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international GVaP is to be commended for recognizing this need and for attempting to bring together the diverse measurement and research communities under a single project devoted to water vapor. The pane' believes that one of GVaP's most enduring legacies may indeed lie in the communication it can establish among the measurement, analysis, and research communities. Comments on the Science and Implementation Plans The overarching goal, approach, and research objectives listed in the draft Science Plan (IGPO, 199Sa) are at once meritorious and extremely ambitious. The GEWEX Pane] believes that some aspects of the proposed program, especially those positioning GVaP as an interface between data providers and users, are more tractable than others. In particular, attempts to deal with cloud-scale processes will be especially difficult given the resolution of the observing systems discussed in the GVaP Implementation Plan. Nevertheless, the microphysics of hydrometeors for example, their re-evaporation in the tropical upper troposphere is critical for understanding water vapor-radiation feedbacks. In this connection, however, GVaP can be especially valuable to the modeling community by synthesizing the larger-scale measurements that are an important component of de- veloping improved parameterizations within global models. Moving forward with its plans, GVaP should pay particular at- tention to the following activities: Gather, assess, and distribute existing water vapor data sets and products. The proposed effort to gather all relevant water vapor data sets should include careful attention to incorporating metadata so that updated versions of data sets can be properly identified and linked to previous versions. Other documentation, such as error estimates, also will be critical to future reprocessing efforts. Because of the large number of existing and planned data sets, a primary initial objective should be to assess their quality so that users are made 10

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aware of the strengths and limitations of each observational product, including its utility in documenting long-term trends. This will also enable the measurement community to define the performance characteristics of prospective observing systems to resolve these deficiencies. . . Coordinate data set intercalibratior' arid comparison with results from validation experiments. The GVaP project should take specific responsibility to examine and understand the systematic differences between available water vapor data sets and those resulting from its own analyses. This effort would be the first step in developing and applying new, consistent calibrations to water vapor data. The pane! recommends that the development and implementation of a reference radiosonde be a high priority. Given the crucial importance to climate research of detecting long-term trends in tropospheric water vapor and the limited lifetime of GVaP, the implementation of a reference radio- sonde is important to ensure the ongoing calibration of different moisture sensors. Highlight upper tropospheric water vapor. Upper level water vapor feedbacks are a major source of uncertainty in global warming simulations. The proposed GVaP focus on global upper tropospheric water vapor could yield especially large benefits in addressing this issue, given the known strengths and weaknesses of the principal data sets to be -assimilated. Because of the growing collection of existing, and often conflicting, measurements for this region, GVaP needs to give additional thought to how it can best contribute to this subject. Close collaboration with the work of the SPARC program should be established, in particular with SPARC's work to better understand upper troposphere-stratosphere interactions. Create new water vapor products, including a merged global water vapor product. The emphasis here should be on a hierarchy of products, including, for example, higher resolution regional products and global products of different space-time resolutions. The pane! recognizes that for many users, however, a single merged, "final" product 11

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would be most useful and should be generated. The pane! strongly recommends that access to the base data used to derive a merged product be maintained, in light of the diverse applications of such data for which properties of "optimal" analyses may differ substantially. The techniques to be employed for a merged product should be drawn from the mathematical tools associated with data assimilation in support of numerical weather prediction. In applying data assimilation approaches to create water vapor products, the direct input of radiances may be preferable to the input of retrievals. Quantification of uncertainties, including bias and error covariance properties, should accompany the creation of any new water vapor data products intended for broad use. . Foster broad community involvement and sponsor workshops. End users of GVaP products must have mechanisms for providing feedback about the quality of these products. The free and open international exchange of GVaP data and products must be supported in order for the measurement, analysis, and modeling communities to be properly engaged. It is especially important for GVaP to engage those responsible for maintaining operational, global climate observing capabilities so that the advances of GVaP can extend beyond its proposed lifetime. GVaP should also involve those responsible for developing new measurement capabilities by documenting and prioritizing deficiencies in current data bases so that performance characteristics of new observing systems (including sampling strategies and optimal mixes of measurements and models) can be designed to remedy the most important problems. Furthermore, in addition to the focused workshops described in the draft Implementation Plan, GVaP ought to help sponsor more broadly based conferences organized by others interested in water vapor-related issues. Community interaction might also be facilitated through the effective use of Web-based tools. To accomplish the goals discussed above, the Data Development activities described in sections 2.2.4 and 2.2.5 of the draft Imple- mentation Plan provide a structure that can deliver meaningful and necessary products to users. The panel believes that the heart of this structure, the Global Processing Center envisioned in the draft 12