**5. Conclusions**

The present work focuses on investigating the impact of assimilating GNSS ZTD observations on the WRF model precipitation forecast skill. The conducted work is the first attempt at applying ZTD data assimilation into a regional atmospheric model over Greece. The evaluation of the impact of ZTD assimilation is performed for seven high-impact rainfall episodes that occurred during the dry (four events) and wet period (three events) of 2018. A substantial effort was devoted to the pre-processing of ZTD observations in order to qualify their adequacy for data assimilation. Then, two sets of model experiments (CTL and ZTD) were performed and the model predicted rainfall was verified against observations, which were collected from over 330 rain gauges provided by the weather monitoring network of the National Observatory of Athens.

The qualitative and quantitative statistical evaluation provides substantial findings on how the assimilation of ZTD observations into the WRF model affects its performance:


The detailed presentation of two case studies (on 10 May and 18 November, 2018) show that the WRF model is capable of satisfactorily capturing the spatial and temporal distribution of the observed rainfall, with the ZTD simulation providing results that are closer to the observations. It also reveals that the

precipitation forecast improvement under the ZTD experiment is induced by the initial conditions' modification at the experiment's initialization time, especially affecting the initial PW field, due to the ZTD assimilation.

The above findings highlight the beneficial impact of assimilating ZTD observations into high-resolution regional scale weather forecasting systems. It is the authors' intention to investigate the impact of variational ZTD bias correction techniques and of ZTD spatiotemporal error correlations in the assimilation framework.

**Author Contributions:** Conceptualization, C.G., V.K., K.L., T.M.G., C.P.; Data curation, C.G., C.P.; Formal analysis, C.G.; Funding acquisition, V.K., K.L., C.P.; Investigation, C.G.; Methodology, C.G., V.K., K.L., T.M.G.; Resources, V.K., K.L., C.P.; Software, C.G., T.M.G.; Supervision, V.K., K.L, T.M.G; Validation, C.G., V.K., K.L., T.M.G; Visualization, C.G.; Writing – original draft, C.G.; Writing – review & editing, C.G., V.K., K.L, T.M.G., C.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** The present research was conducted in the framework of the BeRTISS project which is financed by the Transnational Cooperation Programme Interreg V-B "Balkan-Mediterranean 2014–2020" co-financed by the European Union (European Regional Development Fund–ERDF) and National funds of the participating countries.

**Acknowledgments:** Weekly combined ZTD solutions over AUT1 station used in the current study were kindly provided by the EUREF permanent GNSS network (http://www.epncb.oma.be/).

**Conflicts of Interest:** The authors declare no conflict of interest.
