Report of the Eleventh GCOS Reference Upper Air Network Implementation Coordination Meeting (GRUAN ICM-11) - GCOS-230

This report documents the proceedings and outcomes of the eleventh implementation coordination meeting of the Global Climate Observing System Reference Upper Air Network held in Singapore in May 2019. It reviews progress in developing and maintaining a ground-based network designed to deliver highly accurate upper-air observations supporting long-term climate records, satellite validation, atmospheric process studies and numerical weather prediction. The meeting assessed advances in data products, measurement techniques, and instrument transitions, particularly the shift between radiosonde systems and the development of new observational products. Discussions also addressed operational challenges, network expansion, uncertainty characterisation, data policy and governance. The report records presentations, technical updates, and agreed actions intended to improve data quality, expand observational capacity and strengthen coordination among participating sites and institutions.