Experimental total uncertainty of the derived GNSS-integrated water vapour using four co-located techniques in Finland

Abstract. In this work, we examine data from a Global Positioning System (GPS) ground-based receiver, two co-located ground-based microwave radiometers (MWRs), and radiosondes (RAOBs) to characterize the uncertainties associated with integrated water vapour (IWV) values estimated from the GPS in a sub-Arctic climate region. The experiment was carried out during the Biogenic Aerosols–Effects on Clouds and Climate research campaign conducted using the Atmospheric Radiation Measurement Program's second Mobile Facility (AMF2) in collaboration with the University of Helsinki. The GPS receiver was located about 20 km away from the AMF2 instruments (radiometers and RAOB). The GPS data were processed in Precise Point Positioning mode using the state-of-the-art scientific software GIPSY-OASIS II. Differences between the GPS-derived IWV and that derived from the other three instruments are analysed in terms of bias, standard deviation, and root-mean-square error (RMSE). The availability of three co-located, independently calibrated systems (two MWRs and one RAOB) allows us to isolate issues that may be specific to a single system and to isolate the effects of the distance between the GPS receiver and the remaining instruments. The representativeness error due to the 20-km distance between the GPS and the other systems is of the order of 0.6–1.5 kg/m² and in this study is the dominant effect when the IWV is higher than 20 kg/m². The RMSE between the instruments shows that in the sub-Arctic region, when the IWV variability is less than 20 kg/m², the GPS agrees with other instruments to within 0.5 kg/m². When the variability of water vapour in the 20-km region is higher than 20 kg/m², mostly in the summer months, the GPS agrees with other instruments within 1–2 kg/m².