss of lives. A singular characteristic of heavy precipitation and severe weather is that they are often highly localized events that occur over small areas and for limited times. These mesoscale weather events are the ones that most directly affect humans, yet many elude detection by the present upper- air weather observing network, which is built around 12-hourty balloon soundings at sites some 500 km apart, and even by the surface observing network, which has typical site spacings of 100 to 200 km. Moreover, these events cannot at present be reliably predicted with numerical models. Mesoscale events result from complicated, nonlinear processes that themselves occur across a wide spectrum of spatial and temporal scales extending from variations in the global atmospheric circulation pattern to microscale physical processes in individual clouds. In addition, mesoscale systems, once initiated, affect the larger-scale atmospheric circulations in which they are embedded. For example, Figure 1.1a shows an enhanced infrared image of a mesoscale weather system as seen from a satellite vantage point over Iowa at 0730 UTC on July 16, 1982. Although the cloud cover depicted in this image suggests a single weather entity with a scale of about 300 to 400 km, radar observations of this system made at approximately the same time (Figure 1.1b) and subsequent visible satellite imagery (Figure 1.1c) indicate that the system was composed of several clearly distinguishable elements of distinctly different scales, that is, 1. a broad, roughly circular zone of stratiform precipitation with a diameter of about 200 km, 2. cyclonically curved bands of heavier precipitation («50 km wide and about 200 to 300 km long), and 3. individual thunderstorms, each on the order of 2 to 20 km in scale. In addition to displaying these directly evident multiscale features, this system substantially altered the flow field of its larger-scale environment and produced a large anticyclonic wind perturbation that spanned nearly 1000 km (Figure 1.1d). At smaller scales, high-resolution aircraft observations of such systems suggest that microphysical processes such as freezing, melting, drag, and evaporation of hydrometeors significantly affect the circulations of such systems. These inherently complex systems have so far defied complete scientific understanding, largely because it has not been possible to simultaneously measure how the many different scale components of such systems interact with each other and with their large-scale environment. As a consequence, consistent and accurate prediction of these systems has remained elusive, as in the present example (see Figure 1.1), where over

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a FIGURE 1.1 (a) Enhanced infrared satellite image for 0730 UTC, July 16, 1982. (b) Des Moines, Iowa, radar horizontal display for 0700 UTC, July 16, 1982. Levels of precipitation intensity (VIP) are shown along with axes (dashed) of maximum precipitation, suggesting rotation, (c) Visible satellite image for 2130 UTC, July 16, 1982. Arrows indicate individual

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250km thunderstorms developing within the vortex, (d) 200-mb perturbation wind field produced by the mesoscale convex-live complex shown in (a); valid time, 1200 UTC, July 16, 1982. Full barb = 5 m/s. (Reprinted, with permission, from Murphy and Fritsch, 1989. © 1989 by the American Meteorological Society.)

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8 10 in. of rain with attendant flash floods occurred as the system moved into southern Michigan. This example is just one of many such occurrences of complex mesoscale systems that have resulted in unpredicted severe weather. A massive capital investment in technology now under way as part of the National Weather Service's modernization will permit weather obser- vations of increased detail in space and time. These observations should greatly enhance our ability to observe small-scale weather systems such as the one discussed above. However, as with all new observing systems, research on how they can best be used is required to realize the systems' full potential and bring about improved understanding and prediction of mesoscale weather. Mesoscale systems also significantly affect the exchange of water be- tween the earth and the atmosphere and form a critical link in the hydrologic cycle. The improved basic understanding of the hydrologic cycle resulting from a mesoscale research program will support improved water resource policies and contribute to our understanding of global climate change. HISTORICAL PERSPECTIVE Meteorological observations form the basis of our understanding of the weather. The improvement of numerical weather prediction and the illumination of physical processes depend on the frequency, accuracy, and spacing of available observations. Significant weather events can form on scales as small as 2 km, much smaller than the existing operational networks can resolve, and hence their origins are often not detected. Mesoscale weather events also tend to be episodic rather than periodic, which further decreases the chances of their detection. Furthermore, mesoscale weather phenomena interact importantly with larger-scale weather systems, but the exact nature of these interactions is not well understood. As early as 1969, scientists recognized the need for special programs to observe and study mesoscale weather phenomena and their interactions with larger weather systems. As an outgrowth of this recognition, scientists in 1973 proposed a multiscale program, the Severe Environmental Storms and Mesoscale Experiment (SESAME; NOAA, 1976), which was designed to specifically address the vital scale-interaction problem mentioned above. However, a lack of sufficient resources and observational capability pro- hibited carrying out a multiscale field experiment. Instead, the program was separated into two phases: the first phase concentrated on the larger synoptic and meso-a scales (200 to 2000 km), while the second focused on the smaller meso-/3 (20 to 200 km) and meso-7 (2 to 20 km) scales. Also in 1973, the Cyclonic Extratropical Storms (CYCLES) Project attempted to examine meso-a- and meso-/?-scale disturbances embedded in winter

