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The Need for GVaP Few would argue about the importance of water vapor to the weather and climate on Earth. By modulating the transfer of radiation in and through the atmosphere, water vapor strongly influences the overall energy balance of the planet. The release of latent heat, which accompanies the condensation of water vapor, provides much of the energy driving storms and the atmosphere's general circulation. The presence of water vapor in the atmosphere influences evaporation from the surface, thereby affecting land temperatures and groundwater storage. Moreover, because of its great mobility and brief residence time, water vapor is a central component of the global hydrological cycle. How this cycle may change globally and regionally in the future is a major issue for climate science and society. Because water vapor is vital for Earth's energy and water cycles, it must be monitored in time and space if we are to explain and predict behavior of the climate system. in particular, to properly appraise the response of the climate system to external forcing, the atmospheric transport and cycling/recycling of water vapor must be well understood and modeled. If water vapor transport is to be accurately estimated, then water vapor concentrations and wind velocities are required at sufficient accuracy and resolution to account for the atmosphere's often-distinct vertical gradients of water vapor and wind. Furthermore, in regions like the upper troposphere and stratosphere, where water vapor concentrations can be less than tens of parts per million, water vapor has significant radiative and chemical effects that need to be pro- perly quantified and modeled to address questions about anthropogenic and natural global change, including the feedback of water vapor on changes in radiative forcing. Unfortunately, measuring water vapor sufficiently well to properly understand the processes responsible for its variability has proven disappointingly elusive. This situation results in part because water vapor is not dynamically constrained, and its high spatial variability makes adequate sampling difficult. Problems associated with the various water vapor measurement technologies also have hindered 8

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progress. Standard humidity sensors carried by radiosondes have complex error characteristics up through the mid-troposphere, and their performance is severely diminished at higher levels. Furthermore, the network of radiosonde stations is strongly biased to locations on land, and the number of radiosonde stations has declined in the past few decades. Satellite measurements provide global coverage, but their vertical resolution in the lower troposphere, where water vapor is most abundant, is poor compared with that of radiosondes or other ground- based systems. Long-term water vapor monitoring with satellites can also be problematic due to several factors such as gradual changes in instrument sensitivity and local crossing time, abrupt changes resulting from satellite replacements, and short or intermittent system lifetimes. As a consequence of such difficulties, the research community relies heavily on global analyses of water vapor produced by operational weather centers or similar institutions. All such analyses, however, are model- and/or methodology-dependent and therefore differ notably from each other. While these analyses produce multi- layered output, the accuracy of these data is constrained by the vertical resolution of the inputs of observational data. For instance, in the absence of reliable water vapor measurements in the upper troposphere, operational analyses of this quantity are questionable in this region. Also, in the absence of observed humidity profiles over data sparse regions (e.g., oceans), the vertical detail of model-based analyses tends to exceed the degrees of freedom allowed by satellite observations. Given this state of affairs, the GEWEX Pane! feels that it is appropriate and timely for the international climate research community, acting through GEWEX, to focus a project around water vapor. GVaP may not be able to rectify or overcome some of the important deficiencies in current water vapor measurements (e.g., low vertical resolution from space-based sensors). However, GVaP can and should contribute to developing an improved observing capability by conducting a more detailed and quantitative assessment of the limitations of the current measurements and the potential for new experimental measurement systems that have not yet been fully exploited (e.g., microwave sounders Lincluding the Microwave Limb Sounder], radio occultation, and infrared spectrometers). Careful and coordinated analyses of all of these measurements will be needed to discover how far we can progress in obtaining the information about water vapor required to answer the main research questions. The 9