While the global upper-air observing network has provided useful observations for operational weather forecasting for decades, its measurements lack the accuracy and long-term continuity needed for understanding climate change. Consequently, the scientific community faces uncertainty on such key issues as: the nature of temperature trends in the troposphere and stratosphere; the climatology, radiative effects, and hydrological role of water vapor in the upper troposphere and stratosphere; and the vertical profile of changes in atmospheric ozone, aerosols, and other trace constituents. Radiosonde data provide adequate vertical resolution to address these issues, but they have questionable accuracy and have time-varying biases due to changing instrumentation and techniques. Although satellite systems provide global coverage, their vertical resolution is sometimes inadequate and they require independent reference observations for sensor and data product validation, and for merging observations from different platforms into homogeneous climate records. To address these shortcomings, and to ensure that future climate records will be more useful than the records to date, the Global Climate Observing System (GCOS) program is initiating a GCOS Reference Upper Air Network (GRUAN) to provide high quality observations using specialized radiosondes and complementary remote sensing profiling instrumentation that can be used for validation.

This presentation outlines the scientific rationale for GRUAN, its role in the Global Earth Observation System of Systems, network requirements and likely instrumentation, management structure, current status and future plans. It also illustrates the value of prototype reference upper-air observations in constructing climate records and their potential contribution to the Global Space-Based Inter-Calibration System. We report on the establishment of a GRUAN Lead Centre and the first GRUAN site at the Meteorological Observatory in Lindenberg Germany. We invite constructive feedback on the GRUAN concept and the engagement of the scientific community.

This work is the result of collaborative efforts, over several years, by members of the GCOS Working Group on Atmospheric Reference Observations (Peter Thorne, Chair), the GCOS Secretariat (Dave Goodrich, former Director), the GRUAN Lead Centre (Holger Vömel, Director), and the US GCOS Program (Howard Diamond, Program Manager), a group too long to list individually but too important to neglect to acknowledge.