Rapid estimation of the tropospheric wet delay based on microwave observations of permanent GNSS reference station networks and its potential impact on weather forecast (GNSS-MET)
The importance of high resolution meteorological analysis of the mountain atmosphere has increased in the last years due to local and regional extreme precipitation. A detailed analysis of the humidity field is an important precondition for better monitoring and better forecasting of these events. For this reason, the ZAMG is operating the spatial and temporal high resolution INCA system (INCA = Integrated Nowcasting through Comprehensive Analysis) since beginning of 2005. Errors in this analysis occur mainly in alpine areas where the predicted models do not reproduce the mountain atmosphere correctly, e.g. in Carinthia. The aim of the project presented is to provide GNSS based measurements of the tropospheric water vapour content with an temporal delay of less than 1 hour to use them within the INCA system. The determination of this humidity field requires a carefully the physical properties of the atmosphere. When GNSS (GPS, GLONASS, in future GALILEO) satellite microwave signals are transmitted through the atmosphere they are affected by the media. One of these components is the tropospheric time delay, which is calculated through global delay models, using pressure, temperature and humidity measurements at the observing station. The tropospheric time delay can be split up into the hydrostatic component, which can be determined very well, and into a wet component, describing the rapid variable water vapour content of the troposphere. To separate the hydrostatic part from the wet contribution the exact pressure at the GNSS Sensor Station has to be known or carefully extrapolated from nearby located Meteorological Sensor Stations (in Austria: TAWES network). The remaining Wet Delay (ZWD) may be converted into Integrated Water Vapour (IWV) if the temperature at the GNSS Sensor Station is available, too. In summary the ZWD is an integral value, but available with high temporal resolution and a horizontal resolution, depending on the mean baseline length in the reference station network. This wet component is also of great interest for the meteorological numerical weather predictions. To contribute to operational numerical weather prediction the water vapour content has to be available within 45-60 minutes. This requirement is hard to fulfil taking into account delays in data transfer, the large amount of observation data to be processed and last but not least the accompanying requirements on real-time orbit accuracy. The current proposal aims exactly at defining and setting up operational procedures and investigating more efficient algorithms to speed up the whole data processing to keep the required timelines. In addition the contribution of GLONASS observation data will be investigated for the first time. Till begin of 2005 no accurate GLONASS ephemeris were available in quasi-real time which prevents any use of GLONASS data before. In this context the KELAG reference-station network represents one of the still very rare networks observing both active navigation satellite systems and offers therefore an almost unique environment for this project. We like to establish a fast data transfer and efficient routines based on normal equation stacking to process the data in less than 60 minutes. This goes with an ongoing and almost real-time satellite orbit check. The Water Vapour estimates are forwarded to ZAMG to investigate and validate their potential and usefulness for operational weather forecasting. In addition further investigations aim at monitoring rapid water vapour changes with an extremely high time resolution around the station Sonnblick and at the installation of an GALILEO-IOV(In Orbit Validation) receiver for studying the potential impact of SIS of the new European Satellite Navigation System on the accuracy of Water Vapour estimates.
- Ana Karabatic
- FFG - Österr. Forschungsförderungs- gesellschaft mbH
- KELAG - Kärntner Elektrizitäts- Aktiengesellschaft
- Zentralanstalt für Meteorologie und Geodynamik