Preconditions to GPS water vapour tomography with a resolution relevant for data assimilation in weather forecast models

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Improved knowledge of water vapour fields are necessary for various meteorological and atmospheric research applications. Moisture distributions are used as input for atmospheric models and serve for precise numerical weather forecast. In the last decade, GPS-derived data products made many contributions to provide integrated water vapour (IWV). The knowledge of IWV characteristics is retrieved by analysing delays an the GPS signal caused by atmospheric gradients, using mapping functions and under the assumption of a known dry pressure field. The combination of several stations allow to produce two-dimensional maps of vertical IWV fields. Directly, it is possible to extract the water vapour content along single GPS rays, which state a integrated measurements. Considering a sufficient number of records in a defined area allows tomographic data assimilation. This would be a helpful tool to improve algorithms of moisture and precipitation forecast. A drawback of using only ground-based GPS receivers for water vapour reconstruction is the poor vertical resolution that can be achieved using links of small zenith angle only. In resent times, Low Earth Orbiter (LEO) satellites have been launched carrying an GPS receiver. Therefore, adding GPS-LEO links will significantly improve the vertical resolution, provided a large number of LEO satellite measurements are available. The spatial resolution that should be provided by such threedimensional fields are those used in local/regional weather forecast models. The aim of this study is to investigate the amount and constellation of GPS measurements - both groundbased and spacebased - necessary to perform tomographic reconstructions of water vapour content under the specifications requisite for use in numerical models.