A perspective on the fundamental quality of GPS radio occultation data

Abstract

Abstract. Radio Occultation (RO) is a promising source of observation for weather and climate applications. However, the uncertainties in processing and retrieving RO data may weaken the overall confidence in the data and discourage their use. This study assesses the fundamental quality of RO data, by modeling the "raw" measurement, phase path, through a ray tracing method without the nuisance of retrieval errors. The comparison of phase measurements with the European Center for Medium-Range Weather Forecasts (ECMWF) data made in the observation space shows that the RO measurement is of sufficient accuracy to uncover regional-scale systematic errors in ECMWF's operational analysis and the 45 year reanalysis (ERA40), and to clearly depict the error growth of short-term ERA40 forecasts. In the southern hemispheric stratosphere, in particular, the RO measurement served as a robust reference against which both of the two analyses were significantly biased in opposite directions even though they were produced by the same center using virtually the same set of data. The measurement and ECMWF analyses showed a close agreement in the standard deviation. This confirms the high accuracy of the RO measurement and also indicates that the main problem of the ECMWF analyses lies in their systematic error.