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Executive Summary Significant advances in our understanding of mesoscale1 atmospheric processes are needed in order to increase the accuracy of predictions and warnings of important weather events that occur on this scale. The opportunity exists to improve the 0- to 48-hour prediction of precipitation and severe weather through an enhanced fundamental understanding of precipitation, the hydrologic cycle, and other mesoscale processes, and through full utilization of the advanced observing systems that will soon be available. Mesoscale weather systems play a significant role in global atmospheric circulation through transport of heat, moisture, and chemicals. The effects of clouds generated by mesoscale weather systems are the largest variable in the global energy budget . Therefore, improved understanding of mesoscale processes is essential for improved understanding of climate processes and climate change. A recent report, Meteorological Support for Space Operations: Review and Recommendations (NRC, 1988), recognizes the importance of mesoscale weather events to the nation's space program; thus enhanced understanding of mesoscale processes should contribute in this arena as well. 1 Mesoscale processes are those that occur on scales of 2 to 2000 km, such as thunderstorms, tornado outbreaks, local heavy rain and snowstorms, flash floods, windstorms, downslope winds, and significant air pollution events.

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In order to achieve the goals of improved understanding and pre- diction of mesoscale weather processes, we need to fully exploit new ob- serving systems, such as the Demonstration Wind Profiler Network, the next-generation weather radar (NEXRAD) Doppler radars, instrumented commercial aircraft, and the GOES NEXT satellite; new data-processing methods, such as the Advanced Weather Interactive Processing System for the 1990s (AWIPS-90) and four-dimensional data-assimilation techniques; and a new generation of computers and numerical prediction models. These improved capabilities must be used as an integrated system in order to gain the maximum benefits from each component. The new mesoscale observing systems that will soon be deployed as part of the National Weather Service's modernization have been chosen to greatly enhance the present national weather observing capability. The new systems will be critical for understanding and predicting precipitation and severe weather and for understanding interactions of meteorological processes on different spatial scales. The new observations will also con- tribute to improvement of numerical weather prediction models through use hi initialization and verification of the models. In order to advance understanding and improve weather forecasts, this report recommends that • The planned development and deployment of the new weather ob- serving systems and technologies should continue and be kept on schedule. • The capabilities of new instruments should be systematically as- sessed in order to maximize their utility for short-term forecasting and warning and for initializing and testing numerical prediction models. • Investigation into possible additional applications of the new data streams should be undertaken hi order to realize the full benefits of new observational technologies. • The data from the new observing systems should be processed, archived, and made available to a wide range of users in an efficient, timely, cost-effective, and easy-to-use manner. • Data-assimilation techniques should be developed to combine the many kinds of data into coherent, gridded data sets suitable for a wide range of weather forecast and research tasks. • Numerical prediction models should be improved to more accu- rately account for physical and chemical processes, including those involved in the hydrologic cycle; sensible, radiative, and latent heating throughout the troposphere; and energy exchange at the earth's surface. • Large observing programs, timed to take advantage of the newly deployed observing systems, should be conducted. These programs should make use of the new observing systems to look at the multiscale inter- actions arising from mesoscale weather events. Additionally, observing

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programs that cover smaller regions and specific weather phenomena and that complement the larger-scale programs should also be planned and carried out. • Forecasters should be retrained and, where necessary, university curricula in the atmospheric sciences should be revised, updated, and expanded to produce a sufficient supply of new meteorologists capable of using the new observing systems and scientific concepts to best advantage. The necessary actions to fulfill these recommendations are embodied in the National Stormscale Operational and Research Meteorology (STORM) Program (STORM Program Plan [NCAR, 1990]). Therefore this report recommends that the National STORM Program be implemented as soon as possible. By building on the large investment already committed to new ob- servational technology and capabilities, the National STORM Program will provide the focus for important advances in basic understanding of mesoscale weather processes and in the country's operational abilities to forecast weather events.