Publications

GPS based limb sounding with CHAMP allows for precise sounding of the electric neutral and ionized part of the Earth’s atmosphere on global scale. Nearly 300,000 occultation measurements will be performed as of September 2004 to derive vertical profiles of refractivity, temperature, water vapor and electron density. The CHAMP mission is expected to last until 2007, the first and unique long-term data set of GPS occultation measurements, covering a period of ~6 years, is anticipated. Data and analysis results are available via the Information System and Data Center (ISDC) at GFZ. They are currently in use by more than 40 research groups all over the world to prepare and improve analysis centers for future occultation missions (e.g. COSMIC, METOP or EQUARS), to demonstrate improvement of global weather forecasts, to show ability to detect climate trends, and to improve and calibrate ground and satellite based atmosphere sounders. The analysis results (up to 200 globally distributed atmospheric excess phase data and vertical profiles of refractivity, temperature and water vapour and up to ~150 vertical electron density profiles per day) are generated by automatically working processing systems at GFZ Potsdam and DLR Neustrelitz (Ionosphere). We review the status of GPS radio occultation with CHAMP and review recent results of the scientific data analysis of the neutral gas and ionospheric measure¬ments with emphasis to the neutral atmosphere results. A set of nearly 200,000 CHAMP occultations was analysed with the current version of GFZ’s operational inversion software (005). The data are used for a validation study with meteo¬rological data from ECMWF and the global radiosonde (RS) network. Mean temperature deviations below 1 K and rms errors within the 1-2 K range are observed in the upper troposphere and stratosphere. The refractivity is nearly bias-free (<0.5 %) with rms errors of about 1% within this altitude range. The application of the Full Spectrum Inversion (FSI) technique for the Lower Troposphere (LT) data analysis reduced the known refractivity bias in comparison to the previous product version significantly. The FSI amplitude was used to define a cut-off criterion for the LT data without using additional meteorological and climatological data. Two standard methods (classical iterative, 1Dvar) for the water vapor derivation were implemented and applied to the CHAMP data set. Their performance will be evaluated by comparison with RS data and compared to that of an modified iterative method, which is intended to use for the operational data analysis. Further results, which will be briefly reviewed are: validation of electron density profiles with data of the global ionosonde network, use of the occultation data for calibration of the ENVISAT MIPAS instrument, comparisons of CHAMP analysis results from different processing centers (Radio Occultation Sensor Evaluation activity, ROSE), and long term studies for analysing climate relevant thermal variations in the tropopause region.

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Wickert, J.; Schmidt, T.; Beyerle, G.; Michalak, G.; König, R.; Reigber, C.; Heise, S. (2006): GPS radio occultation with CHAMP and GRACE: Recent results. - In: Foelsche, U.; Kirchengast, G.; Steiner, A. (Eds.), Atmosphere and climate : studies by occultation methods, Springer, 3-16.