GNSS water vapor tomography utilizing the German GNSS networks

The GNSS water vapor tomography is a new remote sensing technique which provides temporally and spatially resolved humidity information for all weather conditions. Such data are not only required by high resolution numerical weather models but also by many meteorological applications such as nowcasting, hazard mitigation or water management. The German Research Center for Geosciences (GFZ) operates a GPS water vapor tomography system which makes use of more than 300 German GPS stations. The GPS data processing runs in near real-time and provides ZTD, IWV and slant delay data operationally, the latter with a sampling rate of 2.5 minutes. This large data set of more than 60000 slant delays per hour provides substantial information about the spatiotemporal water vapor distribution. The tomography system is currently based on iterative reconstruction techniques which can process such a large number of observations within several minutes and has therefore near real-time capabilities. Water vapor fields with a spatial resolution of about 35 km horizontally, several hundred meters vertically and a temporal resolution between 15 and 60 minutes are reconstructed and validated. Reconstructed humidity fields for different weather situations will be compared to analyses of numerical weather models (COSMO-DE) and to radiosonde profiles. The GNSS satellite constellation has a considerable impact on the quality of the tomographic reconstruction. The information contained in a given GNSS slant data set is estimated and related to the reconstruction quality of time series of several hours. The spatial coverage of the atmosphere depends considerably on the number of available GNSS satellites. Currently, only GPS data are analyzed but attempts are made to use also GLONASS data and to prepare the processing system for Galileo. The GNSS tomography will benefit from the increasing number of GNSS satellites. Simulation studies with all three positioning systems will show the impact on the resolution and quality of the reconstructed humidity fields. Networks densified with inexpensive single frequency receivers will further improve the situation and lead to reliable 3D water vapor monitoring systems on a large scale.