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Implementation Plan, ought to assume a broader role, as a Central Processing and Assessment Facility for Water Vapor Data and Analyses, by actively ensuring that the goals cited above are achieved. Within the currently planned structure, GVaP should explicitly involve centers that collect and compare special, high-quality, high- resolution water vapor measurements with the measurements made by various global observing systems. Especially important, too, is the need to involve operational weather centers in GVaP, which will not only benefit the creation of a merged product, but, conversely, will also expose the operational centers to ongoing developments in the measurements. One of the more tangible benefits of the GEWEX Continental-Scale International Project (GCIP) has been the im- provement of the National Centers for Environmental Prediction (NCEP) operational weather analysis system using results obtained from the measurement campaigns (NRC, ] 998~. The pane! believes that the operational centers would accrue similar benefits were they to be linked to GVaP through the Central Facility. This goal could be accomplished by including an Operational Weather Analyses and Data Center on equal footing with the other Centers in the Implementation Plan's Data Development Procedure. The outputs from such a Center could, for instance, be effectively used to inform the process of defining the vertical structure needed for computing water vapor fluxes. As noted in the draft Implementation Plan (IGPO, 199Sb), models for structuring GVaP already exist in both the International Satellite Cloud Climatology Project and the Global Precipitation Climatology Project. There is every reason to believe that a GVaP similarly organized around a scientifically strong, active Central Facility will also be successful. Like these two projects, one of the greatest benefits of GVaP will be the creation of carefully synthesized water vapor data sets to serve the research community. U.S. Opportunities To understand the role of water vapor in the climate system, a comprehensive and ambitious program is required, as recognized by the GVaP Science and Implementation Plans. GVaP could make major 13

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contributions to this effort by focusing on the data collection, processing, and analysis tasks, in a fashion similar to that of other GEWEX Radiation Projects. In making these contributions, GVaP must maintain close ties to the relevant research communities to ensure that its products are being developed with maximum utility. A U.S. national research program would benefit considerably from a foundation grounded thusly on quality-controlled, comprehensive data sets. To realize the key contributions noted above, the panel recommends that U.S. agencies consider targeting support for one or more of the individual Data Centers and/or the Central Facility, as appropriate. For example, NCEP, the only civilian operational meteorological center in the nation, might be selected by the federal agencies to host a center focused on the utilization of operational products related to water vapor. Competitive proposals and bids for operating any of these organizations will help ensure that the best possible expertise is brought to bear on the challenge of measuring and analyzing water vapor. The pane! wishes to emphasize, however, that in no way should such support, including that for a scientifically strong and active Central Facility, be regarded as a substitute for maintaining a vigorous, well-funded extramural research community. Agency support for research outside the GVaP data development structure needs to be part of an overall GVaP effort, and the activities of individual investigators need to be properly considered by GVaP. The first step in advancing understanding of water vapor and its role in the climate system is to obtain an accurate quantitative description of the variations of water vapor in space and time; several global data sets already exist, but, as alluded to at the outset, their quality is less than desirable. The U.S. program can leverage its limited resources to accomplish this step most effectively by participating in the international GVaP project. Contributions that the United States should make in addition to its current activities such as remote sensing include the following: (a) improve instruments, analysis techniques, and sampling strategies to make better water vapor measurements, with special emphasis on efforts to increase vertical and temporal resolution and to improve accuracy in the upper troposphere- lower stratosphere region; (b) implement use of reference radiosondes to facilitate the ongoing calibration of different moisture sensors; (c) provide critical, high quality data sets from the U.S. DOE ARM 14

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program sites and WVSS to validate satellite remote sensing results; (~) lead the effort to compare and evaluate available and new data sets; and (e) undertake a new synthesis of existing and new sources of information about water vapor to produce a global water vapor data set with the needed improvements in accuracy. Although the above are necessary first steps, it is essential that a U.S. GVaP effort recognize the importance of combining water vapor observations with other data to understand the complete water exchange process. For example, a close association between GVaP and the GEWEX Cloud System Study will help advance understanding of cloud-water vapor interactions. In addition, effort should be made to collect improved and more detailed water vapor measurements as part of the GEWEX America Prediction Project. A goal of U.S. hydrologic research programs should be to quantify the rates of evaporation from the land and ocean surface, the rates of water transport by large-scale atmospheric motions, and the rates of conversion of water vapor into clouds and precipitation. Thus, the products of the GVaP program must be capable of being integrated with other analyses of evaporation, clouds, precipitation, convection, and winds, over Earth's entire surface. For instance, improvements in wind observing capabilities, particularly in the lower troposphere, will be required if water vapor transport is to be accurately estimated. It is therefore necessary that the development of new data products within GVaP be undertaken with an awareness of how the data will be used (e.g., for analysis and modeling estimates of large-scale transport, radiative effects, energy cycling) to ensure that the research objectives of the scientific community are met. 15