GRAS SAF RADIO OCCULTATION PROCESSING CENTER

The GRAS SAF is part of EUMETSATs network of Satellite Application Facilities (SAFs) under the EUMETSAT Polar System (EPS). The objective of the GRAS SAF is to deliver operational radio occultation products from the GRAS occultation instruments (Global Navigation Satellite System Receiver for Atmospheric Sounding) onboard the three Metop satellites. The host institute is the Danish Meteorological Institute (DMI) and this will also be the physical location of the operational GRAS SAF processing center. The two other project partners are the IEEC (Institute d’Estudis Espacials de Catalunya, Barcelona, Spain) and the Met Office (Exeter, UK). The GRAS SAF will enter into the operational phase and deliver products from around the end of 2006 (given the current launch plans for Metop). The archiving of GRAS SAF products is done locally at the host institute with a user interface at the UMARF archive at EUMETSAT. The basic principle of the radio occultation (RO) method is that a receiver onboard a low-orbiting satellite tracks GPS signals, as the transmitting satellite sets or rises behind the Earth. Due to refraction in the ionosphere and the neutral atmosphere the signal is delayed and its path bent, enabling calculation of the index of refraction (or refractivity) and subsequently temperature and humidity as a function of height. The operational GRAS SAF Processing and Archiving Center will receive raw and preprocessed GPS radio occultation data from the GRAS instrument, process these into vertical height profiles of refractivity, temperature, pressure, and humidity, and distribute these products continuously in NRT (near real time, within 3 hours from sensing) to numerical weather prediction users. In addition, offline products (improved products, within 30 days from sensing) will be disseminated to e.g. climate monitoring users. Another objective of the GRAS SAF is to supply a software package denoted ROPP (radio occultation processing package) containing tools for 4D-VAR-assimilation of radio occultation data into numerical weather prediction models. The results of such NWP assimilation impact trials using CHAMP data show a clear positive impact on NWP forecasts in the upper troposphere and lower stratosphere. Because raw GPS radio occultation data are calibration free and the assumptions are known, RO data is also well suited for climate investigations and monitoring. We are currently undertaking studies on how to best exploit the GRAS data, both for construction of an accurate single-source climate data base with known error characteristics of the data and for provision of global climate monitoring.