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Summary Water vapor plays a vital role in shaping weather and climate on Earth. Hence, monitoring water vapor is critical if we are to explain and predict the behavior of the climate system. Unfortunately, measuring and analyzing water vapor on the time and space scales needed for this purpose have proven elusive. Therefore, it is appropriate and timely for the international climate research community, through the Global Energy and Water Cycle Experiment (GEWEX), to focus a project around water vapor. To this end, a GEWEX Global Water Vapor Project (GVaP) has been proposed, and drain Science and Implementation Plans have been developed. As requested by the U.S. Global Change Research Program (USGCRP), the National Research Council's (NRC) GEWEX Panel has reviewed these plans with an eye toward U.S. priorities. The pane] commends GVaP for attempting to bring together the diverse water vapor measurement and research communities. The ultimate objectives of GVaP are laudable and should be vigorously pursued, both for their own scientific merit and especially for their potential contributions to the breadth of hydrometeorological research. To achieve its potential, the project should give particular attention to the following activities (described more fully in the body of the report), many of which are already integral elements of the draft GVaP plans: · Gather, assess, and distribute existing water vapor data sets and products. Coordinate clata set intercalibration and comparison with results from validation experiments. · Highlight upper tropospheric water vapor. Create new water vapor products, including a merged global water vapor product.
Foster broad community involvement. In particular, strong interaction with the operational weather community and the broader hydrometeorological research community will also help to ensure that the promise of GVaP is fulfilled. Because water vapor data are needed by the U.S. climate research community, the United States should participate in the international GVaP to better leverage the nation's resources. Contributions that the United States should make in addition to its current activities such as remote sensing include the following: (a) improve instruments and analysis techniques, with special emphasis on efforts to increase vertical and temporal resolution and to improve measurements of water vapor in the upper troposphere-lower stratosphere; (b) implement use of reference radiosondes; (c) provide critical, high quality data sets 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. in addition, the pane] recommends that U.S. agencies consider targeting support for one or more of the individual GVaP Data Centers and/or the Central Facility, as appropriate. 2