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cyclones. These projects were followed in 1975 through 1980 by the High Plains Experiment (HIPLEX) series of field studies and in 1981 by the Cooperative Convective Precipitation Experiment (CCOPE), both of which recognized the importance of mesoscale processes and scale interactions in their plans. However, resources and observational capabilities remained inadequate for detailed study of scale interactions. Over the years, the scientific community has nonetheless remained steadfast in its call for a multiscale, scale-interaction experiment that focuses on understanding and predicting mesoscale weather processes. About a decade ago, the associate administrator of the National Oceanic and Atmospheric Administration (NOAA) requested the National Research Council (NRC) to "conduct a brief survey of the current mesoscale meteorological research being conducted throughout the federal govern- ment and to develop a preliminary assessment of the adequacy of this research in terms of the important opportunities that exist in this area of scientific endeavor." The ensuing NRC report, Current Mesoscale Me- teorological Research in the United States (NRC, 1981), recommended the establishment of a national mesoscale program of basic and applied re- search directed toward better understanding and improved prediction of mesoscale weather events. In 1982, a group of atmospheric scientists met at the National Cen- ter for Atmospheric Research (NCAR). They concluded that significant improvements could be achieved in local weather forecasts and warnings of severe weather as a result of then recent and planned developments in space- and ground-based observing systems. In addition to enhancing early warning capabilities, these new observing systems were considered to have the potential for greatly improving physical understanding of small- scale weather processes. Advances in computer technology, by enhancing the ability to numerically model weather processes, would enable the new observations and improved understanding to be translated into better local weather forecasts and warnings. The scientists recognized that in order to realize the potential for improved understanding and prediction of weather, a coordinated, national effort was needed. The proposed effort was named the National Stormscale Operational and Research Meteorology Program, or simply the National STORM Program (UCAR, 1982). The technical opportunities and scientific basis for a national effort centered on mesoscale meteorology were discussed in detail in The National STORM Program: Scientific and Technological Bases and Major Objectives (UCAR, 1983a). It was recognized that the proposed national program required two parallel efforts: one centered on improved understanding of the physical processes involved, and the other on applying this knowledge to improving weather services. The National STORM Program: A Call to Action (UCAR, 1983b) supported the recommendations of the previous

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10 two documents, urged the community to capitalize on the emerging obser- vational technology, and called for implementation of the two components of the National STORM Program. In 1986, the National Science Foundation's Division of Atmospheric Sciences (ATM) and the University Corporation for Atmospheric Research (LICAR) were advised by a specially appointed committee on the important priorities for atmospheric research over the next decade or so. In the committee's report, The Atmospheric Sciences: A Vision for 1989-1994 (NSF, 1987), progress in mesoscale meteorology was highlighted as one of the top two research priorities. Also, in 1988, the UCAR, in conjunction with the Council of the American Meteorological Society, identified the protection of life and property through improved weather prediction as one of two top priorities for the United States to pursue in the immediate future (AMS, 1988). RECENT DEVELOPMENTS Many of the recommendations found in the documents referenced above have been or are in the process of being implemented. The de- ployment of an advanced national weather observing system has received particular emphasis. This system, representing an investment of more than $1 billion, includes the next-generation weather radar (NEXRAD) Doppler network, the Automated Surface Observing System (ASOS), the Demonstration Wind Profiler Network, the next generation of geostation- ary satellites, and the Advanced Weather Interactive Processing System for the 1990s (AWIPS-90). However, the parallel research and application efforts needed to maximize the benefits to be derived from this impres- sive array of instruments have largely been lacking. To help address these needs, the National STORM Project Office has recently been established to coordinate improvements in the National Weather Service's (NWS) op- erational systems with the nation's weather research program and to plan for a national mesoscale program that will further both understanding and predictive capability. Experts in numerical modeling, in observational research, and in oper- ational forecasting from the government, academia, and the private sector have been involved in planning for the National STORM Program. This process has resulted in a community consensus on goals, objectives, and schedules for the program. The preliminary program plans have been reviewed by this committee and its predecessor, the Panel on Mesoscale Research. The proposed National STORM Program has two equally important goals:

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11 • Improve the 0- to 48-hour prediction of precipitation and severe weather events. • Advance fundamental understanding of precipitation and other mesoscale processes and their role in the hydrologic cycle. As this report makes clear, the atmospheric community is now well positioned to address these goals. The new technologies for observation, data analysis, and information dissemination will soon be operational. An effective administrative framework has been established. The atmospheric sciences community is in agreement on the goals, objectives, and method of attack. Therefore, a well-focused program of basic and applied research di- rected toward achieving the full potential of the new observing technologies for operational prediction and warning is urgently needed. PURPOSE AND SCOPE OF THIS REPORT As noted above, little progress has been made in undertaking a well- focused basic and applied research program that will exploit the full po- tential of the new observing technologies for improved fundamental un- derstanding of mesoscale interactive processes. Without this fundamental understanding, the full benefits of the new technologies for operational prediction and warning cannot be realized. It is the purpose of this report to make recommendations that will assure that the full potential of new observing technologies is realized in terms of improved understanding and better forecasts. In this report, the committee urges the implementation of a national mesoscale program that will provide both the scientific under- standing and the development of the operational techniques required to make the best use of the large investment in instrumentation and technol- ogy, leading to improved weather predictions in all parts of the nation. This report discusses several new technological capabilities relevant to mesoscale meteorology and their relationship to advances in scientific understanding; important relationships between mesoscale meteorology, atmospheric chemistry, and climate are also identified (Chapter 2). The potential implications of the new technologies for improved operational meteorology are summarized (Chapter 3). The report concludes with statements of needs and recommended ac- tions for capitalizing on the opportunities that now exist (Chapters 4 and 5). The recommended actions should lead to major progress in understanding scale interactions, a virtually unexplored frontier in mesoscale meteorol- ogy. This increased understanding should have the direct, positive impact of improving weather forecasts and products delivered to the public. In addition, understanding of the role of mesoscale processes in the global cir- culation, in the transport of greenhouse gases and other trace constituents, and hence in climate should be much improved.

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12 The message is clear. The community has prepared itself well to undertake a national mesoscale program. There is broad agreement on the goals, objectives, and plan of attack, and the technological components are being put into place. This is the optimum time to begin such an effort . The program should result in a major leap forward in understanding of mesoscale processes and in developing markedly improved weather forecasting services for all segments of our society.