Intercomparison of tropospheric datasets from radiosonde and GNSS at co-located GRUAN sites
Résumé
Precipitable water vapour (PWV) is one of the key climate variables according to the Global Climate Observing System (GCOS). The GCOS Reference Upper Air Network (GRUAN) has been providing such measurements from multiple sensors at an increasing number of sites worldwide since 2006. Today, GRUAN consists of 33 sites, nine of which provide a GNSS station that has been co-located with the Vaisala RS41 radiosonde for several years yet. This study aims to detail the discrepancies between the radiosonde and GNSS time series at the nine co-located GRUAN sites from 2021 to 2023. The GNSS time series are computed by the GFZ software EPOS8, following a consistent computation configuration over the whole period and for all sites, set up for the zenith tropospheric delay (ZTD) retrieval for climatology purposes. The biases and standard deviations need to be well explained in order to use GNSS ZTD products for climate studies. The GNSS and radiosonde processing procedures are first described, including the conversion of GNSS ZTD to IWV. The following aspects are then discussed: the height difference between the GNSS antenna and the radiosonde height at the start of the launch, the screening of the IWV time series and the IWV differences, the statistics of the differences between the two time series for each of the nine sites, a different IWV computation strategy from the radiosonde relative humidity (RH) and its impact on the statistics of the differences, and finally the way of comparing the GNSS IWV to the radiosonde time series - the closest GNSS measurement from the radiosonde launch time or the average of the GNSS over one hour from the radiosonde launch time. All these aspects address the issue of data representativeness, i.e. the IWV retrieved by both techniques do not actually come from exactly the same spatio-temporal scale.
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