GRAS-SAF radio occultation data from EPS/Metop

The EUMETSAT network of Satellite Application Facilities (SAFs) will, together with the EUMETSAT central facilities, constitute the future ground segments for the Meteosat Second Generation and EUMETSAT Polar System (EPS)/Metop satellites. SAFs are located in national meteorological offices or other approved institutes of EUMETSAT member states. The scope of the SAF activities will be to deliver products and/or software to derive these products, at the level of meteorological parameters, based primarily on the satellite data. The GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) SAF will receive raw and pre-processed GPS radio occultation data from the GRAS instrument onboard the EPS/Metop satellite, process these into vertical profiles of refractivity, temperature, pressure, and humidity, and distribute these products continuously to numerical weather prediction and climate monitoring users. The GRAS SAF products promise globally distributed high-vertical resolution data on a 24 hour-basis, forming a considerable improvement on today's measurement methods regarding coverage, vertical resolution, frequency, and cost. The accuracy requirements of the products vary with height. The expected root mean square error on temperature will be between 0.5 and 5 K, on pressure between 0.5 and 2 hPa, on specific humidity between 0.025 and 1 g/kg, and on refractivity between 0.1 and 2%. Vertical resolution will be between 0.3 and 3 km in the vertical range covered from surface up to 50 km. A second objective of the GRAS SAF is to supply software for 4D-VAR assimilation of radio occultation data into Numerical Weather Prediction (NWP) models. The GRAS SAF is expected to distribute and archive atmospheric products from around 500 occultations per day at least through the expected project lifetime of 15 years. The main users of the GRAS SAF products will be meteorologists doing NWP data assimilation, and users from the climate research and atmospheric science communities, needing comprehensive, globally distributed temperature, pressure, and humidity information. The products will come in two types: Near-Real Time Products (for NWP), disseminated less than three hours after acquisition, Offline Products (improved, for climate research), available less than 30 days after acquisition. The focus of this paper is on the design of the operational GRAS SAF facility. The GRAS SAF processing system and data products will be described and the user requirements for both Numerical Weather Prediction (NWP) and climate monitoring will be presented.