Tropospheric Parameters from Multi-GNSS and Numerical Weather Models: Case Study of Severe Precipitation in Germany in July 2021

Severe precipitation can lead to destructive flooding causing loss of lives and material damage. In July 2021, a series of storms with prolonged rain episodes took place over central Europe. The flood that accompanied these events was one of the deadliest in Germany, with over 200 casualties. A good understanding and forecasting of such events is thus of uttermost importance.

This study presents a comparisons of tropospheric parameters, i.e. Zenith Total Delays (ZTD), tropospheric gradients and Slant Total Delays (STDs) from Global Navigation Satellite Systems (GNSS) and Numerical Weather Models (NWMs) for a week in July 2021 that contained the strong precipitation leading to the destructive flooding in western Germany. The tropospheric parameters are calculated from three GNSS systems (GPS, GLONASS and Galileo) and three NWMs (ERA5 reanalysis of ECMWF, the regional ICON-D2 model run by the German Weather Service and the global GFS model run by the US Weather Service).

The results show that all considered GNSS solutions have a similar level of agreement with the NWMs. However, for the severe event and the flooding region, the multi-GNSS solutions have better agreement with the NWMs than GPS only, especially in terms of ZTDs. It demonstrates that the multi-GNSS data can clearly show the significant features related to the strong precipitation and can be used for the assimilation into weather models in the future. From the models, ICON has the highest agreement with the GNSS data for all considered tropospheric parameters. The best agreement with ICON is probably due to its high horizontal resolution and thus low representative errors and the fine tuning of DWD's regional model for the specific region (Germany).