# GNSS for Weather Forecast and Climate Research at GFZ

Galina Dick¹, Florian Zus¹, Rohith Thundathil¹,², Aurélie Panetier¹, and Jens Wickert¹,²

(1) GFZ Helmholtz Centre for Geosciences, Potsdam, Germany; (2) Technische Universität Berlin, Germany (Contact: dick@gfz.de)

## Why GNSS for atmospheric water vapor?

* ✓ High resolution in space and time, all-weather capability, low costs

* ✓ High accuracy (about 1mm) of derived precipitable water vapor (PWV), comparable with measurements by meteorological instruments like radiosondes (RS), water vapor radiometers (WVR) and others

* ✓ Long-term stability, continuous time series, water vapor trends with mm accuracy for climate research

<figure><img src="image_1.png" /><figcaption>Radiosonde</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>Water Vapor Radiometer</figcaption></figure>

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$$PWV = PWV (ZTD, T, p)$$

## GNSS for Weather Forecast: E-GVAP and MOSES

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GFZ runs Analysis Centre for E-GVAP, the EUMETNET GNSS Water Vapour Programme

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Automatic processing of hourly GNSS data with GFZ EPOS software

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About 600 stations in near real-time (NRT) processing (German SAPOS +EUREF + IGS)

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Delay <15 minutes after end of each hour for 'Ultra-rapid' products

**Products:**

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Zenith Total Delay (ZTD), Integrated Water Vapor (IWV), Slant Total Delay (STD), Tropospheric Gradients (GN/GE)

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Time resolution: ZTD/IWV/GN/GE 15 min, STD 2.5 min

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GNSS stations network, processed in NRT for E-GVAP: 'Global' solution of GFZ with about 600 stations and a delay of about 50 minutes after end of each hour

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Swabian MOSES 2.0 measurements campaign in summer 2023 on hydro-meteorological extremes in the South of Germany with 9 GNSS receivers installed by GFZ/KIT.

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Integrated water vapor, calculated in NRT by the GFZ for GNSS station Hepsisau during MOSES campaign in July-August, 2023. Good agreement with NWM ERA-5 and ICON

## GNSS for Climate Research: GRUAN

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**GRUAN:** Global Climate Observing System (GCOS) Reference Upper-Air Network for ground-based reference observations of the atmosphere for climate in the frame of GCOS (WMO)

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**GRUAN measurements:** temperature, pressure, upper-air water vapor

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GNSS is priority number one technique for water vapor monitoring

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Central Processing Centre for GNSS-PWV established at GFZ in 2014

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Certification of GFZ GNSS-PWV products as GRUAN Data Product (GDP) on-going

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Validation of GNSS-PWV with Water Vapor Radiometer and NWM ERA-5 for GRUAN station Payerne (Switzerland) for 2023

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Validation of GNSS-PWV with Radiosondes (GDP RS41) for GRUAN station Ny-Ålesund (Norway) for 2021

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**GRUAN GNSS Stations Network**
20 GNSS stations in PWV processing (blue), 2 new stations planned (red)
In yellow GRUAN sites without GNSS

<table><thead><tr><th>SITE ID</th><th>BIAS (mean -0.18/-0.69 kg/m2)</th><th>STDDEV (mean +-1.29/1.26 kg/m2)</th></tr></thead><tbody><tr><td>Graciosa</td><td>+ 0.48 -1.00</td><td>+/- 1.60 1.15</td></tr><tr><td>Lindenberg</td><td>- 1.10 -1.28</td><td>+/- 1.00 1.26</td></tr><tr><td>Lauder</td><td>- 0.40 -0.21</td><td>+/- 1.71 3.62</td></tr><tr><td>Ny-Ålesund</td><td>- 0.20 -0.46</td><td>+/- 0.56 0.64</td></tr><tr><td>Payerne</td><td>- 0.40 -0.39</td><td>+/- 1.10 1.01</td></tr><tr><td>Lamont</td><td>- 0.19 -0.51</td><td>+/- 1.69 1.25</td></tr><tr><td>Singapore</td><td>+1.20 -1.20</td><td>+/- 2.55 0.93</td></tr><tr><td>Sodankylä</td><td>- 0.16 -0.21</td><td>+/- 0.74 0.74</td></tr><tr><td>Barrow</td><td>- 0.86 -0.99</td><td>+/- 0.70 0.72</td></tr></tbody></table>

Validation of GNSS-PWV with Numerical Weather Model ERA5 and Radiosondes (GDP RS41) for GRUAN stations.
Table shows bias and stddev of GNSS-PWV minus ERA5 (blue) and RS41 (red) for 2023

GRUAN
www.gruan.org

<figure><img src="image_21.png" /></figure>

GFZ GNSS receiver on GRUAN station Ny-Ålesund, Norway

## Outlook

### GNSS Meteorology:

* ✓ Operational assimilation of 'Slant Delays' by DWD and other weather services

* ✓ Further assimilation experiments with tropospheric gradients by GFZ

### Climate research:

* ✓ GRUAN: new GNSS sites

* ✓ WegenerNet: GNSS for climate monitoring in cooperation with University Graz, Austria

### References:

Dick, Galina; Wickert, Jens; Zus, Florian; Benjamin, Männel; Bradke, Markus; Ramatschi, Markus; Brandt, Andre (2025): GFZ GNSS-derived tropospheric data products (global). V. 1.0. GFZ Data Services. doi.org/10.5880/GFZ.IRZR.2025.003

Dick, G., Wickert, J., Zus, F., Männel, B., Bradke, M., Ramatschi, M., Brandt, A. (2024): GFZ GNSS-derived precipitable water vapor for climate reference network GRUAN. V. 1.0. GFZ Data Services. https://doi.org/10.5880/GFZ.1.1.2024.003

Thundathil, R., Zus, F., Dick, G., & Wickert, J. (2024). Assimilation of GNSS tropospheric gradients into the Weather Research and Forecasting (WRF) model version 4.4. 1. Geoscientific Model Development, 17(9), 3599-3616.