Evaluating the potential of integrated water vapour data from ground-based, in-situ and satellite-based observing techniques R. Van Malderen 1 , E. Pottiaux 2 , H. Brenot 3 , S. Beirle 4 1 Royal Meteorological Institute of Belgium, Ringlaan 3, B-1180 Brussels, Belgium +32 2 373 05 95, Roeland.VanMalderen@meteo.be 2 Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels, Belgium 3 Belgian Institute of Space Aeronomy, Ringlaan 3, B-1180 Brussels, Belgium 4 Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz,Germany ABSTRACT Water vapour plays a dominant role in the climate change debate. However, observing water vapour for climatological timescales in a consistent and homogeneous manner is challenging. To this end, Integrated Water Vapour (IWV) estimations derived from ground-based observations from Global Navigation Satellite Systems (GNSS) networks such as the International GNSS Service (IGS) network are very promising, with continuous observations spanning over the last 15+ years. Also, the AErosol RObotic NETwork (AERONET) provides long-term and continuous ground- based observations of the IWV performed with standardized and well-calibrated sun photometers. The aim of the present study is to assess the applicability of either dataset for water vapour time series analysis. Therefore, we compare IWV values retrieved (at zenith) from these two techniques, focusing on a selection of 28 sites located worldwide. We show that both techniques agree at the level of -0.26 mm +/- 1.41 mm of IWV. In a case study at the station Uccle (Brussels, Belgium), we further investigate the influence of the clouds on the IWV inter-technique comparison. Additionally, for our selection of 28 sites, we compare the GNSS and sun photometer IWV values with simultaneous and co-located radiosonde and satellite-based IWV measurements (GOME/SCIAMACHY/GOME-2). In particular, we investigate the geographical dependency of the properties of the IWV scatter plots between all these different instruments. 1 Introduction Water vapour is a key variable for climate research. It is the dominant greenhouse gas in the atmosphere and provides the largest known feedback mechanism for amplifying climate change. The knowledge of the temporal and spatial variability of water vapour is of major importance to understand and predict any change in our climate system. Radiosondes have been widely used in the literature to assess the trends in the Integrated Water Vapour (IWV), although these datasets suffer from large inhomogeneities due to humidity sensor improvements between different types of radiosondes (e.g. Van Malderen and De Backer [2010]). The major advantage of radiosonde measurements is their long temporal coverage. Looking for homogeneous datasets of IWV with increasing time coverage, networks of ground-based Global Navigation Satellite Systems (GNSS) receivers (such as the International GNSS Service (IGS) network) or sun photometers (the Aerosol Robotic NETwork (AERONET)) provide very promising estimations of the IWV. Also databases obtained by merging adequately satellite retrievals (e.g. from GOME, SCIAMACHY and GOME-2 instruments) are extending over more than 15 years and moreover, offer a global spatial coverage. In this paper, we examine the capability of each technique to generate homogeneous, unbiased, and long-term IWV time series by comparing simultaneous measurements at co-located sites.