Abstract
Abstract. Ground based GPS zenith path delay (ZPD) measurements are well established as a powerful tool for integrated water vapor (IWV) observation. The International GNSS Service (IGS) provides ZPD data of currently more than 300 globally distributed GPS stations. To derive IWV from these data, meteorological information (ground pressure and mean temperature above the station) are needed. Only a limited number of IGS stations is equipped with meteorological ground sensors up to now. Thus, meteorological data for IWV conversion are usually derived from nearby ground meteorological observations (ground pressure) and meteorological analyses (mean temperature). In this paper we demonstrate for the first time the applicability of ground pressure data from ECMWF meteorological analysis fields in this context. Beside simplified data handling (no single station data and quality control) this approach allows for IWV derivation if nearby meteorological stations are not available. Using ECMWF ground pressure and mean temperature data the new IGS 5-min ZPD data set has been converted to IWV for the first time. We present initial results from selected stations with ground meteorological sensors including pressure and temperature comparisons between ECMWF and local measurements. The GPS IWV is generally validated by comparison with ECMWF IWV. The ECMWF derived station meteorological data are compared with local measurements at all accordingly equipped stations. Based on this comparison, the mean error (in terms of standard deviation) introduced by time interpolation of the 6-hourly ECMWF data is estimated below 0.2 mm IWV.
Highlights
Water vapor plays a key role in the Earth’s atmosphere system
Pressure is calculated by stepwise application (20 m steps) of the barometric height formula and specific humidity for integrated water vapor (IWV) integration is estimated in parallel, assuming that the mean relative humidity of the two lowest European Centre for Medium-Range Weather Forecasts (ECMWF) levels is representative for the atmosphere below
Considering the already mentioned reliability limitations of International GNSS Service (IGS) surface meteorological data we find from the bias results no general impropriety of ECMWF Ps data for zenith path delay (ZPD) to IWV conversion
Summary
Water vapor plays a key role in the Earth’s atmosphere system. It significantly influences the hydrological cycle (water transport, clouds, precipitation) and transports large amounts of latent heat energy. While there are numerous investigations using GPS IWV on a regional scale, only one study describing a long-term IWV data set from the global 2-hourly IGS ZPD product is published up to now (Wang et al, 2007). While NWP analysis fields are widely used to derive the temperature information (e.g., Wang et al, 2005), their accuracy concerning ground pressure data is under discussion (e.g., Hagemann et al, 2003; Nilsson and Elgered, 2008). In this paper we demonstrate for the first time the applicability of ground pressure data from global meteorological analysis fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) in this context. We compare the ECMWF-derived station meteorological data with local measurements at all equipped IGS stations This allows for accuracy estimation regarding temporal interpolation between the 6-hourly ECMWF analysis fields. The GPS IWV is generally validated by comparison with ECMWF IWV at analysis times
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