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Background In 1988 the World Climate Research Programme (WCRP) initiated GEWEX to observe, understand, and mode} the hydrological cycle and energy fluxes in the atmosphere, at the land surface, and in the upper oceans. GEWEX is an integrated program of research, observations, and science activities with the ultimate objective of substantially enhancing the ability to predict global and regional hydrologic processes and water resources and their response to environmental change. In 1990 the GEWEX Science Steering Group (an international advisory body) held a workshop in Easton, Maryland, titled "The Role of Water Vapor in Climate Processes." The discussions and papers presented at that workshop highlighted several key deficiencies in the understanding of water vapor's spatiotemporal characteristics. It was recognized that improvements in this understanding would be necessary to realistically characterize fundamental aspects of the atmospheric system, including radiative heating, precipitation, cloud formation, and horizontal and vertical moisture transport and convergence. Without improved understanding of these critical aspects of the atmospheric system, the ability of models to accurately predict weather and climate at all time scales will be significantly hampered. To address this need, a GEWEX Global Water Vapor Project (GVaP) was initiated, and in May 1991 a strategic research plan was prepared and published by the National Aeronautics and Space Administration (NASA), describing an approach for providing the data to answer some of these critical questions. This initial plan was designed to: (~) improve the accuracy and availability of global water vapor data through the development of a global water vapor data set, (2) establish reference observation stations, and (3) conduct intercomparison studies among the existing water vapor data sets. 3
In November 1991, a meeting was held in Columbia, Maryland, to discuss the implementation of this initial plan. The results of these discussions are described in a document titled "Implementation Plan for the Pilot Phase of the GEWEX Water Vapor Project." This plan included the addition of a fourth element, namely, the improvement and standardization of radiosonde humidity sensors and data r~roc~e.~ina procedures for worldwide use. The cornerstone of this initial phase has been the production of the NASA Water Vapor Project (NVAP) data set (Randel et al., 19961. This is a global, 9-year (1988-1996), 1x1 degree resolution product that quantifies both atmospheric water vapor and liquid water, with daily, pentad (5-day), and monthly temporal resolutions for three layers ~ ~ 000-700, 700-500, and 500-300 mb), as well as the entire atmospheric column. The NVAP data set, which is available on CD- ROM, was constructed through a blending of radiosonde, Special Sensor Microwave/Imager (SSM/I), and TIROS (Television and Infrared Observation Satellite) Operational Vertical Sounder (TOYS) water vapor soundings. In August 1993, a GVaP workshop was held in Breckenridge, Colorado, to discuss the current state of the art in satellite retrievals, radiosonde climatologies, the GVaP pilot phase, and the NVAP data set. In October ~ 994, an American Geophysical Union Chapman Conference on Water Vapor was held on Jekyll Island, Georgia, to review theoretical and observational aspects of water vapor and to identify areas of future research. The development of the NVAP data set and the advancement of validation processes during the initial phase of GVaP led the GEWEX Science Steering Group to conclude that the success of the pilot period, coupled with recognition of the potential importance of upper tropospheric and lower stratospheric water vapor for GEWEX, had set the stage for GVaP to make major contributions to GEWEX. An international planning workshop for GVaP was held in November 1996 in Geneva, Switzerland, at which representatives from WCRP, the National Oceanic and Atmospheric Administration (NOAA), and NASA endorsed the development of plans for the main phase of GVaP. In 1997, the GEWEX Joint Steering Committee recommended a new, 7-year phase of GVaP with the participation of all WCRP programs, including the Arctic Climate Systems Study (ACSYS), the Climate 4
Investigate how the Serologic cycle will change during periods of global warming Characterize the role of clouds and other processes that maintain the vertical distribution of water vapor. Examine the direct and indirect water vapor feedbacks on the climate system, including the relationship of water vapor to other climate variables such as sea surface temperature, cloudiness, andprecipitation. · Document the three-dimensional distribution of water vapor at time scales ranging from interannual down to short-term daily variability. Implementation of the main phase of GVaP will be organized around four major thrusts, discussed in both the Science Plan (IGPO, 199Sa) and the Implementation Plan (IGPO, 1998b): 1. Develop and deploy the tools for serif cation, validation, and calibration of observations for in situ and satellite retrievals of water vapor. Document the climatology of water vapor. Address the GVaP research areas. Develop and test new water vapor observing systems and instruments. GVaP will combine existing data, collected in NVAP, with a number of new data collection systems, including retrievals from satellite systems such as the Global Positioning System (GPS), the Advanced Microwave Sounding Unit (AMSU-B), the Advanced infrared Sounder (AIRS), and the Microwave Humidity Sensor (MHS). In addition, GVaP will take advantage of data from the Water Vapor Sensing System (WVSS), an aircraft-based component of the Meteorological Data Collection and Reporting System (MDCRS). The main phase of GVaP is being initiated with a series of workshops and research meetings to advance knowledge of water vapor data set calibration/validation issues, including algorithm development and data intercomparison. The first of these was the GVaP upper tropospheric humidity (UTH) Intercomparison Workshop held in June 1998 in Darmstadt, Germany, with the objective of quantifying existing differences in top-of-the-atmosphere radiances in the 6.3,um water 6
vapor absorption band as simulated by different radiative transfer codes. Other workshops are planned to steer activities in the following areas: lower tropospheric algorithm intercomparison, instrument validation and intercomparison, GVaP data set development, veri- f~cation of satellite water vapor, and water vapor science and app- lications. Validation-anc! calibration capabilities will be enhanced by the development and testing of new instruments and systems such as Differential Absorption Lidar (DIAL) and Raman Lidar, ground-based microwave and infrared radiometers, GPS measurements, improved radiosonde technologies, very long baseline interferometer (VERBS), and aircraft reporting systems. The Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) program will incorporate, within the ARM structure, responsibility for coordination of the calibration/validation activities and data collection and will undertake fully instrumented calibration/validation measurements at 4-5 locations around the world for verification of satellite retrievals and algorithm development. One of the key foci of GVaP will be documenting the climatology of water vapor. Issues at stake include defining the appropriate spatial and temporal resolutions for improving the understanding of various atmospheric phenomena, such as greenhouse warming and large-scale water vapor transport. in addition, the results of calibration/validation work will be assessed and drawn upon to characterize the accuracy of the data and to assist in developing data merging algorithms. This work will lead to defining the inputs and processing of a second GVaP data set (the follow-on to NVAP), which is proposed to include a minimum of 4-5 vertical levels. It will also lead to the development of an improved suite of water vapor products that include separate satellite- only, ground-only, and blended data sets. Such a hierarchical approach will allow independent comparison amongst the various data sources and merged products. GVaP will also assess the use of new ob- servational sources, such as those mentioned above, as they become available. The Implementation Plan (IGPO, 1998b) calls for these data sets to be used as part of GVaP investigations into the GVaP research objectives. The timeline given in the Implementation Plan indicates that these investigations, as well as the final development of the GVaP data sets, will be completed in 2005. 7
