Validation of refractivity profiles derived from GRAS raw-sampling data

Abstract

Results from GRAS (GNSS Receiver for Atmospheric Sounding) RO (Radio Occultation) data recorded in RS (Raw Sampling) mode processed at the GFZ (German Research Centre for Geoscience) Potsdam are presented. The experimental processing software POCS-X includes FSI (Full Spectrum Inversion) in order to cope with multi-path regions and enables in connection with RS data to retrieve atmospheric refractivity profiles down to the Earths surface. Radio occultation events observed between 30 September and 30 October 2007 are processed and the retrievals are validated against co-located ECMWF (European Centre for Medium-Range Weather Forecasts) profiles. The intercomparison indicates good quality of the retrieved profiles. In the altitude range 8 to 25 km the standard deviation is below 1 %. The mean deviation in this altitude range tends to be negative. At 30 km the negative bias reaches about −0.4 %. Below 8 km the standard deviation increases, reaching 2.5 % at 2 km. Below 2 km the mean deviation tends to be negative, reaching −1.9 % close to the ground. The negative bias mainly stems from the tropical lower troposphere; there, the negative bias reaches −3 %. The tropospheric penetration depth obtained from RS data shows a vast improvement compared to the tropospheric penetration depth typically obtained from CL (Closed Loop) data; 50 % of all retrieved profiles reach 720 m.