*2.3. ZTD Observations*

GNSS applications in meteorology require careful characterization and assessment of tropospheric products derived from ground-based GNSS receivers. This is one of the key specialization areas for the GNSS Quality Control (GNSS-QC) team of the Aristotle University of Thessaloniki (AUTh), which processes and monitors a network of permanent GNSS stations located in Greece. The network consists of over 90 GNSS reference stations deployed by both public and private organizations. Four out of the 90 GNSS stations are included in the EUREF (European reference frame; [54]) permanent network (EPN). The GNSS-QC group has contributed to the E-GVAP project since 2014, providing NRT ZTD estimates to the European NWP centers [55–57].

In the current study, hourly ZTD observations, at 0000 UTC and 0600 UTC, of the AUTh GNSS-QC network were used for the WRF data assimilation experiments. The data collection and analysis were carried out for 2018 in the framework of BERTISS (Balkan-Mediterranean Real Time Severe Weather Service) project. An automated NRT processing scheme was employed using the Bernese software package, version 5.2 [58], and several coding scripts and algorithms for computations, data management, quality control, and database upload [15,59].

Since the observations accuracy and their associated errors is crucial in improving the model analysis, the quality control of the GNSS ZTD estimates and the appropriate treatment of ZTD formal errors within the 3D-var/WRF system are necessary. For this, a thorough observations pre-processing was applied in the present work to prepare the ZTD data for the assimilation process.
