GNSS-radiosonde three-year IWV comparisons in the framework of the GRUAN project

Established in 2006, the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is an international observing network of sites worldwide. Currently, 33 reference sites are designed to detect long-term trends of key climate variables above the Earth’s surface, such as temperature and humidity in the atmosphere. They provide high quality long-term atmospheric measurements using balloon-borne sensors, in particular radiosondes. 

Data from co-located GNSS stations are processed at the GFZ with the EPOS 8 software. This provides GNSS Integrated Water Vapor (IWV) time series co-located with the Vaisala RS41 radiosonde.

This study provides the first three-year comparison, between 2021 and 2023 between GNSS and radiosonde IWV time series at nine sites. The study focuses on small dry bias of GNSS to radiosonde comparisons obtained in previous studies. To this end, several GNSS processing options that may contribute to this bias are discussed. Several approaches to GNSS and radiosonde comparisons are presented, leading to different results in the calculated bias. Finally, the time series provided by the radiosondes are examined in detail, and the IWV is recomputed from the cleaned radiosonde measurements to correct some discrepancies found in the radiosonde-provided IWV profiles.